A text-book on roads and pavements

Front matter

Cornell University Library TE 145.S73

A text-book on roads and pavements.

3 1924 003 881 798

Preface

Successful practice in the construction of highways must depend upon correct reasoning from elementary principles in each instance rather than upon following definite rules or methods of construction.

The aim of this book is to give a brief discussion, from an engineering standpoint, of the principles involved in highway work, and to outline the more important systems of construction, with a view to forming a text which may serve as a basis for a systematic study of the subject.

Details and statistics of particular examples have for the most part been excluded as undesirable in a book of this character. Such information is available in many forms for those having the necessary elementary training and experience to enable them to properly use it.

Considerable space has been given to the location and construction of country roads, as seemed proper in view of the present general public interest in the matter, and the probable development of this new field of activity in engineering work. The improvement of our common roads must come through transferring such work to the charge of those who make it a profession, and not through teaching the public how roads should be constructed.

F. P. S.

Chapter I. General Considerations

FACE

Art. I. Object of Roads i

2. Resistance to Traction 2

3. Tractive Power of Horses 7

4. Desirability of Various Surfaces 9

5. Economic Value 11

6. Healthfulness 14

7. Safety 16

8. Durability 18

CHAPTER H.

DRAINAGE OF STREETS AND ROADS.

Art. 9. Necessity for Drainage 22

10. Surface Drainage 23

11. Subsurface Drainage 26

12. Kinds of Soil 28

13. Types of Drains 3'

14. Culverts 34

I J. Water- breaks 42

CHAPTER HI.

LOCATION OF COUNTRY ROADS.

Art. 16. Considerations governing Location 43

17. Length of Road 47

V

VI CONTENTS.

PAGE

Art. i8. Rise and Fall 49

19. Rate of Grade 51

20. Examination of Country S3

21. Placing the Line 56

22. Comparison of Routes 59

23. Changing Existing Locations 63

CHAPTER IV.

IMPROVEMENT OF COUNTRY ROADS.

Art. 24. Nature of Improvements (>^

25. Earthwork 68

26. Drainage 72

27. Earth-road Surface 73

28. Gravel Roads 76

29. Maintenance of Country Roads 78

30. Width of Country Roads 80

31. Economic Value of Road Improvements 82

32. Systems of Road Management 85

CHAPTER V.

BROKEN-STONE ROADS.

Art. 33. Definition 88

34. Macadam Roads 89

35. Telford Foundations 91

36. Choice of Foundation 93

37. Materials 96

38. Binding Material loi

39. Compacting 103

40. Thickness of Road Covering 105

41. Cross-section 107

42. Maintenance 107

CHAPTER VI.

FOUNDATIONS FOR PAVEMENTS.

Art. 43. Preparation of Road-bed no

44. Purpose of Foundation in

CONTENTS. VU

PAGE

Art. 45. Sand Foundation 112

46. Gravel and Broken Stone 113

47. Concrete 114

49. Sand and Plank 117

50. Depth of Foundation 117

CHAPTER VII.

BRICK PAVEMENTS.

Art. 51. Paving Brick 119

52. Tests for Paving Brick 122

53. Foundations 128

54. Construction 129

55. Maintenance 132

CHAPTER VIII.

ASPHALT PAVEMENTS.

Art. 56. Asphaltum 134

57. Rock Asphalt 140

58. Asphalt Blocks 142

59. Foundations 144

60. Construction 146

61. Vulcanite or Distillate Pavement 148

, 62. Maintenance 149

CHAPTER IX.

WOOD PAVEMENTS.

Art. 63. Wood Blocks 151

64. Foundations 1 54

65. Construction 156

66. Preservation of Wood. 161

67. Maintenance 164

68. Healthfulness 165 via CONTENTS.

CHAPTER X.

STONE-BLOCK PAVEMENTS.

PAGE

Art. 69. Stone for Pavements 168

70. Cobblestone Pavements 170

71. Belgian Blocks 172

72. Granite and Sandstone Blocks 172

73. Construction I73

74. Stone Trackways 176

CHAPTER XI.

