VIII. MATERIALS FOR SEWERS
=90. Materials.=—The materials most commonly used for the manufacture of sewer pipe are vitrified clay and concrete. Cast iron, steel, and wood are also used, but only under special conditions. For pipes built in the trench, concrete, concrete blocks, brick, and vitrified clay blocks are used. Concrete is being used to-day more than bricks or blocks because it is cheaper. A decade or more ago all large sewers were built of bricks. Vitrified clay and concrete are used for manufactured pipe 42 inches and less in diameter. Concrete is used almost exclusively for larger sizes of pipe, particularly for pipe constructed in place, although a brick invert lining is advisable when high velocities of flow are expected.
The character of the external load, the velocity of flow and the quality of sewage are important factors in determining the material to be used in the construction of sewers. Reinforced concrete should be used for large sewers near the surface subjected to heavy moving loads. A high velocity of flow with erosive suspended matter demand a brick wearing surface on the invert. Many engineers consider concrete less suitable than vitrified clay or brick for conveying septic sewage or acid industrial wastes, as concrete deteriorates more rapidly under such conditions. Concrete should be used on soft yielding foundations, whereas a hard compact earth, which can be cut to the form of the sewer, is suitable to the use of brick or concrete.
Cast-iron pipe with lead joints is used for sewers flowing under pressure, or where movements of the soil are to be expected. If the sewage is not flowing under pressure, cement joints are sometimes used in the cast-iron pipe. Movements of the soil are to be expected on side hills, under railroad tracks, etc. Steel pipe is used on long outfalls or under other conditions where external loads are light and the cost is less than for other materials. Because of the thin plates used and the liability to corrosion steel is not frequently used. It should never be deeply buried nor externally loaded because of its weakness in resisting such forces. Like wood pipe, its lightness is favorable to use on bridges, but the greater heat conductivity of steel than wood necessitates protection against freezing in exposed positions. Wood is preferable only where the economy of its use is pronounced and the pipe is running full at all times. It is desirable that the wood pipe should be always submerged as the life of alternately wet and dry wood is short.
Corrugated galvanized iron and unglazed tile have been used for sewers, but usually only in emergencies or as a makeshift. Corrugated iron is not suitable on account of its roughness and liability to corrosion, and unglazed tile because of its lack of strength.
=91. Vitrified Clay Pipe.=—In general the physical and chemical qualities of clays before burning are not sufficient to cause their condemnation or approval by the engineer, as their behavior in the furnace is quite individual and depends greatly on the manner in which they are fired. The engineer is interested in the result and writes his specifications accordingly.
In the manufacture of clay pipe, the clay as excavated is taken to a mill and ground while dry, to as fine a condition as possible. It is then sent to storage bins from which it is taken for wet grinding and tempering. In this process the clay is mixed with water to the proper degree of plasticity. A variation of 1 to 1½ per cent in the moisture content will mean failure. Too wet a mixture will not have sufficient strength to maintain its shape in the kiln. Too dry a mixture will show laminations as it is pressed through the discs.
A press used in the manufacture of clay pipe is shown in cross-section in Fig. 71. With the piston heads in the steam and mud cylinders at their extreme upward positions, the mud cylinder is filled with clay of the proper consistency. Steam is then turned into the steam cylinder under pressure and the clay is squeezed into the space between the inner and outer shells of the die and mandrel to form the hub of the pipe. The pressure on the clay may be from 250 to 600 pounds per square inch. When clay appears at the holes, marked hh at the bottom of the mud cylinder, the bottom plate and the center portion of the die are removed and the remainder or straight portion of the pipe is formed by squeezing the clay between the mandrel and the outer wall of the die. A completely formed pipe can be seen issuing from the press in Fig. 72. Any sized pipe that is desired can be formed from the same press by changing the size of the dies and mandrel.
Curved pipes are made in two ways—by bending directly as they issue from the press, or by shaping by hand in plaster of paris molds. Junctions are made by cutting the branch pipe to the shape of the outside of the main pipe, fastening the branch in place with soft clay and then cutting out the wall of the main pipe the size of the branch. Special fittings are usually made by hand in plaster molds.
After being pressed into shape the pipes are taken to a steam-heated drying room where a constant temperature is maintained in order to prevent cracking of the pipes. They remain in the drying room from 3 to 10 days until dry, when they are taken to the kilns. If taken to the kilns when moist blisters will be produced.
The dried pipes are piled carefully in the kiln so that heat and weight may be as evenly distributed as possible, and the fire is then started in the kiln. The process of burning can be roughly divided into five stages:
1st. Water smoking, which lasts about 72 hours during which the temperature is raised gradually to 350 degrees Fahrenheit.
2nd. Heating, during which the temperature is raised to 800 degrees Fahrenheit in 24 hours.
3rd. Oxidation, during which the temperature is raised to 1,400 degrees Fahrenheit in 84 hours.
4th. Vitrification, in which the temperature is raised to 2,100 degrees Fahrenheit in 48 hours, and finally,
5th. Glazing, during which the temperature is unchanged but salt (NaCl) is thrown in and allowed to burn.
Oxidation must be complete before vitrification is started as otherwise blisters will be raised due to imprisoned carbon dioxide. The important points in vitrification are to make the required temperature within a reasonable time and to maintain a uniform distribution of heat throughout the kiln. When vitrification is complete as shown by a glassy fracture of a broken sample taken from the kiln, glazing is accomplished by throwing a shovelful of salt on the hottest part of the fire. About five to six applications of salt from two to three hours apart may be needed. The kiln is then allowed to cool and the manufacture of the pipe is complete. The completeness of vitrification is indicated by the amount of water that the finished pipe will absorb. Completely vitrified pipe will absorb no moisture. Soft-burned pipe may absorb as much as 15 per cent moisture.
Vitrified clay blocks are made of the same material and in the same manner as vitrified clay pipe.
The following data on vitrified pipe have been abstracted from the specifications for vitrified pipe adopted by the American Society for Testing Materials.
Pipes shall be subject to rejection on account of the following:
(a) Variation in any dimension exceeding the permissible variations given in Table 36.
(b) Fracture or cracks passing through the shell or hub, except that a single crack at either end of a pipe not exceeding 2 inches in length or a single fracture in the hub not exceeding 3 inches in width nor 2 inches in length will not be deemed cause for rejection unless these defects exist in more than 5 per cent of the entire shipment or delivery.
(c) Blisters or where the glazing is broken or which exceed 3 inches in diameter, or which project more than ⅛ inch above the surface.
(d) Laminations which indicate extended voids in the pipe material.
(e) Fire cracks or hair cracks sufficient to impair the strength, durability or serviceability of the pipe.
(f) Variations of more than ⅛ inch per linear foot in alignment of a pipe intended to be straight.
(g) Glaze which does not fully cover and protect all parts of the shell and ends except those exempted in Sect. 31. Also glaze which is not equal to best salt glaze.
(h) Failure to give a clear ringing sound when placed on end and dry tapped with a light hammer.
(i) Insecure attachment of branches or spurs.
Workmanship and Finish
(29) Pipes shall be substantially free from fractures, large or deep cracks and blisters, laminations and surface roughness.
(31) The glaze shall consist of a continuous layer of bright or semi-bright glass substantially free from coarse blisters and pimples.... Not more than 10 per cent of the inner surface of any pipe barrel shall be bare of glaze except the hub, where it may be entirely absent. Glazing will not be required on the outer surface of the barrel at the spigot end for a distance from the end equal to ⅔ the specified depth of the socket for the corresponding size of pipe. Where glazing is required there shall be absence of any well defined network of crazing lines or hair cracks.
(32) The ends of the pipe shall be square with their longitudinal axis.
(33) Special shapes shall have a plain spigot end and a hub end corresponding in all respects with the dimensions specified for pipes of the corresponding internal diameter.
TABLE 36
PROPERTIES OF CLAY SEWER PIPE
Abstracts from Tentative Specifications of the American Society for Testing Materials
─────────┬─────────┬───────────┬───────┬────────┬──────┬──────┬───────── Internal │ Minimum │ Maximum │Laying │Diameter│Depth │Taper │ Minimum Diameter,│Crushing │Absorption,│length,│ of │ of │ of │Thickness Inches │Strength,│ Per Cent │ Feet │ Inside │Socket│Socket│ of │ Pounds │ │ │ of │Inches│ │ Barrel. │ per │ │ │Socket, │ │ │ Inches │ Linear │ │ │ Inches │ │ │ │ Foot. │ │ │ │ │ │ │See Note │ │ │ │ │ │ │ 2 │ │ │ │ │ │ ─────────┼─────────┼───────────┼───────┼────────┼──────┼──────┼───────── │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ ─────────┼─────────┼───────────┼───────┼────────┼──────┼──────┼───────── │ │ │ │ │ │ │ │ │ │ │ │ │ │ ─────────┼─────────┼───────────┼───────┼────────┼──────┼──────┼───────── 6 │ 1430 │ 5 │2, 2½, │ 8¼ │ 2 │1 : 20│ ⅝ │ │ │ 3 │ │ │ │ 8 │ 1430 │ 5 │2, 2½, │ 10¾ │ 2¼ │1 : 20│ ¾ │ │ │ 3 │ │ │ │ 10 │ 1570 │ 5 │2, 2½, │ 13 │ 2½ │1 : 20│ ⅞ │ │ │ 3 │ │ │ │ 12 │ 1710 │ 5 │2, 2½, │ 15¼ │ 2½ │1 : 20│ 1 │ │ │ 3 │ │ │ │ 15 │ 1960 │ 5 │2, 2½, │ 18¾ │ 2½ │1 : 20│ 1¼ │ │ │ 3 │ │ │ │ 18 │ 2200 │ 5 │2, 2½, │ 22¼ │ 3 │1 : 20│ 1½ │ │ │ 3 │ │ │ │ 21 │ 2590 │ 5 │2, 2½, │ 26 │ 3 │1 : 20│ 1¾ │ │ │ 3 │ │ │ │ 24 │ 3070 │ 5 │2, 2½, │ 29½ │ 3 │1 : 20│ 2 │ │ │ 3 │ │ │ │ 27 │ 3370 │ 5 │ 3 │ 33¼ │ 3½ │1 : 20│ 2¼ 30 │ 3690 │ 5 │ 3 │ 37 │ 3½ │1 : 20│ 2½ 33 │ 3930 │ 5 │ 3 │ 40¼ │ 4 │1 : 20│ 2⅝ 36 │ 4400 │ 5 │ 3 │ 44 │ 4 │1 : 20│ 2¾ 39 │ 4710 │ 5 │ 3 │ 47¼ │ 4 │1 : 20│ 2⅞ 42 │ 5030 │ 5 │ 3 │ 51 │ 4 │1 : 20│ 3 ─────────┴─────────┴───────────┴───────┴────────┴──────┴──────┴─────────
─────────┬────────────────────────────────────────────────┬──────── Internal │ Permissible Variations │ Number Diameter,│ │ of Inches │ │Scorings │ │ on │ │ Spigot │ │ and │ │Socket ⅛ │ │ Inch │ │ Deep ─────────┼───────┬─────────────┬────────┬───────┬─────────┼──────── │Length,│ Internal │ Length │ Depth │Thickness│ │ Inches│ Diameter, │ of Two │ of │ of │ │ (-), │ Inches │Opposite│Socket,│ Barrel, │ │ per │ │ Sides, │Inches │ Inches │ │ Foot │ │ Inches │ (-) │ (-) │ ─────────┼───────┼──────┬──────┼────────┼───────┼─────────┼──────── │ │Spigot│Socket│ │ │ │ │ │ (±) │ (±) │ │ │ │ ─────────┼───────┼──────┼──────┼────────┼───────┼─────────┼──────── 6 │ ¼ │ 3/16 │ ¼ │ ⅛ │ ¼ │ 1/16 │ 2 │ │ │ │ │ │ │ 8 │ ¼ │ ¼ │ 5/16 │ ⅛ │ ¼ │ 1/16 │ 2 │ │ │ │ │ │ │ 10 │ ¼ │ ¼ │ 5/16 │ ⅛ │ ¼ │ 1/16 │ 2 │ │ │ │ │ │ │ 12 │ ¼ │ 5/16 │ ⅜ │ ⅛ │ ¼ │ 1/16 │ 2 │ │ │ │ │ │ │ 15 │ ¼ │ 5/16 │ ⅜ │ ⅛ │ ¼ │ 3/32 │ 3 │ │ │ │ │ │ │ 18 │ ¼ │ ⅜ │ 7/16 │ 3/16 │ ¼ │ 3/32 │ 3 │ │ │ │ │ │ │ 21 │ ¼ │ 7/16 │ ½ │ 3/16 │ ¼ │ ⅛ │ 3 │ │ │ │ │ │ │ 24 │ ⅜ │ ½ │ 9/16 │ ¼ │ ¼ │ ⅛ │ 4 │ │ │ │ │ │ │ 27 │ ⅜ │ ⅝ │11/16 │ ¼ │ ¼ │ ⅛ │ 4 30 │ ⅜ │ ⅝ │11/16 │ ¼ │ ¼ │ ⅛ │ 4 33 │ ⅜ │ ¾ │13/16 │ ¼ │ ¼ │ 3/16 │ 5 36 │ ⅜ │ ¾ │13/16 │ ⅜ │ ¼ │ 3/16 │ 5 39 │ ⅜ │ ¾ │13/16 │ ⅜ │ ¼ │ 3/16 │ 5 42 │ ⅜ │ ¾ │13/16 │ ⅜ │ ¼ │ 3/16 │ 5 ─────────┴───────┴──────┴──────┴────────┴───────┴─────────┴────────
NOTE 1. For methods of making tests see Proc. Am. Soc. for Testing Materials.
NOTE 2. Concentrated load at end of vertical diameter.
(a) Slants shall have their spigot ends cut at an angle of approximately 45 degrees with the longitudinal axis.
(b) Curves shall be at angles of 90, 45, 22½, and 11¼ degrees as required. They shall conform substantially to the curvature specified.
(c) ... All branches shall terminate in sockets.
In Fig. 73 are shown the various forms of vitrified pipe and specials which are ordinarily available on the market.
The life of vitrified clay sewers and some observations on the results of the inspection of the sewers in Manhattan are discussed in Chapter XII. The strength of vitrified sewer pipes is shown in Table 37.