CITY STREETS.

Art. 75. Arrangement of City Streets 178

76. Width and Cross-section 183

77. Street Grades 187

78. Street Intersections 189

79. Footways 191

80. Curbs and Gutters 196

81. Crossings 202

82. Street-railway Track 203

83. Trees for Streets 211

84. Alleys 212

A TEXT-BOOK

ON

ROADS AND PAVEMENTS.

CHAPTER I.

GENERAL CONSIDERATIONS.

Art. I. Object of Roads.

The primary object of a road or street is to provide a way for travel, and for the transportation of goods from one place to another. The facility with which traffic may be conducted over any given road depends upon the resistance offered to the passing of vehicles by the surface or the grades of the road, as well as upon the freedom of movement allowed by the width and form of the roadway. In order that a road may offer the least resistance to traffic, it should have as hard and smooth a surface as possible, while affording a good foothold to horses, and should be so located as to give the most direct route with the least gradients.

The expediency of any proposed road construction or improvement depends upon its desirability as affecting the comfort, convenience, and health of residents of the locality, and also upon its economic value which is largely determined by its cost and durability, as well as upon the facility it gives for the conduct of traffic.

The problem of the highway engineer, in designing works of this character, involves the consideration of these various elements and their proper adjustment to give the best results.

The kinds of road surface most commonly employed are as follows : For the streets of cities and towns, pavements of stone blocks, brick, asphalt, and wood ; for suburban streets and important country roads, macadam and gravel surfaces ; for ordinary country roads in general, surfaces of earth or gravel.

Art. 2. Resistance to Traction.

The resistance to traction of a vehicle on a road surface may be divided into three parts : axle friction, rolling resistance, and grade resistance.

Axle friction varies with the nature of the bearing surfaces, and for vehicles of similar construction is directly proportional to the load. It is entirely independent of the nature of the road surface.

Rolling resistance is of two kinds : that due to irregularities in the surface of the road, and that of a wheel to rolling upon a smooth surface, sometimes called rolling friction.

The resistance due to an inequality in the road surface, is the horizontal force necessary, at the axle, to raise the weight upon the wheel to the height of the obstacle to be passed. Thus (Fig. i), by the principle

A

Mto-i ^^

For small inequalities, this resistance will be approximately inversely as the diameter of the wheel. The effect of small ^ irregularities in the surface, however, is due more to the shocks and concussions produced by them than to the direct lifting action of the obstacle, and the resistance due to uneven surface is greater at high ' „ than at low velocities.

Rolling friction is probably due for the most part to the compressibility of the surface of the road, which permits the wheel to indent it to some extent. The wheel is thus always forcing a wave of the surface before it, or climbing an inclination caused by its weight upon the road surface. This rolling friction varies for wheels of differing diameters, being less for large than for small wheels. The experiments of M. Morin, in France, seemed to indicate that the resistance varies inversely as the diameter. Other experiments have indicated a less variation, approximately as the square root of the diameter, while Mr. D. K. Clark (Roads and Streets by Law and Clark ; London, 1890) concludes, from a mathematical discussion based upon the assumption that the material of the surface is homogeneous and the pressure proportional to the depth of penetration, that the resistance to traction is inversely as the cube root of the diameter of the wheel.

For practical purposes it may be considered that, for wheels of ordinary sizes used on road vehicles, the rolling resistances are equal to the load multiplied by a coefficient which depends upon the nature and condition of the road surface, although these coefificients are somewhat affected by the sizes of the wheels.

Many experiments have been made for the purpose of determining the tractive force required for a given load upon various road surfaces. The results show somewhat wide variations, as would be expected when the many elements that may affect them are considered. The following table shows a few average results, which will give some idea of the relative resistances of various surfaces and of the advantage to be derived from a smooth and well-kept road surface :

TRACTIVE RESISTANCES ON VARIOUS SURFACES.

Character of Road. Resistance, Lbs. per ton.