TABLE 37
STRENGTH OF SEWER PIPE
Strength in pounds per linear foot to carry loads from ditch filling material such as ordinary sand and thoroughly wet clay, with the under side of the pipe bedded 60° to 90° by ordinary good methods. From Proc. Am. Society for Testing Materials, Vol. 20, 1920, page 604. ────────┬────────────────────────────────────────────────────────────── Height │ of Fill │ Above │ Top of │ Pipe, │ Feet │ Breadth of the Ditch a Little Below the Top of the Pipe ────────┼───────────┬───────────┬───────────┬────────────┬───────────── │ 1 Foot │ 2 Feet │ 3 Feet │ 4 Feet │ 5 Feet ────────┼───────────┴───────────┴───────────┴────────────┴───────────── │ Ditch Filling Material ────────┼─────┬─────┬─────┬─────┬─────┬─────┬─────┬──────┬──────┬────── │sand │clay │sand │clay │sand │clay │sand │ clay │ sand │ clay ────────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼──────┼──────┼────── 2│ 265│ 280│ 615│ 635│ 970│ 990│ 1330│ 1,350│ 1,690│ 1,710 4│ 400│ 450│ 1055│ 1125│ 1745│ 1825│ 2455│ 2,535│ 3,165│ 3,250 6│ 470│ 545│ 1370│ 1500│ 2370│ 2525│ 3405│ 3,575│ 4,460│ 4,740 8│ 505│ 605│ 1600│ 1790│ 2875│ 3115│ 4215│ 4,495│ 5,595│ 5,890 10│ 525│ 640│ 1765│ 2015│ 3275│ 3610│ 4900│ 5,295│ 6,590│ 7,020 12│ 535│ 660│ 1880│ 2185│ 3600│ 4030│ 5485│ 6,000│ 7,460│ 8,035 14│ 540│ 675│ 1965│ 2320│ 3855│ 4380│ 5975│ 6,620│ 8,225│ 8,950 16│ 545│ 680│ 2025│ 2425│ 4065│ 4675│ 6395│ 7,165│ 8,890│ 9,775 18│ 545│ 685│ 2070│ 2505│ 4230│ 4920│ 6750│ 7,630│ 9,480│10,520 20│ 545│ 690│ 2100│ 2565│ 4365│ 5130│ 7050│ 8,060│ 9,995│11,190 22│ 545│ 690│ 2125│ 2610│ 4470│ 5305│ 7305│ 8,425│10,445│11,795 24│ 545│ 690│ 2140│ 2645│ 4560│ 5445│ 7525│ 8,750│10,840│12,340 26│ 545│ 690│ 2150│ 2675│ 4630│ 5575│ 7705│ 9,035│11,185│12,830 28│ 545│ 690│ 2160│ 2695│ 4685│ 5680│ 7860│ 9,280│11,490│13,270 30│ 545│ 690│ 2165│ 2715│ 4725│ 5765│ 7990│ 9,500│11,755│13,670 Very │ │ │ │ │ │ │ │ │ │ great │ 545│ 690│ 2180│ 2770│ 4910│ 6230│ 8725│11,075│13,635│17,305 ────────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴──────┴──────┴──────
=92. Cement and Concrete Pipe.=—Although there is no general recognition of a difference between cement and concrete pipe, there is a tendency to term manufactured pipe of small diameter cement pipe, and large pipes or pipes constructed in place, concrete pipe. Cement, unlike clay, is used in the manufacture of pipe in the field or by more or less unskilled operators in “one man” plants. Great care should be used in the selection of cement, aggregate, and reinforcement for precast cement pipe since the shocks to which it is subjected in transit are more liable to rupture it than the heavier but steadier loads imposed on it in the trench.
The United States Government, various scientific and engineering societies, and other interested organizations have collaborated in the preparation of specifications for cement and cement tests. These specifications can be found in Trans. Am. Soc. Civil Engineers, Vol. 82, 1918, p. 166, and in other publications.
The following abstracts have been taken from the proposed tentative specifications for Concrete Aggregates, of the Am. Society for Testing Materials, issued June 21, 1921:
1. Fine aggregate shall consist of sand, stone screenings, or other inert materials with similar characteristics, or a combination thereof, having clean, hard, strong, durable uncoated grains, free from injurious amounts of dust, lumps, soft or flaky particles, shale, alkali, organic matter, loam or other deleterious substances.
2. Fine aggregates shall preferably be graded from fine to coarse, with the coarser particles predominating, within the following limits:
Passing No. 4 sieve 100 per cent Passing No. 50 sieve, not more than 50 per cent Weight removed by elutriation test, not more than 3 per cent
Sieves shall conform to the U. S. Bureau of Standards specifications for sieves.
3. The fine aggregate shall be tested in combination with the coarse aggregate and the cement with which it is to be used and in the proportions, including water, in which they are to be used on the work, in accordance with the requirements specified in Section 6....
7. Coarse aggregate shall consist of crushed stone, gravel or other approved inert materials with similar characteristics, or a combination thereof, having clean, hard, strong, durable, uncoated pieces free from injurious amounts of soft, friable, thin, elongated or laminated pieces, alkali, organic or other deleterious matter.
* * * * *
The following Table indicates desirable gradings, in percentages, for coarse aggregate for certain maximum sizes.
GRADINGS OF COARSE AGGREGATES
─────────┬─────────────────────────────────────────┬─────────────────── Maximum │ Circular Openings, Inches │ Passing Screen Size of │ │ Having Circular Aggregate│ │Openings ¼ Inch in Inches │ │diameter, not more │ │ than ─────────┼───┬───┬───┬─────┬─────┬─────┬─────┬─────┼─────────────────── │ 3 │2½ │ 2 │ 1½ │ 1¼ │ 1 │ ¾ │ ½ │ ─────────┼───┼───┼───┼─────┼─────┼─────┼─────┼─────┼─────────────────── 3 │100│ │ │40–75│ │ │ │ │ 15 per cent 2½ │ │100│ │ │40–75│ │ │ │ 15 per cent 2 │ │ │100│ │ │40–75│ │ │ 15 per cent 1½ │ │ │ │ 100 │ │ │40–75│ │ 15 per cent 1¼ │ │ │ │ │ 100 │ │ │35–70│ 15 per cent 1 │ │ │ │ │ │ 100 │ │40–75│ 15 per cent ¾ │ │ │ │ │ │ │ 100 │ │ 15 per cent ─────────┴───┴───┴───┴─────┴─────┴─────┴─────┴─────┴───────────────────
The manufacture of small size cement pipe requires relatively more skill than equipment. As a result great care must be observed in the inspection of cement pipe and in the enforcement of specifications. For large size concrete pipe and reinforced concrete pipe the difficulty of holding the pipe together during transportation and lowering into the trench aid in insuring a good product.
Cement pipe is made by ramming a mixture of cement, sand, and water into a cylindrical mold and allowing it to stand until set. The mold is then removed and the pipe stands for a further period of time to become cured. The selection and proportion of materials, the amount of water, the method of ramming, the period of setting, the length of time of curing, and the control of moisture and temperature during this period are of great importance in the resulting product. E. S. Hanson[52] states that the most conservative engineers recommend a mixture of one sack of cement to 2½ cubic feet of aggregate measured as loosely thrown into the measuring box. In making up the aggregate, clean gravel or broken stone up to ¼ inch in size is used. The American Concrete Institute recommends that 100 per cent pass a ½-inch screen, 70 per cent a ¼-inch screen, 50 per cent a No. 10, 40 per cent a No. 20, 30 per cent a No. 30, and 20 per cent a No. 40. The materials should be carefully graded by experiment and not guessed at, as the behavior of all aggregates is not the same. Too coarse an aggregate is difficult to handle in manufacturing. It causes loss of pipe when the jacket or mold is removed and results in rough pipe, stone pockets, and pin holes through which water spurts when pressure tests are applied. Too fine an aggregate causes loss of strength and with ordinary mixtures tends to produce a pipe which will show seepage under internal pressure tests. The amount of water in the mixture will vary, from 15 to 20 per cent. The mixture should appear dry but should ball in the hand under some pressure.
The mixture can be rammed into the molds by hand or machine. A machine-made pipe is preferable as it produces a more even and stronger product. There are two types of machines for this purpose. One type consists of a number of tamping feet which deliver about 200 blows to the minute with a pressure of about 800 pounds per square inch of area exposed. In the other type a revolving core is drawn through the pipe, packing and polishing the concrete as it is pulled through, with special provision for packing the bell of the pipe. The tamping machines can make 1,500 feet of small size pipe to 300 feet of 24–inch pipe in a day. Machines of the second type can make 750 feet of 8–inch to 200 feet of 30–inch pipe in 30–inch lengths in 9 hours. The inside and outside forms for a 24–inch pipe are shown in Fig. 74 as used with the tamping machines. The forms are swabbed with oil before being filled in order to facilitate their removal. In making a Y-branch or other special, a hole is cut in the pipe or mold the size of the joining pipe which is then set in place and the joint wiped smooth with cement.
TABLE 38
PROPERTIES OF CEMENT CONCRETE SEWER PIPE
1917 Specifications of American Society for Testing Materials, with Subsequent Revisions
─────────┬───────┬────────┬───────┬───────┬──────┬───────── │Laying │Diameter│Normal │ Depth │Taper │ Minimum │Length,│ at │Annular│ of │ of │Thickness │ Feet │ Inside │Space, │Socket,│Socket│ of │ │ of │Inches │Inches │ │ Barrel, │ │Socket, │ │ │ │ Inches │ │ Inches │ │ │ │ Internal │ │ │ │ │ │ Diameter,│ │ │ │ │ │ Inches │ │ │ │ │ │ ─────────┼───────┼────────┼───────┼───────┼──────┼───────── │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ ─────────┼───────┼────────┼───────┼───────┼──────┼───────── │ │ │ │ │ │ │ │ │ │ │ │ ─────────┼───────┼────────┼───────┼───────┼──────┼───────── │2, 2½, │ 8¼ │ ½ │ 2 │1 : 20│ ⅝ 6 │ 3 │ │ │ │ │ │2, 2½, │ 11 │ ⅝ │ 2¼ │1 : 20│ ¾ 8 │ 3 │ │ │ │ │ │2, 2½, │ 13¼ │ ⅝ │ 2½ │1 : 20│ ⅞ 10 │ 3 │ │ │ │ │ │2, 2½, │ 15⅝ │ ⅝ │ 2½ │1 : 20│ 1 12 │ 3 │ │ │ │ │ │2, 2½, │ 19¼ │ ⅝ │ 2½ │1 : 20│ 1¼ 15 │ 3 │ │ │ │ │ │2, 2½, │ 22¾ │ ⅝ │ 2¾ │1 : 20│ 1½ 18 │ 3 │ │ │ │ │ │2, 2½, │ 26½ │ ¾ │ 2¾ │1 : 20│ 1¾ 21 │ 3 │ │ │ │ │ │2, 2½, │ 30¼ │ ¾ │ 3 │1 : 20│ 2⅛ 24 │ 3 │ │ │ │ │ 27 │ 3 │ 34 │ ⅞ │ 3¼ │1 : 20│ 2¼ 30 │ 3 │ 38 │ 1 │ 3½ │1 : 20│ 2½ 33 │ 3 │ 41½ │ 1 │ 4 │1 : 20│ 2¾ 36 │ 3 │ 45½ │ 1¼ │ 4 │1 : 20│ 3 39 │ 3 │ 49 │ 1¼ │ 4 │1 : 20│ 3¼ 42 │ 3 │ 53 │ 1½ │ 4 │1 : 20│ 3½ ─────────┴───────┴────────┴───────┴───────┴──────┴─────────
─────────┬──────────────────────────────────────┬─────────┬─────────── │ Limits of Permissible Variations │ Minimum │ Maximum │ │Crushing │Absorption, │ │Strength,│ Per Cent │ │ Pounds │ │ │ per │ │ │ Linear │ Internal │ │ Foot at │ Diameter,│ │ End of │ Inches │ │Diameter │ ─────────┼───────┬─────────────┬──────┬─────────┼─────────┼─────────── │Length,│ Internal │Depth │Thickness│ │ │ Inch │ Diameter, │of Hub│of Barrel│ │ │ per │ Inches │ (-) │ (-) │ │ │ Foot │ │Inches│ Inches │ │ │ (-) │ │ │ │ │ ─────────┼───────┼──────┬──────┼──────┼─────────┼─────────┼─────────── │ │Spigot│Socket│ │ │ │ │ │ (±) │ (±) │ │ │ │ ─────────┼───────┼──────┼──────┼──────┼─────────┼─────────┼─────────── │ ¼ │ 3/16 │ 3/16 │ ¼ │ 1/16 │ 1430 │ 8 6 │ │ │ │ │ │ │ │ ¼ │ ¼ │ ¼ │ ¼ │ 1/16 │ 1430 │ 8 8 │ │ │ │ │ │ │ │ ¼ │ ¼ │ ¼ │ ¼ │ 1/16 │ 1570 │ 8 10 │ │ │ │ │ │ │ │ ¼ │ ¼ │ ¼ │ ¼ │ 1/16 │ 1910 │ 8 12 │ │ │ │ │ │ │ │ ¼ │ ¼ │ ¼ │ ¼ │ 3/32 │ 1960 │ 8 15 │ │ │ │ │ │ │ │ ¼ │ ¼ │ ¼ │ ¼ │ 3/32 │ 2200 │ 8 18 │ │ │ │ │ │ │ │ ¼ │ 5/16 │ 5/16 │ ¼ │ ⅛ │ 2590 │ 8 21 │ │ │ │ │ │ │ │ ⅜ │ 5/16 │ 5/16 │ ¼ │ ⅛ │ 3070 │ 8 24 │ │ │ │ │ │ │ 27 │ ⅜ │ 5/16 │ ⅜ │ ¼ │ ⅛ │ 3370 │ 8 30 │ ⅜ │ ⅜ │ ⅜ │ ¼ │ ⅛ │ 3690 │ 8 33 │ ⅜ │ ⅜ │ ⅜ │ ¼ │ 3/16 │ 3930 │ 8 36 │ ⅜ │ ½ │ ½ │ ¼ │ 3/16 │ 4400 │ 8 39 │ ⅜ │ ½ │ ½ │ ¼ │ 3/16 │ 4710 │ 8 42 │ ⅜ │ ½ │ ½ │ ¼ │ 3/16 │ 5030 │ 8 ─────────┴───────┴──────┴──────┴──────┴─────────┴─────────┴───────────
After the removal of the mold the pipe may be cured by the water or the steam process. Hanson states:
By the former the pipe are simply set on the floor of the plant and as soon as they are sufficiently strong so that they can be sprinkled with water without falling down; sprinkling is commenced and continued at such intervals for 6 or 7 days that the pipe will be moist at all times. This is a slower process than steam curing. It is also less uniform and less subject to control than where the product is cured by steam.
In the steam process the pipe is exposed to low-pressure steam with plenty of moisture in a closed receptacle for 24 hours, or until hardened. It has been found by tests that pipes sprinkled for 28 days are as strong as steam-cured pipes.
The dimensions of cement concrete sewer pipe as recommended by the Am. Society for Testing Materials are shown in Table 38.
The following has been abstracted from the description of the manufacture of one form of concrete pipe by G. C. Bartram.[53] All pipe are manufactured in 4–foot lengths near the site at which they are to be installed because of their great weight, for example, 36–inch pipe weighs one ton. The plant for the manufacture of the pipe consists of cast-iron bottom and top rings for each size to be used on the job, and inside and outside steel casings. There are three bases for each steel casing as the pipes stand on the bases for 72 hours and the steel casing remains on for only 24 hours after the concrete has been poured. The pipes are then lifted off the bases and stored for aging. The pipes are cast with the spigot end up.
The concrete is ordinarily mixed in the proportions of 1 : 2 : 4. The materials are placed in the mixer in the following order: first, the stone, then the sand, then the cement, and finally the water. Sufficient water is added to make the concrete flow freely. In cold weather or for a hurry-up job the molds are covered with canvas and are steamed for 2 or 3 hours immediately after the concrete is poured. The molds are then removed but the pipe should be steamed before use. Otherwise they are allowed to stand 72 hours, as explained above. In cold weather the steam is used to prevent freezing and not to hasten the completion of the pipe.
One layer or ring of reinforcement is used for sizes from 24 to 48 inches and two layers or rings for larger pipe. A type of reinforcement sometimes used is the American Steel and Wire Company’s Triangular Mesh, an illustration of which is shown in Fig. 75. The wire mesh is cut to fit and is placed in a slot in the cast-iron base. The slot is then filled with sand so that the concrete cannot enter, thus leaving a portion of the reinforcement exposed. The inside reinforcement extends through and out of the spigot of the completed pipe. In the trench the two reinforcements overlap in the key-shaped space left on the inside of the pipe by the design of the bell and spigot. This space is shown in Fig. 76 A. When the pipe is placed in the trench the key-shaped space is plastered with mortar and a piece is knocked out of the bell to receive the grout with which the joint is closed. A spring steel band is then put on the outside of the joint and grout poured into the hole at the top. The band is removed as soon as the joint materials have set.
The rules for the reinforcement of concrete pipe recommended in Volume XV, 1919, of the Transactions of the Concrete Institute are as follows:
No reinforcement is approved for pipe between 30 and 60 inches in diameter or in rock or hard soils. For pipe 36 inches in diameter or less the minimum thickness of shell shall be 5 inches. For 60–inch pipe the minimum thickness shall be 7 inches with intermediate sizes in proportion. Reinforcement for circular pipe shall consist of one or two rings of circular wire fabric or rods of the areas shown in Table 39. All sewers near the surface and subject to vibration should be reinforced. For sewers 6 feet or less in diameter the reinforcement should consist of at least ½ of 1 per cent of the area of the concrete. It should be placed near the inside at the crown and near the outside at the haunches. If large horizontal pressures are expected the pipe should be reinforced for these reverse stresses, which involves placing the reinforcement near the outside at the crown and near the inside at the haunches. The minimum thickness of the walls of sewers greater than 6 feet in diameter with flat bottom and arch, with or without side walls, should be 8 inches.
TABLE 39
REINFORCEMENT FOR CIRCULAR CONCRETE SEWER PIPE
(See Vol. XV, Proceedings Am. Concrete Institute) ─────────────────┬─────────────────┬─────────────────┬───────────────── Diameter in │Minimum Thickness│ Number of Rings │ Cross Sectional Inches │ of Shell in │ │Area of Each Ring │ Inches │ │ ─────────────────┼─────────────────┼─────────────────┼───────────────── 24 │3 │1 │.058 27 │3 │1 │.068 30 │3½ │1 │.080 33 │4 │1 │.107 36 │4 │1 │.146 39 │4 │1 │.146 42 │4½ │1 │.153 48 │5 │2 │.107 54 │5½ │2 │.123 60 │6 │2 │.146 66 │6½ │2 │.168 72 │7 │2 │.180 84 │8 │2 │.208 96 │9 │2 │.245 ─────────────────┴─────────────────┴─────────────────┴─────────────────
Three methods for the reinforcement of concrete sewers are shown in Fig. 76 B.
=93. Proportioning of Concrete.=—In the proportioning of concrete questions of strength, of permeability, and of workability[54] may need consideration. All of these qualities are affected by the amount of cement, the nature and gradation and relative proportions of the fine and the coarse aggregate, and the amount of mixing water used.
Other things being equal the strength varies with the amount of cement put into the concrete. For the same amount of cement and the same consistency of the mixture, the strength increases with increased density of concrete (that is, with decreased voids), and the effort should be made so to proportion the fine and coarse aggregates as to produce the densest concrete (least voids) with the aggregates available. For the same consistency, the strength then will vary with the ratio of the amount of cement to the amount of the voids.
So far as the mixing water is concerned, the greatest strength in the concrete will be attained at a rather dry mix; that which produces the least volume of concrete. The addition of more water results in a concrete of less strength; 40 per cent more water may give a concrete of less than half the normal strength. The reduction in strength is then very marked for the wetter mixes, and the water content used is a feature of considerable importance in the design of concrete mixtures.
Permeability is affected by the same elements as strength, but the size and discontinuity of the pores have a greater influence.
Workability is an important quality; in some respects it will have to be obtained at the expense of strength. Increasing the amount of mixing water increases the workability of the mixtures, with a resulting decrease in strength which may have to be accepted or else overcome by increasing the cement in the mix.
An excess of water is often used unnecessarily through ignorance of the injurious results. A high proportion of coarse aggregate, up to a certain limit, will give concrete of high strength, but the mixture will be harsh-working and not easy to place. Lower proportions of coarse aggregate will give greater workability and better uniformity of product, the latter being an important matter. It is apparent that the degree of workability of the mixture needed will depend upon the nature of the construction—for a pavement where the concrete will receive substantial tamping or working the water content may be much less than that which may need to be used in placing concrete around reinforcement in narrow members, or where little tamping or spading can be done. The nature of the work will affect the standard of consistency to be specified.
The proportioning of the concrete should then be dependent upon the needs of the structure and the manner of placing the concrete. The proportions selected should be carefully adhered to and especially should care be taken to see that the right quantity of mixing water is used.
The materials are commonly measured volumetrically (by bulk). Because of the variations which are introduced by volumetric measurement of the materials by the presence of varying degrees of moisture, measurements by weight would be more accurate, but these would also be affected by differences in the specific gravity of the materials. The methods of measuring, the allowance for moisture, as well as the proportions of the materials, should be specified.
The methods for proportioning concrete are:
(1) Arbitrarily selected proportions.
(2) Proportions based on minimum voids.
(3) Proportions based on trial mixtures.
(4) Proportions based on a sieve analysis curve.
(5) Proportions based on the surface area of the aggregates.
(6) Proportions based on the water-cement ratio and the fineness modulus.
(7) Proportions based on mortar-voids and cement-voids ratio.
Arbitrarily selected proportions are in quite general use; they are intended to apply to the materials most commonly used in the vicinity of the work. The most common practice is to use twice as great a volume of coarse aggregate as fine aggregate, as for instance 1 part cement, 2 parts fine aggregate, and 4 parts coarse aggregate. Decreasing the ratio of coarse aggregate to fine aggregate may give a more easily worked mix or require relatively less water for a given workability, and in some cases it will be proper to increase this ratio and thus secure an increase of strength. Judgment and experience with given materials may warrant changes from a stated ratio. The proportions are now frequently given as one part cement to a certain number of parts of the mixed aggregate, leaving the proportions of the fine to coarse to be determined otherwise, since small variations in the relation of these will not greatly affect the strength. Proportions in common use are:[55]
Mortar for Laying brick and stone masonry from 1 : 0 to 1 : 3 Filling joints in sewer pipe 1 : 0 to 1 : 2 Surfaces, floors, sidewalks, pavements 1 : 0 to 1 : 2 Waterproof linings 1 : 0 to 1 : 2 Cement, bricks, and blocks 1 : 2½ to 1 : 4 Concrete for Gravity retaining walls, heavy foundations, structures needing mass more than strength from 1 : 3 : 6 to 1 : 4 : 8 Retaining walls, piers, sewers, pavements, foundations, and work requiring strength. (Compressive strength in 28 days, 1,500 to 2,000 pounds per square inch) from 1 : 2 : 4 to 1 : 3 : 6 Floors, beams, pavements, reinforced concrete, arch bridges, low-pressure tanks. (Compressive strength in 28 days, 2,000 to 3,000 pounds per square inch) from 1 : 1½ : 3 to 1 : 2½ : 4½ Reinforced concrete columns, conduit pipe, impervious concrete. (Compressive strength in 28 days, 3,000 to 4,000 pounds per square inch) from 1 : 1 : 2 to 1 : 1½ : 3
The usual method of proportioning based on minimum voids is to assume that the particles of fine aggregate should fill the voids in the coarse aggregate and that the particles of the cement will fill the voids in the fine aggregate. About 5 to 10 per cent additional fine aggregate is generally added to push the particles of the coarse aggregate apart and thus give a more easily worked concrete and one freer from void spaces. This method is inaccurate, principally because of the effect of the moisture on the volume of the voids, and because the effect on the volume by the addition of water is unknown.
Trial mixtures may be made by carefully weighing each of the ingredients and then combining them to give a workable concrete. Using a given amount of cement, the proportion of ingredients, of the same total weight, which will give the least volume and therefore the densest concrete is adopted. When making the comparison the consistency of the mixes must be maintained constant.
Proportioning may be based on an ideal sieve analysis curve of the mixed cement and aggregates. The sieve analysis of the aggregates is made by screening a predetermined weight of the sample through a series of 5 to 8 sieves graded in size from slightly below the size of the largest particle to slightly above the smallest particle of the aggregate. The analysis is then expressed in the form of a curve. The ideal curve, according to Fuller,[56] is shown in Fig. 77.
The method of proportioning concrete by surface areas is based on the theory that the strength of a concrete depends on the amount of cement used in proportion to the surface area of the aggregates.[57]
The proportioning of concrete on the basis of a water-cement ratio and a fineness modulus was introduced by Prof. D. A. Abrams.[58] It is based on the theory that with fixed conditions of aggregate, moisture, etc., the ratio of water to cement determines the strength of the concrete.
A method of proportioning concrete by determining experimentally the voids in mortars made up with a given amount of sand and definite proportions of cement, and then calculating the voids in the concrete made up by adding a definite amount of coarse aggregate to the mixture, has been developed.[59] The method is based on the theory that the strength of the concrete is a known function of the ratio of the volume of cement to the volume of the voids in the concrete. The effect of varying the proportion of the ingredients, including an increase in the amount of mixing water beyond that required to give the densest mixture, may be found by the method, and a comparison may be made of results obtainable with different classes of fine and coarse aggregates.
Arbitrarily selected proportions, proportions based on voids, and proportions based on trial mixtures are usually satisfactory for small jobs where the amount of materials involved is not large. Where the saving in materials will permit, more accurate methods should be used. The methods can be studied more fully by reference to the original articles quoted in the footnotes, or to the following texts:
Materials of Construction, Johnson, 5th Edition, 1918. Materials of Engineering, H. F. Moore, 2d Edition, 1920. Masonry Construction, I. O. Baker, 10th Edition, 1912. Concrete Engineer’s Handbook, Hool and Johnson, 1918. Concrete, Plain and Reinforced, Taylor and Thompson, 1916.
=94. Waterproofing Concrete.=—The waterproofing of concrete is most satisfactorily done by making dense mixtures. In practice such substances as hydrated lime, clay, alum and soap, and proprietary compounds such as Ceresit, Medusa, etc., are frequently mixed with the concrete under the theory that these very fine substances will fill any remaining voids and render the concrete impervious. The specifications of the Joint Committee issued on June 4, 1921, are much briefer and contain less detailed instruction than those issued earlier.[60] The earlier instructions follow.
Many expedients have been resorted to for making concrete impervious to water. Experience shows, however, that when mortar or concrete is proportioned to obtain the greatest practicable density and is mixed to the proper consistency, the resulting mortar or concrete is impervious under moderate pressure.
On the other hand concrete of dry consistency is more or less pervious to water, and, though compounds of various kinds have been mixed with the concrete or applied as a wash to the surface, in an effort to offset this defect, these expedients have generally been disappointing, for the reason that many of these compounds have at best but temporary value, and in time lose their power of imparting impermeability to the concrete.
In the case of subways, long retaining walls, and reservoirs, provided the concrete itself is impervious, cracks may be so reduced, by horizontal and vertical reinforcement properly proportioned and located, that they will be too minute to permit leakage, or will be closed by infiltration of silt.
Asphaltic or coal tar preparations applied either as a mastic or as a coating on felt cloth or fabric, are used for waterproofing, and should be proof against injury by liquids or gases.
For retaining and similar walls in direct contact with the earth, the application of one or two coatings of hot coal tar pitch, following a painting with a thin wash of coal tar dissolved in benzol, to the thoroughly dried surface of concrete is an efficient method of preventing the penetration of moisture from the earth.
Tar paper and asphaltic compounds are not often used in sewer work as absolute imperviousness is seldom necessary.
=95. Mixing and Placing Concrete.=—Careful workmanship is desirable in the mixing and placing of concrete in sewers since water-tight construction is desired. Because of the difficulty of inspecting concrete in wet, dark and crowded excavations, and the careless habits of workmen experienced in concrete sewer construction, the highest class of concrete work cannot be expected. The situation is met by designing thick walls as shown in the sections illustrated in Fig. 22 and 23.
In the report of the Joint Committee on Concrete and Reinforced Concrete in Transactions of the American Society of Civil Engineers for 1917, on page 1101 the recommendation is made concerning the mixing and placing of concrete as follows:[61]
The mixing of concrete should be thorough and should continue until the mass is uniform in color and is homogeneous. As the maximum density and greatest strength of a given mixture depends largely on thorough and complete mixing, it is essential that this part of the work should receive special attention and care.
Inasmuch as it is difficult to determine by visual inspection whether the concrete is uniformly mixed, especially where aggregates having the color of cement are used, it is essential that the mixing should occupy a definite period of time. The minimum time will depend on whether the mixing is done by machine or hand.
(a) Measuring Ingredients: Methods of measurement of the various ingredients should be used which will secure at all times separate and uniform measurements of cement, fine aggregate, coarse aggregate and water.
(b) Machine Mixing: The mixing should be done in a batch machine mixer of a type which will insure the uniform distribution of the materials throughout the mass, and should continue for the minimum time of 1½ minutes after all the ingredients are assembled in the mixer. For mixers of 2 or more cubic yards capacity, the minimum time of mixing should be 2 minutes. Since the strength of the concrete is dependent on thorough mixing, a longer time than this minimum is preferable. It is desirable to have the mixer equipped with an attachment for automatically locking the discharging device so as to prevent the emptying of the mixer until all the materials have been mixed together for the minimum time required after they are assembled in the mixer. Means should be provided to prevent aggregates being added after the mixing has commenced. The mixer should also be equipped with water storage, and an automatic measuring device which can be locked if desired. It is also desirable to equip the mixer with a device recording the revolutions of the drum. The number of revolutions should be so regulated as to give at the periphery of the drum a uniform speed. About 200 feet per minute seems to be the best speed in the present state of the art.
(c) Hand Mixing: Hand mixing should be done on a water-tight platform and especial precautions taken after the water has been added, to turn all the ingredients together at least 6 times, and until the mass is homogeneous in appearance and color.
(d) Consistency: The materials should be mixed wet enough to produce a concrete of such a consistency as will flow sluggishly into the forms and about the metal reinforcement when used, and which at the same time can be conveyed from the mixer to the forms without separation of the coarse aggregate from the mortar. The quantity of water is of the greatest importance in securing concrete of maximum strength and density; too much water is as objectionable as too little.
(e) Retempering: The remixing of concrete and mortar that has partly reset should not be permitted.
Placing Concrete
(a) Methods: Concrete after the completion of the mixing should be conveyed rapidly to the place of final deposit; under no circumstances should concrete be used that has partly set.
Concrete should be deposited in such a manner as will permit the most thorough compacting such as can be obtained by working with a straight shovel or slicing tool kept moving up and down until all the ingredients are in their proper place. Special care should be exercised to prevent the formation of laitance; where laitance has formed it should be removed, since it lacks strength and prevents a proper bond in the concrete.
Care should be taken that the forms are substantial and thoroughly wetted (except in freezing weather) or oiled, and that the space to be occupied by the concrete is free from all debris. When the placing of concrete is suspended, all necessary grooves for joining future work should be made before the concrete has set.
When work is resumed concrete previously placed should be roughened, cleansed of foreign material and laitance, thoroughly wetted and then slushed with a mortar consisting of one part Portland cement and not more than 2 parts of fine aggregate.
The surfaces of concrete exposed to premature drying should be kept covered and wet for at least 7 days.
Where concrete is conveyed by spouting, the plant should be of such a size and design as to insure a practically continuous stream in the spout. The angle of the spout with the horizontal should be such as to allow the concrete to flow without separation of the ingredients; in general an angle of about 27 degrees or 1 vertical to 2 horizontal is good practice. The spout should be thoroughly flushed with water before and after each run. The delivery from the spout should be as close as possible from the point of deposit. Where the discharge must be intermittent, a hopper should be provided at the bottom. Spouting through a vertical pipe is satisfactory when the flow is continuous; when it is checked and discontinuous it is highly objectionable unless the flow is checked by baffle plates.
(b) Freezing Weather: Concrete should not be mixed or deposited at a freezing temperature, unless special precautions are taken to prevent the use of materials covered with ice crystals or containing frost, and to prevent the concrete from freezing before it has set and sufficiently hardened.
As the coarse aggregate forms the greater portion of the concrete, it is particularly important that this material be warmed to well above the freezing point.
The enclosing of a structure and the warming of a space inside the enclosure is recommended, but the use of salt to lower the freezing point is not recommended.
(c) Rubble Concrete: Where the concrete is to be deposited in massive work, its value may be improved and its cost materially reduced by the use of clean stones saturated with water, thoroughly embedded in and completely surrounded by concrete.
(d) Under Water: In placing concrete under water, it is essential to maintain still water at the place of deposit. With careful inspection the use of tremies, properly designed and operated, is a satisfactory method of placing concrete through water. The concrete should be mixed very wet (more so than is ordinarily permissible) so that it will flow readily through the tremie and into place with practically a level surface.
The coarse aggregate should be smaller than ordinarily used and never more than one inch in diameter. The use of gravel facilitates the mixing and assists the flow. The mouth of the tremie should be buried in the concrete so that it is at all times entirely sealed and the surrounding water prevented from forcing itself into the tremie. The concrete will then discharge without coming in contact with the water. The tremie should be suspended so that it can be lowered quickly when it is necessary either to choke off or to prevent too rapid flow. The lateral flow preferably should not be over 15 feet.