Earth — ordinary in fair condition. 125 to 140

dry and hard 60 " 100

Macadam — very good 40 " 60

ordinary 60 " 80

Granite-block pavement — good. . . 25 • 4c

ordinary 50 " 80

Asphalt pavement 15 " 25

Wooden-block pavement 20 " 30

On earth roads or smooth pavements the tractive force is independent of the velocity ; but on rough pavements, where concussions take place, the tractive force increases as the speed increases.

Grade Resistances. — Tractive resistance due to grade is independent of the nature of the road surface, or of the size of the wheels. It is equal to the load multiplied by the sine of the angle made by the grade with

GENERAL CONSIDERATIONS.

the Jiorizontal. Thus in Fig. 2 the tractive force P,

due to the grade, is the force necessary to prevent the

wheel from rolling down

the slope under the ac- / v»'l \ p

tion of the weight W, or

it is the component of

fFparallel to the slope ac.

Fig. 2.

Grades are ordinarily expressed in terms of rise or fall in feet per hundred, or as percentage of horizontal distance.

For all ordinary cases of small inclinations ab is approximately equal to ac, and we may take

P=W ab '

or the tractive force necessary to overcome any grade equals the load multiplied by the percentage of grade.

The total tractive force necessary to haul a load up an inclined road equals the sum of the force necessary to haul the load upon the same surface when level, and the force necessary to overcome the grade resistance. Thus, if we wish to find the tractive effort necessary to haul a load of 2 tons up a grade of 3 ft. in 100 over a good macadam road. Taking the resistance of the road surface when level at 60 lbs. per ton, we have for the total resistance

In going down the grade, the for(^e due to grade becomes a propelling force, and the tractive effort required is the difference between the surface resistance and grade force. In case the grade force be the greater, the resulting tractive force becomes negative, or it will be necessary to apply the force as a resistance to prevent acceleration of the velocity in the descent.

The angle for which the tractive force required for a given surface equals the grade resistance is called the Angle of Repose for that surface. In the case given above, 2 X 60 — 4000 X tIt = O) o^" the angle of repose for a surface whose level resistance is 60 lbs. per ton is a 3^ grade. If a vehicle were left standing upon that inclination, it should remain standing with the forces just balanced. If it were started down the grade, it should continue to move at a uniform rate, without the application of any other force.

In a series of experiments made by the Studebaker Brothers Manufacturing Company (see The Engineering Record for Dec. 16, 1893) upon the traction necessary upon various surfaces with American wagons, it was found that the width of wheel-tire has little if any effect upon a hard surface ; that there was a small difference in favor of the wide tire upon soft ground, and upon sod the narrow tire would cut through where the wide one would pass over.

The wheels used in the tests had tires if, 3, and 4 inches wide, and varied from 3 ft. 6 in. to 4 ft. 6 in. in diameter.

The general results show a variation in tractive force required, depending upon the construction of the vehicle, of from 30 to 65 pounds per ton for a stone-block pavement, from 120 to 175 pounds per ton for a good sand road, from 60 to 100 pounds per ton for a gravel road, and from 240 to 325 pounds per ton on a muddy road.

It was also found that the force necessary to start the load was from 125 to 200 pounds per ton greater than that necessary to keep it in motion ; the load starting easier on wheels of large diameter than upon small ones, but the diameter seemingly having less effect upon the traction when in motion.

Art. 3. Tractive Power of Horses.

The loads that a horse can pull upon various road surfaces will not necessarily be proportional to the resistance offered by the surface to traction, as the tractive force that the horse can exert depends upon the foothold afforded by the surface. The ability of a horse to exert a tractive force depends upon the strength of the animal, upon his training for the particular work, and whether he be accustomed to the surface upon which he is travelling. The work of different animals is therefore subject to considerable variations, and only very rough approximations are possible in giving average values of the work a horse may do under differing conditions.

The tractive force that may be exerted by a horse, at moderate speeds, varies approximately inversely as the rate of speed ; or, in other words, the power that a horse can exert through any considerable time is nearly constant for varying velocities. Thus it may be assumed, as an average value, that a horse working regularly ten hours per day can put forth a tractive effort of 80 pounds at a speed of 259 feet per minute on an ordinary level road surface.