The flow should be continuous in order to produce a monolithic mass and to prevent the formation of laitance in the interior.
In case the flow is interrupted it is important that all laitance be removed before proceeding with the work.
In large structures it may be necessary to divide the mass of concrete into several small compartments or units to permit the continuous filling of each one. With proper care it is possible in this manner to obtain as good results under water as in the air.
A less desirable method is the use of the drop bottom bucket. Where this method is used the bottom of the bucket should be released when in contact with the surface of the place of deposit.
Concrete sewers should be constructed in longitudinal sections in a continuous operation without interruption for the entire invert, side walls, or arch. In pouring the concrete it should be kept level in the forms and should rise evenly on each side of the sewer. All rough places in the concrete should be finished smooth by brushing with a grout of neat cement and water and honeycombs should be filled with neat cement or a one-to-one mortar.
=96. Sewer Brick.=—The quality of brick used in sewers is seldom specified with the minute care that is taken in the specifications for concrete, iron, and certain other materials of construction, as inferior materials in brick are more easily detected. The specifications of the Baltimore Sewerage Commission for sewer brick are:
Sewer brick shall be whole, new bricks of the best quality, of uniform standard size, with straight and parallel edges and square corners: they shall be of compact texture, burned hard and entirely through, free from injurious cracks and flaws, tough and strong, and shall have a clear ring when struck together. The sides, ends and faces of all bricks shall be plane surfaces at right angles and parallel to each other. Bricks of any one make shall not vary more than 1/16th of an inch in thickness, nor more than 1⅛th of an inch in width or length, from the average of the samples submitted for approval.
The truest bricks shall be used in the face of the masonry and the exposed surfaces shall be true and smooth planes.
All bricks delivered for use shall be culled by the Contractor when required. No brick thrown out in the culling shall be used in any work done under any contract of the Sewerage Commission, except that the best of the culls may be used in manholes, above the level of the top of the sewer, if permitted by the Engineer.
The average amount of water absorbed by the bricks, after being thoroughly dried and then immersed for 24 hours, shall not exceed 6 per cent. All bricks shall be uniform in quality and percentage of absorption.
Whenever vitrified bricks are required in the invert of the sewer, they shall be smooth, hard, tough, and of such durability as will fit them for this use. They shall be of standard size, well and uniformly burned, thoroughly vitrified throughout, and free from warps, cracks, and other defects. The surfaces and edges shall be true and straight and the corners sharp and square. They shall be in every respect satisfactory to the Engineer, and in all respects equal to the sample in the office of the Engineer.
The remaining paragraphs of the specifications deal with the manner in which samples shall be submitted and the necessity for conformity between the samples submitted and the bricks used.
A common size of brick in use for sewers is 2¼ × 4 × 8¼ inches, but the variations in size are many. The bricks in use on any one job should be as near the same size as possible as the extra mortar filling necessary to make up for small brick detracts from the strength of the sewer. Small brick are undesirable as the cost of laying small and large bricks is the same, but the thickness of the finished sewer is less. Sewer brick should not absorb more than 10 to 20 per cent moisture by volume, in 24 hours; except the special paving brick used to prevent erosion at the invert which should absorb less than 5 per cent moisture.
=97. Vitrified Sewer Block.=—Blocks and bricks are manufactured in a manner similar to the manufacture of vitrified sewer pipe described in Art. 91. J. M. Egan describes two types of sewer blocks[62] as follows:
There are on the market two designs of blocks, one being a single-ring block and the other a double-ring block. The former has a ship-lap joint on the ends and a tongue-and-groove joint on the sides. In the double block the laps and joints are made in the construction of the sewer and the blocks are placed one on top of the other as in a two ring brick sewer. The blocks are hollow longitudinally with web braces. They are made for sewers from 30 inches to 108 inches in diameter and weigh from 40 to 120 pounds. They are 18 inches to 24 inches long, 9 to 15 inches wide, and 5 to 10 inches thick. Short lengths are made for convenience in construction and for use on sharp curves. Special blocks are made for connections and junctions.
A special block is also made for inverts, which has occasionally been used with brick sewers to avoid the difficulty of constructing with brick at this point. Such blocks are objectionable, as they leave a line of weakness along the longitudinal joint so formed. They are not used frequently in present day practice.
Vitrified blocks are generally cheaper than bricks, but they do not make so strong a structure. In some cases it is possible to lay vitrified block without the expense of high-priced bricklayers, thus saving on the cost of the sewer and obtaining a conduit with a smoother interior finish.
=98. Cast Iron, Steel, and Wood.=—Cast iron, steel, and wood pipe belong more to the field of waterworks than of sewerage, as they are not extensively used in the construction of sewers. There are, however, some special conditions under which these materials may be serviceable.
The iron used in cast-iron pipe for sewers, and in castings for manhole covers, inlet frames, etc., is seldom carefully or definitely specified. The standard specifications of the American Water Works Association with regard to the quality of iron for water pipe are:
All pipe and special castings shall be made of cast iron of good quality and of such character as shall make the metal of the castings strong, tough, and of even grain and soft enough to satisfactorily admit of drilling and cutting. The metal shall be made without the admixture of cinder iron or other inferior metal, and shall be remelted in a cupola or air furnace.
The specifications of the Sanitary District of Chicago for the quality of iron to be used in manhole covers, etc., are given on page 101.
Although sewer pipes are not ordinarily subjected to internal pressure, cast-iron pipe for sewers should be as heavy or heavier than water pipe to resist the corrosive action of the sewage and the external stresses that are to be imposed upon it. The sizes and details of standard cast-iron pipe used for both water works and sewerage can be found in specification of the American and New England Water Works Associations.
The quality of steel used for reinforcing concrete should be carefully specified because of the possibility of the substitution of inferior material. The specifications for “Billet Steel Concrete Reinforcement Bars,” of the American Society for Testing Materials[63] are the standard for engineering practice, or the following specifications may be used:
All reinforcement shall be free from excessive rust, scale, paint, or coatings of any character which will tend to destroy the bond. The bars shall be rolled from new billets. No rerolled material will be accepted. All reinforcement bars shall develop an ultimate tensile strength of not less than 70,000 pounds per square inch. The test specimen shall bend cold around a pin, whose diameter is two times the thickness of the bar, 180 degrees without cracking on the outside portion. The reinforcing bars shall in all respects fulfill the requirements of the standard specifications of the American Society for Testing Materials for Billet Steel Concrete Reinforcing Bars serial designation A 15–14.
The steel used in pipe should be a soft, open-hearth steel with an ultimate tensile strength of 60,000 pounds per square inch, an elastic limit of 30,000 pounds per square inch, an elongation in 8 inches before fracture between 22 and 25 per cent, and a reduction in area before fracture of 50 per cent. The working strength of the steel is taken at 16,000 to 20,000 pounds per square inch in tension, 10,000 to 12,000 pounds per square inch in shear, and 20,000 to 24,000 pounds per square inch in bearing. A liberal allowance should be made for corrosion. The standard specifications for Open-Hearth Boiler Plate and Rivet Steel of the American Society for Testing Materials, Aug. 16, 1919, include “flange steel,” which is suitable for the manufacture of plates, and extra soft steel which is suitable for rivets.
Steel pipe should be coated both inside and out to protect it against corrosion. The various proprietary coatings are mainly coal tar pitches, or mixtures of coal tar pitch and asphalt. A coal tar pitch is a distillate of coal tar from which the naphtha has been removed and to which about one per cent of heavy linseed oil has been added. The coating is applied to the pipe at a temperature of about 300 degrees Fahrenheit, by dipping hot pipe in the heated coating material. The pipe should be carefully cleaned and all rust and scale removed before it is dipped. In some cases the steel is pickled before dipping. This consists in rolling the cold plates to a short radius to loosen the scale, heating them to about 125 degrees, and dipping them in a warm 5 per cent acid solution for about 3 minutes, and finally rinsing in a weakly basic wash water.
The woods commonly used for the manufacture of wood pipe are spruce, Oregon fir, Douglas fir, and California redwood. Wood pipe lines have been constructed of other kinds of lumber but only in more or less unusual conditions. The following has been abstracted from the specifications for California redwood given by J. F. Partridge.[64]
The staves shall be of clear, air-dried, California redwood, seasoned at least one year in the open air, and shall be free from knots (except small knots appearing on one face only), sap, dry rot, wind shakes, pitch, pitch seams, pitch pockets, or other defects which would materially impair their strength or durability. The sides of the staves shall be milled to conform to the inside and outside radii of the pipe; and the edges shall be beveled to true radial planes. The staves shall be milled from stock sizes of lumber, the net finished thickness of the stave, for the various diameters of pipe, shall be as given in Table 40. The ends shall be cut square and slotted to receive the metallic tongues which form the butt joints. The slots shall appear in the same position on each stave, and shall be cut to make a tight fit with the tongues in all directions. The staves shall have an average length of at least 15 ft. 6 in. and not more than one per cent shall have a length of less than 9 ft. 6 in. Staves shorter than 8 ft. will not be accepted.
The bands shall be spaced on the pipe with a factor of safety of at least four, and shall consist of round, mild steel rods, connected with malleable iron shoes. Either open-hearth or Bessemer steel may be used.... The ultimate strength shall be from 55,000 to 65,000 lb. per sq. in.
The original reference should be consulted for complete details and for specifications for various kinds of wood and classes of pipe. The discussion following the specifications is of value.
Machine-made wood pipe is superior to stave pipe put together in the field. It is seldom manufactured in sizes large enough for use in sewers, which results in the almost exclusive use of field constructed stave pipe. The steel bands used to hold the staves together should be coated similarly to steel plates. All lumber, except California redwood should receive a preservative coating of creosote[65] or other material. One of the best methods of preserving the wood is to keep it submerged and to maintain the pipe under internal pressure.
TABLE 40
DETAILS OF DESIGN FOR CONTINUOUS STAVE WOOD PIPE
CLASSES A, B, AND C
(By J. F. Partridge, Trans. A. S. C. E., Vol. 82, page 461) ───────────┬───────────┬───────────┬───────────┬───────────┬─────────── │ Stave │Stock Size │ Size of │ Top Width │Spacing of │Thickness, │of Lumber, │ Band, │of Staves, │ Bands for Diameter, │ Standard, │ Inches │ Inches │ Standard, │ 100 Feet Inches │ Inches │ │ │ Inches │ Head ───────────┼───────────┼───────────┼───────────┼───────────┼─────────── 12│ 1⅜ │ 2 × 4 │ ⅜ │ 3.56 │ 6.38 18│ 1–7/16 │ 2 × 4 │ 7/16 │ 3.66 │ 5.76 24│ 1–7/16 │ 2 × 4 │ 7/16 │ 3.70 │ 4.34 30│ 1½ │ 2 × 6 │ ½ │ 5.48 │ 4.53 36│ 1–9/16 │ 2 × 6 │ ½ │ 5.62 │ 3.77 42│ 1⅝ │ 2 × 6 │ ½ │ 5.51 │ 3.23 │ 1⅝ │ 2 × 6 │ ½ or ⅝ │ 5.60 │ 2.84 or 48│ │ │ │ │ 4.41 60│ 2½ │ 3 × 6 │ ⅝ │ 5.56 │ 3.54 │ 3½ │ 4 × 6 │ ⅝ or ¾ │ 5.69 │ 2.95 or 72│ │ │ │ │ 4.24 84│ 3½ │ 4 × 6 │ ¾ │ 5.65 │ 3.63 120│ 3⅝ │ 4 × 6 │ ¾ │ 5.68 │ 2.54 │ 3⅝ │ 4 × 6 │ ¾ or ⅞ │ 5.64 │ 2.12 or 144│ │ │ │ │ 2.89 ───────────┴───────────┴───────────┴───────────┴───────────┴───────────
IX. DESIGN OF THE SEWER RING
=99. Stresses in Buried Pipe.=—The stresses which sewer pipe should be designed to resist are: internal bursting pressure, for sewers flowing under pressure; stresses due to handling, for precast pipe; temperature stresses; and external loads. The latter is by far the most important and frequently is the only stress considered in design.
The thickness of a pipe to resist internal stress should be
(PR)⁄_f__{t},
in which P = the intensity of internal pressure;
R = the radius of the inside of the pipe, and
_f__{t} = the unit-strength of the material in tension
The derivation of this expression is simple. The stresses due to handling cannot be computed and are cared for by a thickness of material dictated by experience. These thicknesses are given for vitrified clay and cement pipe in the specifications in the preceding chapter. Temperature stresses are not allowed for in the design of the pipe ring, but allowance must be made for them in long rigid pipe lines exposed to wide variations in temperature. Such a condition seldom exists in sewerage works.
The external forces are ordinarily the controlling features in the design of sewer rings. The simplest problems arise in the design of a circular pipe. If the external loading is uniform about the circumference of the pipe the internal stresses will all be compression. Almost all other forms of loading will cause bending moments resulting in tension and compression in different parts of the pipe. The maximum bending is caused by two concentrated loads diametrically opposed. As such a condition is extreme it is not cared for in ordinary design, but a loading between this condition and perfect distribution is assumed, as explained in Art. 103.
=100. Design of Steel Pipe.=—The stresses which may occur in steel sewer pipes are commonly caused by the internal or bursting pressure of the contained liquid. Occasionally a steel pipe may be used as a bridge or as a stressed member of a bridge, but steel pipes should not be used to withstand compression normal to the axis. In order to avoid such stresses the bursting tensile stresses should exceed the external compressive stresses. Such a condition in design requires that buried pipes shall never be emptied, a condition that cannot always be fulfilled. Precaution should be taken, by the installation of proper valves, to prevent the emptying of the pipe at so rapid a rate that a vacuum is created resulting in the collapse of the pipe.
Steel pipes are ordinarily made of plates curved to the proper diameter, the edges being held together by rivets. The design of the pipe consists in the determination of the thickness of the plate and the design of the riveted joint. The longitudinal joint and the thickness of the plate are first designed. The design of the joint consists in determining the diameter and pitch of the rivets and the thickness of the plate so that the full strength of the uncut metal shall be developed as nearly as possible under bearing, tearing, and shearing. This is done by making the efficiency of the joint the same under all stresses. The efficiency of the joint is the ratio of the strength of the joint under any kind of stress to the strength in tension of the unpunched plate. Properties of riveted joints are given in Table 41.
The diameter of the rivet holes should be computed as 1/16 of an inch larger than the diameter of the rivets. Rivets and plates should be designed for the nearest or next largest commercial size, and a generous allowance for corrosion should be made in determining the thickness of the plate. The distance from the edge of the plate to the side of the rivet should not be less than 1½ times the diameter of the rivet. The unit-strengths of the metal are given in the preceding chapter.
The transverse joint must be designed empirically as the stresses in it are indeterminate. The common form of joint for pipes less than 48 inches in diameter is a single-riveted lap joint, and for larger pipes or for pipes exposed to unusual stresses, a double riveted lap joint is used. The same size rivets are used as in the longitudinal joint. The maximum permissible distance between rivets should be used in the transverse joint.
TABLE 41
PROPERTIES OF RIVETED JOINTS
(Chicago Bridge and Iron Works) ──────────────────────┬─────────┬────────┬────────┬──────────┬───────── Type of Joint │Thickness│Diameter│ Pitch, │Efficiency│Thickness │ Plate, │ of │ Inches │of Joint, │ Butt │ Inch │ Rivet, │ │ Per Cent │ Plate, │ │ Inch │ │ │ Inches ──────────────────────┼─────────┼────────┼────────┼──────────┼───────── Single-riveted lap │ ¼ │ ⅝ │ 1.88 │ 49 │ │ ¼ │ ¾ │ 2.25 │ 50 │ │ 5/16 │ ⅞ │ 2.63 │ 50 │ ──────────────────────┼─────────┼────────┼────────┼──────────┼───────── Double riveted lap │ ¼ │ ⅝ │ 2.50 │ 70 │ │ 5/16 │ ¾ │ 3.00 │ 71 │ │ ⅜ │ ⅞ │ 3.40 │ 71 │ ──────────────────────┼─────────┼────────┼────────┼──────────┼───────── Triple riveted lap │ ¼ │ ½ │ 2.39 │ 74 │ │ 5/16 │ ⅝ │ 2.96 │ 74 │ │ ⅜ │ ¾ │ 3.53 │ 75 │ │ 7/16 │ ⅞ │ 4.09 │ 76 │ ──────────────────────┼─────────┼────────┼────────┼──────────┼───────── Quadruple riveted lap │ ⅜ │ ⅝ │ 3.20 │ 77 │ │ 7/16 │ ¾ │ 3.90 │ 78 │ ──────────────────────┼─────────┼────────┼────────┼──────────┼───────── Double riveted butt │ ½ │ ⅞ │ 3.62 │ 72 │ ⅜ │ 9/16 │ ⅞ │ 3.62 │ 72 │ ⅜ │ ⅝ │ ⅞ │ 3.62 │ 72 │ ⅜ │ 11/16 │ ⅞ │ 3.62 │ 72 │ 7/16 │ ¾ │ 1 │ 4.12 │ 73 │ 7/16 │ ⅞ │ 1 │ 3.82 │ 71 │ ½ │ 1 │ 1 │ 3.48 │ 68 │ 9/16 ──────────────────────┼─────────┼────────┼────────┼──────────┼───────── Triple riveted butt │ ⅝ │ ⅞ │ 4.94 │ 80 │ ½ │ ¾ │ 1 │ 5.62 │ 80 │ 9/16 │ ⅞ │ 1 │ 5.16 │ 78 │ 9/16 │ 1 │ 1 │ 4.66 │ 76 │ 9/16 ──────────────────────┼─────────┼────────┼────────┼──────────┼───────── Quadruple riveted butt│ ¾ │ 1 │ 7.13 │ 84 │ ¾ │ ⅞ │ 1 │ 6.51 │ 83 │ 11/16 │ 1 │ 1 │ 5.84 │ 81 │ ⅝ ──────────────────────┴─────────┴────────┴────────┴──────────┴─────────
Pipes used as compression members of a bridge are stiffened by riveting standard rolled steel sections longitudinally on the pipe.
Lock Bar Pipe is a steel pipe with a special form of joint made by the East Jersey Pipe Corporation. It is arranged as shown in Fig. 78 and has the advantage of developing the full strength of the plate. It is equivalent to a joint with 100 per cent efficiency, which permits the use of thinner plates.
=101. Design of Wood Stave Pipe.=—In the design of wood stave pipe[66] the entire bursting pressure is taken up by steel bands wrapped around the outside of wood staves which make up the shell of the pipe. The pipe is not designed to resist external loads except those which may be overcome by the internal pressure in the pipe. The thickness of the staves is fixed by experience. The sizes of staves and bands recommended by J. F. Partridge[67] are given in Table 40. The size of the steel bands can be determined from the expression;
S = Cr(R + t)
in which S = the total stress in the band;
R = the radius of the inside of the pipe;
t = the thickness of the stave;
r = the area of bearing per unit length of the band on the wood. For circular bands it is assumed as the radius of the band;
C = the crushing strength of wood, usually taken at 650 pounds per sq. in.
The preceding expression can be derived easily by the application of the laws of mechanics, and from it the expression for the distance between bands follows logically. It is,
p = S⁄(PR + kt)
in which S = the strength of the band;
p = the distance between bands;
P = the intensity of bursting pressure in the pipe;
R = the radius of the inside of the pipe;
t = the thickness of the staves;
k = the swelling strength of wood, usually taken at 100 pounds per sq. in.
Transverse joints between staves are closed by inserting metal strips between them, or by shaping the edges irregularly so that they fit closely together with an irregular joint. Transverse joints between all staves at any one point are avoided by splitting the joints between staves. Longitudinal joints between staves are usually made smooth and are closed by steel bands which are drawn tight about the pipe by inserting the ends in coupling shoes as shown in Fig. 79.
=102. External Loads on Buried Pipe.=—Prof. Anston Marston and H. C. Anderson published[68] the results of a series of experiments on the loads on buried pipes which are of extreme value in the design of sewer pipe. The load on the pipe is given by the empirical expression W = CwB^2, in which w is the weight of the backfilling material in pounds per cubic foot, B is the width of the trench in feet at the elevation of the end of a radius making an angle of 45 degrees upwards with the horizontal diameter of the pipe as illustrated in Fig. 80, and C is a coefficient dependent on the character of the backfill and the ratio of the width to the depth of the trench. Values of C are given in Table 42. The weights of various classes of backfilling are given in Table 43.
TABLE 42
APPROXIMATE SAFE WORKING VALUES OF C IN THE EXPRESSION W = CwB^2
From Bulletin No. 31 of the Engineering Experiment Station, Iowa State College of Agriculture. ──────────────┬──────────────────────────────────────────────────────── Ratio of Depth│ Approximate Values of C to Width │ ──────────────┼──────────────┬─────────────┬─────────────┬───────────── │Damp Top Soil │Saturated Top│ Damp Yellow │ Saturated │ and Dry and │ Soil │ Clay │ Yellow Clay │ Wet Sand │ │ │ ──────────────┼──────────────┼─────────────┼─────────────┼───────────── 0.5 │ 0.46 │ 0.47 │ 0.47 │ 0.48 1.0 │ 0.35 │ 0.86 │ 0.88 │ 0.90 1.6 │ 1.16 │ 1.21 │ 1.25 │ 1.27 3.0 │ 1.47 │ 1.51 │ 1.56 │ 1.62 2.6 │ 1.70 │ 1.77 │ 1.83 │ 1.91 3.0 │ 1.90 │ 1.99 │ 2.08 │ 2.19 3.6 │ 2.08 │ 2.18 │ 2.28 │ 2.43 4.0 │ 2.22 │ 2.35 │ 2.47 │ 2.65 4.6 │ 2.34 │ 2.49 │ 2.63 │ 2.85 6.0 │ 2.45 │ 2.61 │ 2.78 │ 3.02 6.5 │ 2.54 │ 2.72 │ 2.90 │ 3.18 6.0 │ 2.61 │ 2.81 │ 3.01 │ 3.32 6.6 │ 2.68 │ 2.89 │ 3.11 │ 3.44 7.0 │ 2.73 │ 2.95 │ 3.19 │ 3.55 7.5 │ 2.78 │ 3.01 │ 3.27 │ 3.66 8.0 │ 2.82 │ 3.06 │ 3.33 │ 3.74 8.5 │ 2.85 │ 3.10 │ 3.39 │ 3.82 9.0 │ 2.88 │ 3.14 │ 3.44 │ 3.89 9.5 │ 2.90 │ 3.18 │ 3.48 │ 3.96 10.0 │ 2.92 │ 3.20 │ 3.52 │ 4.01 11.0 │ 2.95 │ 3.25 │ 3.58 │ 4.11 12.0 │ 2.97 │ 3.28 │ 3.63 │ 4.19 13.0 │ 2.99 │ 3.31 │ 3.67 │ 4.25 14.0 │ 3.00 │ 3.33 │ 3.70 │ 4.30 15.0 │ 3.01 │ 3.34 │ 3.72 │ 4.34 ∞ │ 3.03 │ 3.38 │ 3.79 │ 4.50 ──────────────┴──────────────┴─────────────┴─────────────┴─────────────
TABLE 43
APPROXIMATE WEIGHTS OF DITCH FILLING MATERIAL TO BE USED IN THE EXPRESSION W = CwB^2[69]
───────────────────────────────────┬─────────────────────────────────── Ditch Filling │ Pounds per Cubic Foot ───────────────────────────────────┼─────────────────────────────────── Partly compacted top soil (damp) │ 90 Saturated top soil │ 110 Partly compacted damp yellow clay │ 100 Saturated yellow clay │ 130 Dry sand │ 100 Wet sand │ 120 ───────────────────────────────────┴───────────────────────────────────
Where surface loads are to be carried on the sewer trench the proper proportion of the load to be carried by the sewer is determined by the expression _L__{p} = CL, in which _L__{p} is the equivalent backfill load per unit length of the trench, L is the surface load per unit length of the trench, and C is a coefficient in which allowance is made for the character of the backfilling, the ratio of depth to width of trench, and the character of the load, whether long or short. A long load is a load extending along the length of the trench such as a pile of building material. A short load is one extending across the trench and for only a short distance along it, such as that caused by a street car or road roller crossing the trench. Values of C are given in Table 44 for long loads, and in Table 45 for short loads. Values of long and short loads occasionally met in practice are given in Tables 46 and 47 respectively.
TABLE 44
RATIO OF LOAD ON PIPE TO LONG LOAD ON TRENCH[70]
──────────────┬──────────────┬──────────────┬──────────────┬────────────── Ratio of Depth│Sand and Damp │Saturated Top │ Damp Yellow │ Saturated to Width │ Top Soil │ Soil │ Clay │ Yellow Clay ──────────────┼──────────────┼──────────────┼──────────────┼────────────── 0.0│ 1.00│ 1.00│ 1.00│ 1.00 0.5│ 0.85│ 0.86│ 0.88│ 0.89 1.0│ 0.72│ 0.75│ 0.77│ 0.80 1.5│ 0.61│ 0.64│ 0.67│ 0.72 2.0│ 0.52│ 0.53│ 0.59│ 0.64 2.5│ 0.44│ 0.48│ 0.52│ 0.57 3.0│ 0.37│ 0.41│ 0.45│ 0.51 4.0│ 0.27│ 0.31│ 0.35│ 0.41 5.0│ 0.19│ 0.23│ 0.27│ 0.33 6.0│ 0.14│ 0.17│ 0.20│ 0.26 8.0│ 0.07│ 0.09│ 0.12│ 0.17 10.0│ 0.04│ 0.05│ 0.07│ 0.11 ──────────────┴──────────────┴──────────────┴──────────────┴──────────────
For example, let it be desired to determine the load on a 72–inch concrete sewer with a 9–inch shell under the following conditions: depth of backfill over the top of the pipe, 15 feet; character of backfill, saturated yellow clay; superimposed load, pile of building brick 6 feet high. The ratio of the depth of backfill to the width of the trench is 15 ÷ 9 or 1.67. The coefficient in the expression CwB^2 is 1.39, from Table 42. The weight of saturated yellow clay is 130 pounds per cubic foot, from Table 43. Therefore the load per foot length of the sewer due to the backfill is:
W = CwB^2 = 1.39 × 130 × 81 = 14,600 pounds.
TABLE 45
RATIO OF LOAD ON PIPE TO SHORT LOAD ON TRENCH[71]
───────┬───────────────┬───────────────┬───────────────┬─────────────── Ratio │ │ │ │ of │ │ │ │ Height │ │ │ │ to │ │ │ │ Width │ │ │ │ of │ Sand and Damp │ Saturated Top │ Damp Yellow │ Saturated Trench │ Top Soil │ Soil │ Clay │ Yellow Clay ───────┼───────────────┴───────────────┴───────────────┴─────────────── │ Length of Load Equal to ───────┼───────┬───────┬───────┬───────┬───────┬───────┬───────┬─────── │ Width │⅒ Width│ Width │⅒ Width│ Width │⅒ Width│ Width │⅒ Width │ of │ of │ of │ of │ of │ of │ of │ of │Trench │Trench │Trench │Trench │Trench │Trench │Trench │Trench ───────┼───────┼───────┼───────┼───────┼───────┼───────┼───────┼─────── 0.0│ 1.00│ 1.00│ 1.00│ 1.00│ 1.00│ 1.00│ 1.00│ 1.00 0.5│ 0.77│ 0.12│ 0.78│ 0.13│ 0.79│ 0.13│ 0.81│ 0.13 1.0│ 0.59│ 0.02│ 0.61│ 0.02│ 0.63│ 0.02│ 0.66│ 0.02 1.5│ 0.46│ │ 0.48│ │ 0.51│ │ 0.54│ 2.0│ 0.35│ │ 0.38│ │ 0.40│ │ 0.44│ 2.5│ 0.27│ │ 0.29│ │ 0.32│ │ 0.35│ 3.0│ 0.21│ │ 0.23│ │ 0.25│ │ 0.29│ 4.0│ 0 12│ │ 0.12│ │ 0.16│ │ 0.19│ 5.0│ 0.07│ │ 0.09│ │ 0.10│ │ 0.13│ 6.0│ 0.04│ │ 0.05│ │ 0.06│ │ 0.08│ 8.0│ 0.02│ │ 0.02│ │ 0.03│ │ 0.04│ 10.0│ 0.01│ │ 0.01│ │ 0.01│ │ 0.02│ ───────┴───────┴───────┴───────┴───────┴───────┴───────┴───────┴───────
TABLE 46
WEIGHTS OR COMMON BUILDING MATERIAL WHEN PILED FOR STORAGE. POUNDS PER CUBIC FOOT
────────────────────────────────────────┬────────────────────────────── Brick │ 120 Cement │ 90 Sand │ 90 Broken stone │ 150 Lumber │ 35 Granite paving │ 160 Coal │ 50 Pig iron │ 400 ────────────────────────────────────────┴──────────────────────────────
The pressure of the pile of brick per square foot of trench area is, from Table 46, 120 × 6 = 720 pounds per square foot. The value of C from Table 44, is about 0.70. Therefore L{p}_ is 0.7 × 9 × 720 = 4536 pounds. The equivalent depth of backfill weighing 130 pounds per cubic foot is (4536)⁄130 × 9 = 3.88 foot. The total equivalent depth of backfill is therefore 3.88 + 15 = 18.88 feet. The ratio of depth to width is 18.88⁄9 = 2.98. The coefficient C in the expression W = CwB^2 is 2.17. The total load per foot length of sewer is therefore W = 2.17 × 130 × 81 = 22,800 pounds.
TABLE 47
WEIGHTS OF SHORT LOADS ON SEWER TRENCHES
(Adapted from Specifications of the American Bridge Company for Bridges) ──────────────────────────────┬──────────────────────────────────────── Street railways, heavy │A load of 24 tons on 2 axles on 10 foot │ centers. Street railways, light │A load of 18 tons on 2 axles on 10 foot │ centers. For city streets, heavy │A load of 24 tons on 2 axles 10 feet traffic │ apart and 5 foot gage. For city streets, moderate │A load of 12 tons on 2 axles 10 feet traffic │ apart and 5 foot gage. For city streets, light │A load of 6 tons on 2 axles 10 feet traffic or country roads │ apart and 5 foot gage. │ Road rollers │Total weight 30,000 pounds. Weight on │ front wheel, 12,000 pounds, and on │ each of two rear wheels, 9,000 pounds. │ Width of front wheel, 4 feet and of │ each of two rear wheels 20 inches. │ Distance between front and rear axles │ 11 feet. Gage of rear wheels, 5 feet, │ c. to c. ──────────────────────────────┴────────────────────────────────────────
=103. Stresses in Circular Ring=—In Fig. 81_a_ the loads shown indicate the distribution ordinarily assumed in sewer design, the forces being uniformly distributed across the diameter. To find the bending moment in the pipe caused by this loading, let ab in Fig. 81_b_ represent a section of a pipe loaded with equally distributed horizontal and vertical forces. Then the vertical component on a strip of differential length ds is wds cos Θ and the horizontal component is wds sin Θ and resolving, the resultant normal to the surface is wds, in which w is the intensity per unit length of the horizontal and vertical forces and Θ is the angle which the tangent to ds makes with the horizontal. Thus the loading of the nature shown in Fig. 81_b_ is equivalent to a loading of equally distributed normal forces which give no moment in the ring.