For the power of the horse we then have

Power = force X velocity = 80 X 250 = 20000 foot-lbs. per minute.

For any other rate of speed, as 200 feet per minute, we would have 20000 -e- 200 = 100 pounds as the tractive force exerted by the horse.

If the period of daily work be lessened, the power that may be developed will be increased, either by increasing the load or the velocity.

The tractive force that a horse is able to exert decreases very rapidly as the rate of inclination increases. This is due both to the expenditure of power by the horse in lifting his own weight up the grade, and to the less firm footing on the inclination. The effect of differences in the foothold afforded by various pavements is very marked in the loss of tractive power upon grades.

In the table below are given the loads that an average horse may be expected to continuously haul up different inclinations, on various road surfaces, at slow speed. These figures, while of little value as an absolute measure of what may be done in any particular case, are of use as a rough comparison of the relative tractive properties of different surfaces and grades. The effect of grades upon tractive effort will also depend upon the condition in which the surface is maintained, and upon the weather. Snow and ice in winter, or the damp and muddy condition of some pavements in wet weather, have a very considerable effect to diminish tractive power.

GENERAL CONSIDERATTONS.

LOADS IN POUNDS THAT A HORSE CAN DRAW UPON VARIOUS SURFACES AND GRADES.

Kinds of Surface.

Rate of Grade.

Level.

sin

TOO.

10 in

TOO.

15 in

TOO.

Earth road — good lOOOO

2400 I zoo

ZIOO

4500 2300

1400 500 2200

IIOO

I3CXJ

1200 500 1200

1700 900 1000 poor

Broken-stone — good

poor

Stone Blocks — good

poor

Asphalt— clean and dry

On steep grades (more than 8 or 10 in 100) special forms of block pavements are sometimes employed to increase the tractive power by affording better foothold to horses. Sheet asphalt is not usually employed on grades of more than 4%. Ordinary wood blocks and brick are used up to grades of y% or 8%, and granite blocks to 10^.

A horse may frequently exert for a short time a tractive force about double that which he can exert continuously ; hence, when short grades occur steeper than the general grades of the road, loads may often be taken over them much heavier than could be carried if the steeper grade prevailed upon the road.

Art. 4. Desirability of Various Road Surfaces.

The desirability of a road surface for any particular use depends both upon its fitness for the service required of it and upon its durability in use.

Upon a country road, the problem of improvement ordinarily consists simply in providing the hardest and lO A TEXT-BOOK ON ROADS AND- PAVEMENTS.

most durable surface consistent with an economical expenditure of available funds, the object being to lighten the cost of transportation by reducing the resistance to traction, and to render travel easy and comfortable.

Upon city streets, however, several other factors may be of importance in the design of highway improvements.

The comfort both of those using the street and of the occupants of adjoining property will be largely affected by the freedom of the surface from noise and dust.

The safety of the pavement in use, its effect upon the health of residents of the locality, and its economic value must in each case be considered.

To adjust to the best advantage these various elements, frequently quite discordant with each other, is a matter which can only be accomplished by the exercise of good judgment. Local conditions and necessities must always be considered — such as the difificulties of drainage, the availability of various materials, the nature of the trafific to be carried, and the needs of the business or property interests of the neighborhood. Thus, for heavy hauling of a large city, the durability and resistance to wear of the pavement may be the paramount consideration ; for an office district, quiet may be very important ; for the lighter driving of a residence street, the elements of comfort and healthfulness may properly be considered as of greater force than the purely economic ones ; while in all of the cases the necessary limitation of first cost will largely determine what may or may not be done.

general considerations. ii

Art. 5. Economic Value.

The determination of the economic value of any proposed road or street improvement is always a matter of difHculty, as it embraces so many items which cannot be exactly evaluated. The factors to be considered in this connection are :

1. Cost of construction.

2. Cost of maintenance and repairs.

3. Cost of conducting transportation.

4. Effect upon land values or business interests. For the purpose of comparing various pavements,

or of considering the advisability of any proposed improvement, we may sum the interest on the cost of construction with the annual charges representing the other items, and find which improvement will make the total annual cost a minimum or annual benefit a maximum. This process, in any case in practice, simply amounts to a use of the judgment, having properly in view the various interests to be affected, as to what expense may legitimately be allowed in order to secure a certain benefit. The outlay is usually quite tangible and easily estimated, while the advantages cannot be directly estimated and are often overlooked. It must not, however, be supposed that they have no financial value, or that the pavement which can be constructed and maintained for the least money is necessarily the most economical to use.