Considering a ring subjected to vertical forces only, the moments will be as shown in Fig. 81_c_ and if loaded with horizontal forces only, the moments will be as shown in Fig. 81_d_. Because of the symmetry of the figure, moment (1) equals moment (4) but is opposite in direction and moment (2) equals moment (3) but is opposite in direction. When the horizontal and vertical forces are combined on the same ring as in Fig. 81_b_ these moments cancel each other as has been proven. Therefore moment (1) equals moment (2) and moment (3) equals moment (4). Then in Fig. 81_e_, M{a}_ = M{b}_. Now ∑M = O for conditions of equilibrium, therefore M{a}_ + M{b}_ + (W⁄2)(d⁄4) = O and solving M{a}_ = (Wd)⁄16. This moment occurs at the ends of the horizontal and vertical diameters and causes tension on the inside of the pipe at the top and on the outside at the ends of the horizontal diameter. There will also be compression at each end of the horizontal diameter equal to one-half of the total load on the pipe. If the material of the pipe is homogeneous, the maximum fiber stress f can be found through the expression f = (My)⁄I ± P⁄A in which M is the bending moment, y is the distance from the neutral axis to the extreme fiber of a cross-section of the shell of the pipe of unit length, I is the moment of inertia of this cross-section about its neutral axis, P is one-half the total load on the pipe, and A is the area of the cross-section. For reinforced concrete, the standard formulas should be used with this expression for M. The stresses in a circular ring subjected to other distributions of loads are shown in Table 48. An exhaustive study of the stresses in circular rings was published by Prof. A. N. Talbot in Bulletin No. 22 of the Engineering Experiment Station at the University of Illinois, 1908.
TABLE 48
MAXIMUM STRESS IN FLEXIBLE RINGS DUE TO DIFFERENT LOADINGS
(From Marston) ──────────────────┬───────────────┬───────────────┬─────────── Symmetrical │Moment at Crown│ Moment at End │Compressive Vertical Loadings │ of Sewer │ of Horizontal │ Thrust at │ │ Diameter │ Crown │ │ │ │ │ │ ────────────┬─────┼───────────────┼───────────────┼─────────── Character │Width│ │ │ ────────────┼─────┼───────────────┼───────────────┼─────────── Concentrated│ 0°│+ .318_R__W_/12│- .182_R__W_/12│ 0.000 Uniform │ 60°│+ .207_R__W_/12│- .168_R__W_/12│ 0.000 Uniform │ 90°│+ .169_R__W_/12│- .154_R__W_/12│ 0.000 Uniform │ 180°│+ .125_R__W_/12│- .125_R__W_/12│ 0.000 ────────────┴─────┴───────────────┴───────────────┴───────────
──────────────────┬────────────┬───────────┬────────── Symmetrical │Compressive │ Shear at │ Shear at Vertical Loadings │ Thrust at │ Crown │ End of │ End of │ │Horizontal │ Horizontal │ │ Diameter │ Diameter │ │ ────────────┬─────┼────────────┼───────────┼────────── Character │Width│ │ │ ────────────┼─────┼────────────┼───────────┼────────── Concentrated│ 0°│+ .500_W_/12│0.500_W_/12│ 0.000 Uniform │ 60°│+ .500_W_/12│0.000_W_/12│ 0.000 Uniform │ 90°│+ .500_W_/12│0.000_W_/12│ 0.000 Uniform │ 180°│+ .500_W_/12│0.000_W_/12│ 0.000 ────────────┴─────┴────────────┴───────────┴──────────
R = the radius of the pipe, W = total weight of ditch filling and superimposed load plus ⅝ of the weight of the pipe itself (usually neglected), expressed in pounds per foot length of pipe. Moments are inch-pounds per inch length of pipe. Shears and thrusts are in pounds per inch length of pipe.
=104. Analysis of Sewer Arches.=—The preceding method for the determination of the stresses in a sewer ring has referred only to a circular pipe uniformly loaded. Other methods must be used if the pipe is not circular or the load is not uniformly distributed. The simplest method, is the static or so-called vouissoir method. In this method the arch is assumed to be fixed at both ends, presumably at the springing line or line of intersection between the inside face of the arch and the abutment, and it is so designed that the resultant of all the forces acting on any section shall lie within the middle third of that section.
To design an unreinforced sewer arch by the vouissoir method, a desired arch is drawn to scale in apparently good proportions for the loadings anticipated. The arch is then divided into any number of sections of equal or approximately equal length called vouissoirs, and the line of action of the resultant load, including the weight of the vouissoir is drawn above each vouissoir as shown in Fig. 82. The forces are assumed to act as shown in the figure. In symmetrically loaded sewer arches there is no vertical reaction at the crown. The resultant R is assumed to act at the lower middle third of the skewback, which is the inclined joint between the arch and the abutment. The upper horizontal force H is assumed to act at the upper middle third of the middle or crown section. The magnitude of H is computed by equating the sum of the moments of all forces about the point of application of R at the skewback to zero, and solving. The force polygon is then drawn as shown in Fig. 83, and the equilibrium polygon is completed in Fig. 82 with its rays parallel to the corresponding strings drawn from the end of H as origin in Fig. 83. If the equilibrium polygon line, called the resistance line, lies wholly within the middle third of each vouissoir, the arch is satisfactory to support the assumed load without reinforcement. If any portion of the resistance line lies outside of the middle third, an attempt should be made to find a resistance line which lies wholly within the middle third. The true resistance line is that which deviates the least from the neutral axis of the arch. To approximate more nearly the true resistance line find two points at which the resistance line already drawn deviates the most from the neutral axis of the arch. Select points M and N on these joints, M being nearer the crown than N. Then let W{1}_ and W{2}_ be the sum of all the loads between the crown and M and N respectively, y represent the vertical distance from the crown to N, and y′ represent the vertical distance between M and N, and x{1}_ and x{2}_ represent the horizontal distance from W{1}_ and W{2}_ to M and N respectively. Then the horizontal thrust, H, and a, the distance from the crown to the point of application of H, are,
H = ((W{2}x_{2}_ − W{1}x_{1})_)⁄y′,
a = y − (W{2}x_{2}_)⁄H.[72]
A resistance line should be drawn with this new horizontal thrust. If no resistance line can be found lying wholly within the middle third, new sections should be designed until a resistance line can be drawn lying wholly within the middle third—unless the arch is to be reinforced. A number of satisfactory arches should be designed and the easiest one to build should be selected. This method is limited in its application to sewer arches with rigid side walls and it cannot be extended to include the invert. Although an approximate method it is accurate within less than 10 per cent of the true stresses and is usually quite close.
The elastic method for the design of arches locates the true line of resistance without approximations and is more accurate though not so simple to apply as the static or vouissoir method. In this method a desired form of arch is drawn as in the static method and subdivided into vouissoirs so that the distance S along the neutral axis between joints is such that the ratio I⁄S shall be the same for all vouissoirs. I is the average of the moments of inertia of the surfaces of the two limiting joints about the neutral axis. If the thickness of the arch is constant the distance between joints will be the same. The method for dividing the arch into sections such that the ratio I⁄S shall be a constant[73] is as follows: divide the half arch axis into any number of equal parts; measure the radial depth at each point of division; lay off the length of the arch axis to scale on a straight line; divide this line into the same number of equal parts as the half arch, as shown in Fig. 84; at each point erect a perpendicular equal in length by scale to the moment of inertia at the corresponding point on the arch section; draw a smooth curve through the tops of these lines; draw a line ab at any slope from the center of the original straight line to the curve, and then a line bc back to the straight line to form an isosceles triangle abc; continue forming these triangles in a similar manner thus dividing the original straight line in the required ratio. The distance between joints is represented by the bases of the triangles. By construction the altitude of the triangle represents the average moment of inertia between the two limiting joints. The base of each isosceles triangle is S, and I⁄S = ½ tan α in which α is the base angle of all the isosceles triangles.
The following steps in the procedure are taken from the second edition of the American Civil Engineers Pocket Book, p. 634:
In Fig. 85 let the middle points of the joints be marked 1, 2, 3, etc. and the coordinates x and y from the crown be found for each by computation or measurement. For a load W placed at one of these points, let z denote the distance from it, toward the nearest skewback, to another middle point. Let ∑zx be the sum of the products of all the values of z by the corresponding x, and ∑zy be the sum of all the products of z by the corresponding y; that is, each z in the last two summations is multiplied by the x or y of the point back of W which corresponds to z.
For a single load W on the left semi-arch of Fig. 85 the following formulas are deduced from the elastic theory, n being the number of parts into which the semi-arch is divided.
Horizontal thrust, H = (W⁄2)(n∑zy − ∑y·∑z)⁄(n∑y^2 − (∑y)^2) (1)
Moment at Crown, _M__{0} = (½_W_∑z − H∑y)⁄n (2)
Shear at Crown, _V__{0} = (½_W_∑zx)⁄∑x^2 (3)
For symmetrical loading such as W on the left and W on the right the horizontal thrust and crown moment due to both loads are double those found by the above formulas, while the crown shear _V__{0} is zero. For several loads unsymmetrically placed the formulas are to be applied to each in succession and the results added algebraically, the value of _V__{0} being taken as negative for the left semi-arch and positive for the right semi-arch.
For any joint whose middle point is at a distance x from the crown
M = _M__{0} + Hy + _V__{0}x − ∑Wz,
V = _V__{0} − ∑W,
where ∑W is the sum of all the loads between the joint and the crown and ∑Wz is the sum of the moments of those loads with respect to the middle of the joint. The components of the resultant thrust normal and parallel to the joints are,
N = H cos θ − V sin θ,
F = H sin θ + V cos θ,
in which θ is the angle which the plane of the joint makes with the vertical.
The distances from the neutral axis to the resistance line are,
at the crown, _e__{0} = _M__{0}⁄H,
at the joint, e = M⁄N.
The resistance line should be located as in the vouissoir method and if not within the middle third a new design should be studied.
=105. Reinforced Concrete Sewer Design.=—The method to be followed in the design of reinforced concrete arches is similar except that the moment of inertia should include both the concrete and the steel, that is,
I = I{c}_ + nI{s}_,
in which I is the moment of inertia to be employed, I{c}_ is the moment of inertia of the concrete, I{s}_ is the moment of inertia of the steel, and n is the ratio of their moduli of elasticity, generally taken as 15. All of the moments of inertia are referred to the neutral axis of the beam. The reinforcement called for in precast circular pipes is given in Table 39. Sewers cast in place are ordinarily designed to avoid reinforcement, except where the depth of cover is small and the sewer may be subjected to superimposed loads.
Concrete sewers are sometimes reinforced longitudinally, with expansion joints from 30 to 50 feet apart. This reinforcement is to reduce the size of expansion and contraction cracks by distributing them over the length of a section. The pipe is divided into sections to concentrate motion due to expansion or contraction at definite points where it can be cared for.
The amount of longitudinal reinforcement to be used is a matter of judgment. It varies in practice from 0.1 to 0.4 per cent of the area of the section. Since the coefficients of expansion of concrete and of steel are nearly the same, movements of the structure are as important as the stresses due to changes in temperature.
Because of the uncertain and difficult conditions under which concrete sewers are frequently constructed it is advisable to specify the best grade of concrete and not to stress the concrete over 450 pounds per square inch in compression, with no allowable stress in tension. The concrete covering of reinforcing steel should be thicker than is ordinarily used for concrete building design, because of the possibility of poor concrete allowing the sewage to gain access to the steel, resulting in more rapid deterioration than would be caused by exposure to the atmosphere. A minimum covering of about 2 inches is advisable, except in very thin sections not in contact with the sewage. A minimum thickness of concrete of about 9 inches is frequently used in design, although crown thicknesses of 4½ inches have been used with success. Greater thicknesses should be used near the surface, particularly in locations subjected to heavy or moving loads.
Brick linings are often provided for the invert where moderately high velocities of about 10 feet per second when flowing full are to be expected. For velocities in the neighborhood of 20 feet per second the invert should be lined with the best quality vitrified brick. Although concrete may erode no faster than brick under the same conditions, brick linings are more easily replaced and at a smaller expense.
X. CONTRACTS AND SPECIFICATIONS
=106. Importance of the Subject.=—Sewers may be constructed by day labor or by contract. Under the day labor plan a city official or commission is charged with the purchase of material, the hiring and firing of employees, and the management of the work. Under the contract system a private individual or company contracts to supply all the material and labor necessary for the completion of the work.
Under the day labor plan all persons engaged are “working for the City.” There is not the same sense of individual responsibility, the same incentive to economize, the same feeling of loyalty that is inspired by work under the personality of a contractor. Under either the day labor or contract plan unscrupulous politics are likely to enter into the relations of the employees of the city and the city officials or between the contractor and the city officials. Neither the day labor nor the contract plan offer a sure cure for unscrupulous political misdealings. Under the contract plan the contractor is led to keep his bid as low as possible, realizing the competition of other bidders, and during construction he will obtain greater efficiency from his labor because of their realization of the different conditions under which they are working. In some states and cities it is illegal for the municipality to do sewer construction except under the contract method.
The contract method is therefore used in the majority of cases, and it is to the interest of the engineer that he be acquainted with the essentials of contracts and specifications necessary for the proper prosecution of sewer construction.
=107. Scope of Subject.=—The making of a contract is one of the most common episodes of every day life. The contract may be an informal verbal agreement to meet at a certain place at a certain time, or it may be a formal document hedged about by confusing legal phraseology and bearing varieties of penalties and dire consequences in the event of its breach. The purpose of this chapter is to explain only those general features of an engineering contract which have particular bearing upon sewerage construction. Only the most essential points can be touched in the limited space available to this subject, it being presumed that the engineer is previously grounded in the principles of business law.[74]
=108. Types of Contracts.=—Contracts are known as lump sum, cost-plus, unit-price, and by other titles indicating the method of payment.
A lump sum contract is one in which a stated amount is fixed upon, before the execution of the contract, to be paid for all the work to be done and materials to be furnished under the contract. Such an arrangement is not advisable for a sewer contract, as the cautious contractor will bid high enough to protect himself in the event of any probable emergency. The principal must therefore pay whether the emergency or unforeseen difficulty is met or not. The advantage of this type of payment is that the principal knows exactly the cost of the work to him before construction is commenced.
Cost-plus contracts are those in which the cost of the work to the contractor is to be paid by the principal, plus, (a) a fixed sum of money, (b) a percentage of the cost of the work, (c) a percentage of the cost of the work but with a fixed limit, (d) a percentage of the difference between the cost of the work and some fixed sum, or other variations of this principle. Such contracts have the advantage that the principal assumes all the risk in construction and therefore pays for only those contingencies which actually arise. Except for the last named form, they have the disadvantage that there is little or no incentive for the contractor to keep the cost of the work down. They are most successful where the contractor can be selected by the principal, but where it is necessary to let contracts to the lowest bidder, the “cost-plus” contract is not easily managed. In most states a municipality cannot make a cost-plus contract.
A unit-price contract is one in which the amount to be paid is fixed in proportion to the amount of work done or materials supplied. This type of contract is the most suitable for sewer construction for a municipality where the contract must be let to the lowest bidder. The contractor is protected in the event of many unforeseen emergencies and the principal is protected against a raise in bids to cover such emergencies and against increase in the cost of the work in order to increase the profits under a “cost-plus” contract.
It is sometimes desirable for the principal to furnish a portion of the materials, the bidders being notified beforehand that this material will be furnished. In this manner the quality of material is assured, contractors with the necessary skill but small capital may be attracted to bid, and uncertainties in the procuring of materials is eliminated.
=109. The Agreement.=—A contract is an agreement between two or more interested parties to do a certain thing. A contract for the construction of a sewer is an agreement between a municipality or individual desiring sewerage facilities and a company or individual engaged in the construction of sewers. The latter promises to construct a sewer in return for which the former promises to pay a certain amount of money.
The various portions of the agreement which are bound together as the complete contract are: I. The Advertisement, II. Information and Instructions for Bidders, III. Proposal, IV. General Specifications, V. Technical Specifications, VI. Special Specifications, VII. Contract, VIII. Bond, and IX. Contract Drawings. These should be fastened together in pamphlet form and constitute the complete instrument called the contract. No binding contract and specifications can be drawn upon logical deductions alone as legal precedent and tried methods must be followed to insure success. To draw up an original contract requires the combined knowledge of an engineer and a lawyer. The engineer of to-day writes his specifications by copying copiously from specifications used on work which has been completed successfully. In order that selections may be made with judgment and discrimination some examples have been selected from existing published specifications and contracts.
=110. The Advertisement.=—This should contain: (1) A heading indicating the type of work, (2) A statement as to when, where and how bids will be received and opened, (3) A brief description of the character and amount of work to be done, (4) The method of payment, (5) The conditions under which further information can be obtained, (6) A statement as to the amount of money which must be deposited with the bid, and (7) Any other pertinent facts concerning the work.[75] An example of an advertisement follows;
Sewer Construction
Construction Turkey Creek Sewer
Kansas City, Missouri.
Bids for the construction of the Turkey Creek Sewer, two sewage pumping stations to be used in connection therewith, and certain laterals and extensions of existing sewers thereto, for Kansas City, Missouri, will be received up to 2 p.m. August 19, 1919, at the office of the Board of Public Works, City Hall, Kansas City, Missouri.
The main sewer will be about one and one-fifth miles long, and the laterals and extensions about three and one-half miles: the main sewer will be constructed of reinforced concrete, the laterals and extensions will consist of concrete, segment blocks, and clay pipe.
This work is estimated to cost from $1,500,000 to $1,750,000. Payment for the work will be made in four year special tax bills, bearing 7 per cent interest, payable one-fourth each year. Time 600 working days, barring strikes, bad weather, etc.
Bidders are required to deposit $15,000 in cash or a certified check with bid, to insure signing of contract when let. Same to be returned on execution of the contract or rejection of bid.
Complete plans and specifications for the work may be had and all information obtained by seeing or writing to A. D. Ludlow, Engineer of Sewers, City Hall, Kansas City, Missouri. Twenty-five ($25.00) Dollars will be required to be deposited for a set of the plans, but $20.00 thereof will be refunded upon return of the plans in good condition.
BOARD OF PUBLIC WORKS,
Kansas City, Missouri,
by F. E. McCabe, Secretary.
There are usually legal restrictions which require that the advertisement be inserted a certain number of times in specified newspapers or other advertising mediums before the opening of bids. If the contract is of sufficient size to attract outside contractors, the advertisement should be inserted in engineering and contracting journals of wide circulation. Although the advertisement appears separately from the other portions of the contract, a copy is usually bound in as the first page of the pamphlet containing the contract and specifications and is made an integral part thereof.
=111. Information and Instructions for Bidders.=—This is somewhat on the order of an introduction to the pamphlet in which the specifications, contract, and contract drawings are published. As examples of the type of information and instructions given to prospective bidders the abstracts below have been taken from the “Contract, Specifications, Bond, and Proposal for the North Shore Sanitary Intercepting Sewer” by the Sanitary District of Chicago. The information and instructions to bidders can be divided into the following sections: 1st. Examination of Site, 2nd. Character and Quantity of Work, 3rd. Qualification for Bidding, 4th. Instructions for Making out Proposal, 5th. Certified Check, and 6th. Rejection of Bids.
REQUIREMENTS FOR BIDDING AND INSTRUCTIONS TO BIDDERS
Bidders are required to submit their bids upon the following express conditions:
Bidders must carefully examine the entire sites of the work and the adjacent premises, and the various means of approach to the sites, and shall make all necessary investigations to inform themselves thoroughly as to the facilities for delivering and handling materials at the sites and to inform themselves thoroughly as to all the difficulties that may be involved in the complete execution of all work under the attached contract in accordance with the specifications hereto attached.
Bidders are also required to examine all maps, plans, and data mentioned in the specifications, contract or proposal as being on file in the office of the Chief Engineer, for examination by bidders. No plea of ignorance of conditions that exist or that may hereafter exist or of conditions or difficulties that may be encountered in the execution of the work under this contract, as the result of a failure to make the necessary examinations and investigations, will be accepted as an excuse for any failure or omission on the part of the Contractor to fulfill in every detail all of the requirements of said contract, specifications and plans, or will be accepted as a basis for any claims whatsoever for extra compensation. Upon application all information in the possession of the Chief Engineer will be shown to bidders, but the correctness of such information will not be guaranteed by the Sanitary District.
The following schedule of quantities, although stated with as much accuracy as is possible in advance, is approximate only, and is assumed solely for the purpose of comparing bids.
Then follows an itemized schedule of the quantity of work to be done after which comes the following:
Bidders must determine for themselves the quantities of work that will be required, by such means as they may prefer, and shall assume all risks as to variations in the quantities of the different classes of work actually furnished under the contract. Bidders shall not at any time after the submission of this proposal, dispute or complain of the aforesaid schedules of quantities or assert that there was any misunderstanding in regard to the amount or the character of the work to be done, and shall not make any claims for damages or for loss of profits because of a difference between the quantities of the various classes of work assumed for comparison of bids and the quantities of work actually performed.
Proposals that contain any omissions, erasures, or alterations, conditions or items not called for in the contract and plans attached hereto, or that contain irregularities of any kind, will be rejected as informal.
Bids manifestly unbalanced will not be considered in awarding the contract.[76]
No bid will be accepted unless the party making it shall furnish evidence satisfactory to the Board of Trustees of the Sanitary District of Chicago of his experience and familiarity with work of the character specified and of his financial ability to successfully and properly prosecute the proposed work to completion within the specified time.
Each bid shall be accompanied by a certified check, or cash, to the amount of ten (10) per cent of the total amount of said bid figured on the quantities given herewith, the lowest alternative total being allowed. Said amounts deposited with bids, shall be held until all of the bids have been canvassed and the contract awarded and signed. The return of said check or cash to the bidder to whom the contract for said work is awarded will be conditioned upon his appearing and executing a contract for the work so awarded and giving bond satisfactory to said Board of Trustees, for the fulfillment of each contract in the amount of fifty (50) per cent of the amount of each contract.
The said Board of Trustees reserves the right to reject any or all bids.
Accompanying the contract form are plans which, together with the specifications, show the work on which said tenders are to be made.
The proposal must not be detached herefrom or from the contract by any bidder when submitting a bid.
=112. Proposal.=—The proposal is a blank printed form on which the bidder is required to enter the prices for which he proposes to do the work. The proposal blank is necessary in order that the bids may be sufficiently uniform for proper comparison. Sewers are often paid for, particularly for small sizes, per foot of completed sewer as measured along the center line of the pipe parallel to the surface of the ground with the exterior length of manholes and other structures deducted. Sometimes, under other conditions, a different rate is allowed for each additional two feet of depth of sewer, and special structures, such as manholes, catch-basins, flush-tanks, etc., are paid for at a unit price according to the depth. Water connections to flush-tanks are paid for per foot of length of service pipe laid. In especially large or difficult work, materials are paid for at a unit-price, for example, per cubic yard of excavation, per cubic yard of concrete, per thousand feet board measure of lumber, etc.
The following example is taken from the contract for the North Shore Intercepting Sewer previously quoted, to indicate the type of Proposal used:
PROPOSAL
FOR THE CONSTRUCTION OF THE NORTH SHORE INTERCEPTING SEWER
To the Honorable, the President and the Board of Trustees of the Sanitary District of Chicago:
Gentlemen:__The undersigned hereby certi____ that ____ ha____ examined the specifications and form of contract and the accompanying plans for the construction of the North Shore Intercepting sewer, and ha____ also examined the premises at and adjacent to the sites of the proposed work, as herein described, and the means of approach to the said sites.
The undersigned ha____ also examined the foregoing “Requirements for Bidding and Instructions to Bidders” and propose ____ to do all the work called for in said specifications and contract, and shown on said plans, and to furnish all materials, tools, labor and all appliances and appurtenances necessary to the full completion of said work at the rates and prices for said work as follows, to_wit:
(1_a_) For six (6) by nine (9) foot concrete sewer, complete in place, as specified, the sum of ____ Dollars and ____ cents ($ ____ ) per linear foot.
(6_a_) For manholes, concrete, complete in place, as specified the sum of ____ Dollars and ____ cents ($ ____ ) each.
The following plans showing the work to be performed in accordance with the attached specifications, have been examined by the undersigned in preparing the foregoing proposal, to-wit: ____ ____ In accordance with the requirements set forth in the attached Information and Instructions for Bidders, there is deposited herewith the sum of ____ ____ Dollars and ____ cents ($ ____ ) which under the terms therein mentioned entitle ____ to bid on said work, the said sum to be refunded to ____ ____ upon the faithful performance of all conditions set forth in the Information and Instructions for Bidders.
Name ____ Address ____
Blanks are provided for each item. No place is left at the end for a summary. The proposal ends with an acknowledgment that the contract has been examined completely and all preliminary directions therein have been complied with. A blank is prepared for inserting the amount of the required certified check, and finally for the signature of the bidders.
=113. General Specifications.=—The specifications, both general and technical, are occasionally incorporated in the contract form, but more frequently they are printed separately and are bound in the pamphlet preceding the contract. The general specifications relate to the conditions under which all work must be performed and are as applicable to the construction of a pumping station as to the smallest lateral, unless otherwise specified. It is not possible to include a complete set of General Specifications in the limited space of this text, but the more important specifications will be emphasized by examples taken from specifications in use.[77]
The subjects covered in General Specifications are:
(1) Definitions of doubtful terms.
(2) The Engineer to settle disputes.
(3) Duties of the Engineer.
(4) Duties of the Contractor.
(5) Hours and days of work.
(6) No work to be done in the absence of an inspector.
(7) Contractor to be represented at all times.
(8) Time of commencing and completing the work.
(9) Liquidated damages for delay in completion.
(10) The City may change the plans.
(11) The City may increase the amount of the work.
(12) Inspection and its conduct.
(13) The Contractor to be acquainted with laws relating to the work.
(14) Contractor responsible for damages to persons or property.
(15) City to be protected against patent claims.
(16) Abandonment of contract and its remedy.
(17) Estimates of work done and moneys due.
(18) Payments for extra work.
(19) Character of workmen to be employed.
(20) City may reserve a sum for repairs during stipulated term after completion.
(21) City may use money due Contractor to pay claims for labor or materials used on the work and not paid for by the Contractor.
(22) The Contractor shall have no claim for damages on account of delay or unforeseen difficulties.
(23) The Contractor may not assign nor sublet the contract without the City’s consent.
(24) Cleaning up after completion.
(25) The Contractor’s relations to other contractors.
(26) The portions composing the contract.
The following examples cover the subjects named in the preceding titles:
1. Definitions. The word Engineer whenever not qualified shall mean the Chief Engineer of the Commission, acting either directly or through his properly authorized agents, such agents acting severally within the scope of the particular duties entrusted to them.
This article may include words that may be in dispute or ambiguous such as: Board of Trustees, Elevation, City, Contractor, Rock, Earth, etc., etc.
2. Disputes. To prevent disputes and litigations, the Engineer shall in all cases determine the amount, quality, and acceptability of the work which is to be paid for under the contract; shall decide all questions in relation to said work and the performance thereof, and shall in all cases decide every question which may arise relative to the fulfillment of the contract on the part of the Contractor. His determination, decision and estimate shall be final and conclusive, and in case any question shall arise between the parties touching the contract, such determination, decision, and estimate shall be a condition precedent to the right of the Contractor to receive any moneys under the contract.
3. Duties of the Engineer. The Engineer shall make all necessary explanations as to the meaning and intentions of the specifications and shall give all orders and directions, either contemplated therein or thereby, or in every case in which a difficulty or unforeseen condition shall arise in the performance of the work. Should there be any discrepancies in or between, or should any misunderstanding arise as to the import of anything contained in the plans and specifications, the decision of the Engineer shall be final and binding. Any errors or omissions in plans and specifications may be corrected by the Engineer, when such corrections are necessary for the proper fulfillment of their intentions as construed by him.
4. Duties of the Contractor. The Contractor shall do all the work and furnish all the labor, materials, tools and appliances necessary or proper for performing and completing the work required by the contract, in the manner called for by the specifications, and within the contract time. He shall complete the entire work at the prices agreed upon and fixed therefor to the satisfaction of the Commission and its Chief Engineer and in accordance with the specifications, the drawings, and such detailed drawings as may be furnished from time to time, together with such extra work as may be required for the performance of which written orders may be given and received as hereinafter provided.
The Contractor shall place sufficient lights on or near the work and keep them burning from twilight to sunrise; shall erect suitable railings, fences or other protections about all open trenches, and provide all watchmen on the work, by day or night, that may be necessary for the public safety. The Contractor shall, upon notice from the Engineer that he has not satisfactorily complied with the foregoing requirements, immediately take such methods and provide such means and labor to comply therewith as the Engineer may direct, but the Contractor shall not be relieved of this obligation under the contract by any such notice or directions given by the Engineer, or by neglect, failure, or refusal on the part of the Engineer to give such notice and directions.
The Contractor shall furnish such stakes and the necessary labor for driving them as may be required by the Engineer. He shall maintain the stakes when set, with reasonable diligence, and stakes misplaced due to the carelessness of the Contractor or his workmen shall be reset under the direction of the Engineer, at the Contractor’s expense.
5. Night, Sunday, and Holiday Work:[78] No night, Sunday, nor holiday work requiring the presence of an engineer or inspector will be permitted except in case of emergency, and then only to such an extent as is absolutely necessary and with the written permission of the Engineer; provided that this clause shall not operate in the case of a gang organized for regular and continuous night, Sunday, or holiday work.
6. Absence of Engineer or Inspector. Any work done without lines, levels, and instructions having been given by the Engineer or without the supervision of an assistant or inspector, will not be estimated or paid for except when such work is authorized by the Engineer in writing. Work so done may be ordered removed and replaced at the Contractor’s sole cost and expense.
7. Absence of Contractor. During the absence of the Contractor he shall at all times have a duly authorized representative on the work. The Contractor shall give written notice to the Commission of the name and address of said representative and shall state where and how such representative can be reached, at any and all hours, whether by day or night.
Whenever the Contractor or his representative is not present at any place on the work where it may be necessary to give orders or directions, such orders or directions will be given by the Engineer and they shall be received and promptly obeyed by the superintendent or foreman who may have immediate charge of the particular work in relation to which the order may be given.
8. Commencing Work. The Contractor agrees to begin the work covered by this contract within —— days of the execution of the contract and to prosecute the same with all due diligence and to entirely complete the work within —— days.
It is understood and agreed that time is of the essence of this contract, and that a failure on the part of the Contractor to complete the work herein specified within the time specified will result in great loss and damage to said Sanitary District and that on account of the peculiar nature of such loss it is difficult, if not impossible, to accurately ascertain and definitely determine the amount thereof.
9. Liquidated Damages. It is therefore covenanted and agreed that in case the said Contractor shall fail or neglect to complete the work herein specified on or before the date hereinbefore fixed for completion, the said Contractor shall and will pay the said Sanitary District the sum of —— Dollars for each and every day the Contractor shall be in default in the time of completion of this contract.
Said sum of —— Dollars per day is hereby agreed upon, fixed and determined by the parties hereto as the liquidated damages which said Sanitary District will suffer by reason of such defaults, and not by way of a penalty.
10. Changes in Plans. The Board reserves the right to change the alignment, grade, form, length, dimensions or materials of the sewers or any of their appurtenances, whenever any condition or obstructions are met that render such changes desirable or necessary. In case the alterations thus ordered make the work less expensive to the Contractor a proper deduction shall be made from the contract prices and the Contractor shall have no claim on this account for damages or for anticipated profits on the work that may be dispensed with. In case such alterations make the work more expensive, a proper addition shall be made to the contract prices. Any deduction or addition as aforesaid shall be determined and fixed by the Engineer.
11. Extensions and Additions. In the event that any material alterations or additions are made as herein specified which in the opinion of the Engineer will require additional time for execution of all the work under this contract, then, in that case the time of completion of the work shall be extended by such a period or periods of time as may be fixed by said Engineer and his decision shall be final and binding upon both parties hereto, provided that in such case the Contractor, within four (4) days after being notified in writing of such alterations and additions, shall request in writing an extension of time, but the provisions of this paragraph shall not otherwise alter the provisions of this contract with reference to liquidated damages, and the said Contractor shall not be entitled to any damages or compensation from the said Sanitary District on account of such additional time required for the execution of the work.