The cost of construction must, of course, include everything connected with the original construction of the road and all necessary expenses leading to the improvement under consideration.

The cost of maintenance and repairs includes an estimate of the average cost of keeping the road in good condition over a term of years, taking into account the necessity of renewing the surface at the expiration of the life of the pavement, the cost of cleaning and sprinkling, and such minor repairs as may be necessary from time to time to maintain a uniformly good surface.

An approximate estimate upon these points may usually be made by examining the records of the same kind of construction under similar conditions elsewhere. The cost of maintenance of each kind of pavement varies widely in different localities and under differing treatment, and no general rules can be stated as to the relative costs of the various systems.

All road surfaces will require maintenance, the same as any other class of engineering constructions subjected to wear in use ; and as a rule the cost of maintenance will be less as the care used in keeping the surface always clean and in good condition is greater.

The cost of transportation is affected by the nature and condition of the road over which the traffic must pass, both because the resistance to traction offered by the surface determines the load that may be hauled over it, and because the roughness of the surface serves both to limit speed and to cause wear upon horses, harness, and vehicles. The evaluation of these items is a matter of difficulty, on account of the practically indeterminate nature of the data upon which they should be based.

A rough idea of the relative cost of transportation over different road surfaces may sometimes be obtained by observing or estimating the extent and nature of the traffic that is likely to pass over the road and estimating the cost of carrying tliis traffic over each surface. The portion of the traffic which consists in hauling maximum loads will be directly affected by differences in tractive resistances ; the number of loads necessary to move the traffic, and hence the cost, being, for this portion, approximately proportional to the resistance. For the lighter portion of the traffic the greater speed with a smooth surface and easy grades will be of value in the saving of time, although difficult to state in money values.

The effect of a smooth surface is also very appreciable in the cost of wear and tear upon horses, vehicles, and harness. The value of this item is variously estimated, and probably ranges from one to ten cents per mile travelled.

Earth roads, in good condition, and wood pavements seem most favorable to horses, although asphalt and broken-stone roads are commonly considered most advantageous as to general wear. Brick would not differ greatly from asphalt. On earth roads in poor condition the wear is severe, and on stone blocks it is estimated to be three or four times as great as on asphalt. The financial value of the saving in this wear and tear is difficult to ascertain, but it is undoubtedly sufficient to make it an important item in the cost of highway transportation.

Land Values. — The effect of highway improvements upon the value of adjoining property is dependent upon the nature of the uses to which the property may be put, and the extent to which various characteristics of the road surfaces, such as dust or noise, may affect the occupations or comfort of the occupants. A pavement may thus sometimes have a direct effect upon rental values. In general, however, the effect is difficult to estimate, although it is commonly recognized in the practice of assessing a portion of the cost of improvements against abutting property.

The value of comfort, convenience, safety, and healthfulness to a community, as affected by the condition of their roads and streets, cannot readily be stated in figures ; but they have a money value, both in their effect upon the general life and business of the community and in the attraction presented to outside business enterprises or home-seekers.

Art. 6. Healthfulness.

The effect of a pavement upon the health of the residents of its locality will be affected by the tendency of the materials composing it to decay, by its permeability, and by its degree of freedom from noise and dust.

The permeability of a road surface is important on account of the tendency of surface-water and refuse matter to penetrate and saturate it, and thus cause it to become dangerous to health. A continuous sheet pavement is the most desirable in this particular, and a block pavement with open joints the least so. When, however, the joints of a block pavement are properly cemented, the pavement may be made nearly impervious. If the material of which the pavement is composed be permeable, it may gradually become saturated with street refuse, even though the joints be made tight, and where the material is liable to decay it may of itself become obnoxious to health.