12. Inspection. All materials of whatsoever kind to be used in the work shall be subject to the inspection and approval of the Engineer and shall be subject to constant inspection before acceptance. Any imperfect work that may be discovered before its final acceptance shall be corrected immediately, and any unsatisfactory materials used in the work or delivered at the site shall be rejected and removed on the requirement of the Engineer. The inspection of any work shall not relieve the Contractor of any of his obligations to perform proper and satisfactory work as herein specified, and all work which, during the progress and before the final acceptance, may become damaged from any cause, shall be removed and replaced by good and satisfactory work without extra charge therefor. The Engineer and his assistants shall have at all times free access to every part of the work and to all points where material to be used in the work is manufactured, procured or stored and shall be allowed to examine any material furnished for use in the work under this contract.
All inspection of any and all material furnished for use in work to be performed under this contract shall be made at the site of the work after the delivery of the material, provided, that, if requested by the Contractor the Engineer may at his option perform, or have performed, inspection of materials at points other than the site of the work. In any such case the Contractor shall pay the Sanitary District the extra cost of such inspection, including the necessary expenses of the inspector for the extra time expended in performing any such inspection at said other points.
13. Legal Requirements. The Contractor shall keep himself fully informed of all existing and future national and state laws and local ordinances and regulations in any manner affecting those engaged or employed in the work, or the materials used in the work, or of all such orders and degrees of bodies or tribunals having any jurisdiction or authority over the same, and shall protect and indemnify the party of the first part against any claim or liability arising from or based on the violation of such law, ordinance, regulation, order or decree, whether by himself or his employees.
14. Damages. If any damage shall be done by the Contractor or by any person or persons in his employ to the owner or occupants of any land or to any real or personal property adjoining, or in the vicinity of the work herein contracted to be done or to the property of a neighboring contractor the Engineer shall have the right to estimate the amount of said damage and to cause the Sanitary District to pay the same to the said owner, occupant, or contractor, and the amount so paid shall be deducted from the money due said Contractor under this contract. Said Contractor covenants and agrees to pay all damages for any personal injury sustained by any person growing out of any act or doing of himself or his employees that is in the nature of a legal liability, and he hereby agrees to indemnify and save the Sanitary District harmless against all suits or actions of every name and description brought against said Sanitary District, for or on account of any such injuries, or such damages received or sustained by any person or persons; and the said Contractor further agrees that so much of the money due to him under this contract, as shall be considered necessary by the Board of Trustees of said Sanitary District, may be retained by the Sanitary District until such suit or claim for damages shall have been settled, and evidence to that effect shall have been furnished to the satisfaction of said Board of Trustees.
15. Patents. It is further agreed that the Contractor shall indemnify, keep and save harmless said Sanitary District from all liabilities, judgments, costs, damages and expenses which may in any wise come against said Sanitary District, or which may be the result of an infringement of any patent by reason of the use of any materials, machinery, devices, apparatus, or process furnished or used in the performance of this contract, or by reason of the use of designs furnished by the Contractor and accepted by the Sanitary District, and in the event of any claim or suit or action at law or in equity of any kind whatsoever being made or brought against said Sanitary District, then the Sanitary District shall have the right to retain a sufficient amount of money in the same manner and upon the conditions as hereinafter specified.
16. Abandonment of Contract. If the work to be done under the contract shall be abandoned by the Contractor, or if at any time the Engineer shall be of the opinion, and shall so certify, in writing, to the Commission, that the performance of the contract is unnecessarily or unreasonably delayed, or that the Contractor is willfully violating any of the conditions of the specifications, or is executing the same in bad faith, or not in accordance with the terms thereof, or if the work be not fully completed within the time named in the contract for its completion, the Commission may notify the Contractor to discontinue all work thereunder, or any part thereof, by a written notice served upon the Contractor, as herein provided; and thereupon the Contractor shall discontinue the work, or such part thereof, and the Commission shall thereupon have the power to contract for the completion of said work in the manner prescribed by law, or to procure and furnish all necessary materials, animals, machinery, tools and appliances, and to place such and so many persons as it may deem advisable to work at and complete the work described in the specifications, or such part thereof, and to charge the entire cost and expense thereof to the Contractor. And for such completion of the work or any part thereof, the Commission may for itself or its contractors, take possession of and use or cause to be used any or all such materials, animals, machinery, tools and implements of every description as may be found on the line of the said work. The cost and expense so charged shall be deducted from, and paid by the City out of such moneys as may be due or may become due to the Contractor, under and by virtue of the contract. In case such expense shall exceed the amount which would have been payable under the contract, if the same had been completed by the Contractor, he shall pay the amount of such excess to the City. When any particular part of the work is being carried on by the Commission, by contract or otherwise, under the provisions of this clause of the contract, the Contractor shall continue the remainder of the work in conformity with the terms of his contract, and in such manner as in no wise to hinder or interfere with the persons or workmen employed by the Commission by contract or otherwise as above provided, to do any part of the work or to complete the same under the provisions hereof.
17. Estimates. The Engineer shall from time to time as the work progresses, on or about the last day of each month, make in writing an estimate, such as he shall believe to be just and fair, of the amount and value of the work done and the materials incorporated into the work by the Contractor under the specifications, provided however that no such estimate shall be required to be made when, in the judgment of the Engineer the total value of the work done and the materials incorporated into the work since the last preceding estimate is less than —— dollars. Such estimates shall not be required to be made by strict measurements, but they may be approximate only.
The Contractor shall not be entitled to demand from the Commission as a right, a detailed statement of the measurements or quantities entering into the several items of the monthly estimates, but he will be given such opportunities and facilities to verify the estimates as may be deemed reasonable by the Commission.
When in the opinion of the Engineer, the Contractor shall have completely performed the contract on his part, the Engineer shall make a final estimate, based on actual measurements, of the whole amount of the work under and according to the terms of the contract, and shall certify to the Commission in writing, the amount of the final estimate at the completion of the work. After the completion of the work the City shall pay to the Contractor the amount remaining after deducting from the total amount or value of the work, as stated in the final estimate, all such sums as have theretofore been paid to the Contractor under any of the provisions of the contract, except such sums as may have been paid for extra work, and also any sum or all sums of money which by the terms thereof the City is or may be authorized to reserve or retain; provided that nothing therein contained shall affect the right of the City, hereby reserved, to reject the whole or any portion of the aforesaid work, should the said certificate be found or known to be inconsistent with the terms of the contract or otherwise improperly given. All monthly estimates upon which partial payments have been made, being merely estimates, shall be subject to correction in the final estimate, which final estimate may be made without notice thereof to the Contractor, or of the measurements upon which it is based.
18. Extra Work. The Contractor shall do any work not herein otherwise provided for, when and as ordered in writing by the Engineer or his agents specially authorized thereto in writing, and shall when requested by the Engineer so to do, furnish itemized statements of the cost of the work ordered and give the Engineer access to accounts, bills, vouchers, etc. relating thereto. If the Contractor claims compensation for extra work not ordered as aforesaid, or for any damages sustained, he shall within one week after the beginning of any such work or the sustaining of any such damage, make a written statement of the nature of the work performed or the damage sustained, to the Engineer, and shall, on or before the fifteenth day of the month succeeding that in which any such extra work shall have been done or any such damage shall have been sustained, file with the Engineer an itemized statement of the details and amount of any such work or damage; and unless such statement shall be made as so required, his claim for compensation shall be forfeited and he shall not be entitled to payment on account of any such work or damage.
For all such extra work the Contractor shall receive the reasonable cost of said work, plus fifteen (15) per cent of said cost.
19. Competent Employees. The Contractor shall employ only competent skillful men to do the work; and whenever the Engineer shall notify the Contractor, in writing, that any man employed on the work is, in his opinion unsatisfactory, such man shall be discharged from the work and shall not again be employed on it, except with the consent of the Engineer.
20. Money Retained. Upon the completion of the work and its acceptance by the City, the City shall reserve and retain five (5) per cent of the total value of the work done under the contract as shown by the final estimate, over and above any and all other reservations which the city by the terms thereof is entitled or required to retain and shall hold the said five (5) per cent for a period of nine (9) months from and after the date of completion and acceptance, and the City shall be authorized to apply such part of said five (5) per cent so retained to any and all costs of repairs and renewals as may become necessary during such period of nine (9) months, due to improper work done or materials furnished by the Contractor, if the Contractor shall fail to make such repairs or renewals within twenty-four (24) hours after receiving notice from the City so to do.
Upon the expiration of said nine (9) months from and after the completion and acceptance of the work, the City shall pay to the Contractor the said five (5) per cent hereby retained, less such sums as may have been retained hereunder.
21. Unpaid Claims against Contractor. The Contractor shall furnish the City with satisfactory evidence that all persons who have done work or furnished materials under the contract, and have given written notices to the City, before and within ten (10) days after the final completion and acceptance of the whole work under the contract, that any balance for such work or materials is due and unpaid, have been fully paid or satisfactorily secured. And in case such evidence is not furnished as aforesaid, such amount as may be necessary to meet the claims of the persons aforesaid shall be fully discharged or such notices withdrawn.
22. Delays and Difficulties. The Contractor shall not be entitled to any claims for damages on account of postponement or delay in the work occasioned by forces beyond the control of the City, nor for postponement or delay in the work where ten (10) days written notice has been given the Contractor of such postponement or delay, nor where unforeseen difficulties are encountered in the prosecution of the work. In the event of a postponement or delay ordered in writing by the City the time of completion of the contract shall be extended a number of days equal to the number of days that the work has been postponed or delayed.
23. Assignment of Contract. The Contractor shall not assign by power of attorney or otherwise, nor sublet the work or any part thereof, without the previous written consent of the party of the first part, and shall not either legally nor equitably assign any of the moneys payable under this agreement or his claim thereto unless by and with the consent of the party of the first part.
24. Cleaning Up. On or before the completion of the work, the Contractor shall, without charge therefor, tear down and remove all buildings and other structures built by him, shall remove all rubbish of all kinds from any grounds which he has occupied, and shall leave the line of the work in a clean and neat condition.
25. Access to Work and Other Contractors. The Commission and its engineers, agents and employees may at any time and for any purpose enter upon the work and the premises used by the Contractor, and the Contractor shall provide proper and safe facilities therefor. Other contractors of the Commission may also when so authorized by the Engineer, enter upon the work and the premises used by the Contractor for all the purposes which may be required by their contracts. Any differences or conflicts which may arise between this Contractor and other contractors of the Commission in regard to their work shall be adjusted and determined by the Engineer.
26. The Contract. It is understood and agreed by the City and the Contractor that the terms of this contract are embodied and included in the Advertisement, Information and Instructions to Bidders, Proposal, Specifications of every nature, the Bond and the contract drawings hereto attached.
These few articles have been given as examples of some of the essential subjects to be treated in general specifications. It is to be understood that these examples do not represent a complete set of general specifications and items have been omitted the absence of which in a complete contract might be injurious to the successful completion of the work.
=114. Technical Specifications.=—These ordinarily follow the general specifications and have to do with the quality of materials, the manner of putting them together, and the method of doing the work. The subject headings in the Technical Specifications on the Baltimore Sewerage Commission are:
Excavation Tunneling Rock Excavation Sheeting Sheet Piling Sheeting and Bracing Piles Blasting Pumping and Drainage Foundations Refilling Repaving Underdrains Buildings Inlets and Catch-Basins Cement Mortar Concrete Brick Masonry Reinforced Concrete Vitrified Pipe Concrete and Brick Sewers Vitrified Pipe Sewers and Drains Manholes Iron Castings House Connections Obstructions Fences Flush-Tanks
Each of these subjects is treated in the appropriate section of this book.
An important part of each section of the technical specifications is the clause providing for the method of payment for the work specified. This is usually the last clause in the section. For example, the last clause in the Baltimore Specifications relating to Rock Excavation, is:
“Payment will be made for the number of cubic yards of rock measured and allowed as above specified at the price of four dollars and fifty cents ($4.50) per cu. yd., measured in place. Payment for rock excavation will be made in addition to the prices bid for excavation.”
=115. Special Specifications.=—These have to do with problems, methods of construction, or materials peculiar to certain contracts or certain portions of the work. It frequently occurs that the construction of sewerage works will be let out under a number of contracts, or bids will be called for on different alternatives to which the entire Advertisement, Information and Instructions for Bidders, Proposal, and General Specifications are applicable. The special specifications will apply only to the contract in question, e.g., in some work done under the direction of the author, the sewer on one contract came within twelve inches of the surface of a highway. The special specification relating to this piece of construction, was:
“Where crossing under the Chicago Road the pipe sewer shall be embedded in concrete as shown on the contract drawings. The concrete for this purpose shall be mixed in the proportions of one (1) part cement, three (3) parts fine aggregate, and six (6) parts coarse aggregate. Payment for the concrete so used will be made at the unit price stated in the accompanying Proposal.”
In order to avoid confusion the special specifications are either incorporated directly in the Contract form, or follow the Technical Specifications and are grouped according to the contracts to which they apply.
=116. The Contract.=—The contract is a brief instrument which includes a simple statement of the obligations of each party involved. The following is an example of a form in successful use:
CONTRACT
This agreement made and entered into this ____ day of ____ in the year one thousand nine hundred and ____ by and between the City of ____ by its duly constituted or elected authorities herein acting for the City of ____ without personal liability to themselves, party of the first part, hereinafter designated as the City, and ____ party of the second part hereinafter designated as the Contractor.
WITNESSETH, that the parties to these presents each in consideration of the undertakings, promises and agreements on the part of the other herein contained, have undertaken, promised and agreed, and do hereby undertake, promise and agree, the party of the first part for itself, its successors and assigns, and the part ____ of the second part for ____ and ____ heirs, executors, administrators and assigns as follows, to-wit:
Art. I. To be bounden by all the articles of the General, Technical, and Special Specifications applicable, and by the terms of the Advertisement, Information and Instructions for Bidders, Proposal and Contract Drawings hereto attached, and which are understood and acknowledged to be an integral part of this contract.
Art. II. The work to be completed under this contract is ____
Art. III. The City shall pay and the Contractor shall receive as full compensation for everything furnished and done by the Contractor under this contract, including all work required but not specifically mentioned in the following items, and also for all loss or damage arising from the nature of the work aforesaid, or from the action of the elements, or from any unforeseen obstruction or difficulty encountered in the prosecution of the work and for well and faithfully completing the work as herein provided, as follows:
Then follows a copy of the Proposal with the prices bid. The contract closes with the final clause:
In witness whereof the said City of ____, party of the first part have hereunto set their hands and seals, and the Contractor has also hereunto set his hand and seal and the party of the first part and the Contractor have executed this agreement in duplicate, one part to remain with the party of the first part and one to be delivered to the Contractor this ____ day of ____ in the year one thousand nine hundred and ____
City of ____ ____ ____
Contractor ____ ____ ____
=117. The Bond.=—The bond called for in the Information and Instructions for Bidders is bound in the pamphlet following the Contract. No uniform practice is followed in the amount of the bond required. It varies from 50 to 100 per cent of the contract price and may be stated as a lump sum before the contract price is known. There is a possibility that the Contractor may fail before he has commenced work and the City may be unable to procure another contractor to take up the work. The City should then be protected by a 100 per cent bond. Such a contingency is remote. The Contractor seldom fails until work is well under way, and other contractors are usually available, although the failure of one contractor tends to increase the bids of other contractors for the same work. In fixing the amount of the bond the judgment of the Engineer is called into play in order that the amount may be as low as possible in fairness to the Contractor, and high enough to protect the interests to the City. By reducing the amount of the bond the expense to the City is also reduced as the City ultimately must pay its cost.
Upon the acceptance of the bond and the execution of the Contract, the Engineer’s duties take him out of the designing office and into the construction field.