Both these objections are raised to the use of wood pavements, and probably in many cases with justice. This is a matter, however, concerning which authorities differ. The extent of the danger to health involved in the use of wood for pavements in any particular case probably depends largely upon the wood selected for use, and the method of construction adopted. It is at least questionable whether the permeability of the material used for pavements is in practice ever as objectionable on the ground of health as that caused by open-joint construction of block pavement, even though the material of the blocks be impervious to moisture.

The noise made by traffic upon a pavement is important not only because of its effect upon the comfort of the people using it or living adjacent to it, but also because to it are frequently attributed many nervous disorders to which people in some cities are subject.

Stone-block pavements are the most objectionable in this particular, causing a continual roar, due both to the rumbling of wheels over them and the blows of the horses' feet upon them. Upon asphalt the noise is only that due to the horses' feet, giving a sharp, chcking sound. Upon wood the horses produce no appreciable sound ; but wheels give a dull rumble, generally considered the least objectionable of any of the noises made by the more common pavements. The noise of wood pavements is diminished by making the joints between blocks small. A brick surface gives a combination of the sounds of wood and asphalt, the clicking being much less sharp than on asphalt, and the rumble less noticeable than on wood. On any block pavement the noise is lessened as the foundation is made more firm and the joints more close and well cemented.

l6 A TEXT-BOOK ON ROADS AND PAVEMENTS.

An earth or broken-stone road is usually less noisy than any of the hard pavements.

The giving off of dust by a pavement under the action of traffic is also objectionable on the score of health as well as of comfort. All pavements produce more or less dust, the amount depending more upon the method of construction and care used ia forming the surface and filling the joints than upon the material of the pavements. For the most part, however, the presence of dust is dependent rather upon the maintenance, cleaning, and sprinkling of the pavement than upon its nature, and the dirt upon the surface of a hard pavement is usually carried there from the outside and not due to the pavement.

Earth and broken-stone roads wear rapidly, and make dust freely in dry weather, requiring frequent sprinkling and cleaning to keep the road clear of it, and are on this account objectionable for use on the streets of towns under any considerable traffic.

Art. 7. Safety.

The safety of a road surface depends upon the foothold afforded by it to horses under normal conditions, and also upon the degree of slipperiness that it may take in wet weather, or under the influence of ice and snow in winter.

A dry earth road in good condition gives the best and surest foothold, with broken-stone and gravel roads nearly as good.

The relative safety of the various pavements used in city streets is a matter upon which there is considerable difference of opinion amongst authorities. Local conditions affect the pavement in this regard to an important degree. The dampness of the cHmate, the shade from buildings, the cleanliness of the streets, and the prevalence of snow and ice in winter are all important.

Statistics upon the question of relative safety of wood, asphalt, and granite have been collected by Capt. Greene in this country and by Col. Haywood in London, the attempt being made to determine the number of miles travelled by horses upon each kind of pavement to each accident due to slipperiness.

The results of Col. Haywood seem to show that of the three wood is the safest and granite the most dangerous, while the results of Capt. Greene show asphalt to be the best and wood the worst in this particular.

Col. Haywood's observations were all taken on London streets, and are as follows :

Miles travelled to each fall on

Granite.

Asphalt.

Wood.

In dry weather,

" damp "

" wet

All observations.

The observations were made when dry weather prevailed, and therefore are somewhat unfavorable to granite, which is safest when wet.

Capt. Greene's observations were made in several American cities, and showed the distance travelled to each fall to be, on granite 413 miles, on asphalt 583 miles, and on wood 272 miles. The observations on wood in this series were too few to give a reliable indication, and it is to be observed with regard to all of them that slipperiness is largely affected by the condition in which the surface is maintained, and it is therefore difficult to draw any general conclusions which would fit all cases.

All hard pavements are slippery when muddy and wet, and cleanliness is the necessary condition of safety.

Wood and asphalt, if clean, are least slippery when dry and most so when simply damp. Granite, after the surface becomes worn and polished, is most slippery when dry and least so when wet.