Text-book on roads and pavements

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.

Ithaca, N.Y., July, 1894.

m

NOTE TO THIRD EDITION.

The methods employed in the construction and maintenance of highways have changed so greatly since the first publication of this book that, in the preparation of this edition, it has been found necessary to practically rewrite the entire book.

An effort has been made to briefly represent the best recent practice in highway work, and the book has necessarily expanded considerably beyond its former limits.

F. P. S.

Columbia, Mo., July, 1908.

CHAPTER I. Road Economics and Management

PAGE

Art. i. Object of Roads i

2. Resistance to Traction , 3

3. Tractive Power of Horses 8

4. Benefits Derived from Good Roads 10

5. Cost of Wagon Transportation 12

6. Economic Value of Road Improvement 16

7. Sources of Revenue for Road Improvement 19

8. Systems of Road Management , . . . , , . . . 2-2

CHAPTER II. Drainage of Streets and Roads

Art. 9. Necessity for Drainage 26

10. Surface Drainage 28

11. Sub-drainage 29

12. Tile Drains ^

13. Stone Drains 36

14. Culverts „ 38

15. Concrete Culverts 45

CHAPTER III. Location of Country Roads

Art. 16. Considerations Governing Location 49

17. Length of Road 52

18. Rise and Fall 55

19. Rate of Grade , 57

20. Examination of Country * 59

21. Placing the Line ...*... 63

22. Comparison of Routes 65

23. Changing Existing Locations 69

vii

Vlll CONTENTS.

CHAPTER IV. Improvement and Maintenance of Country Roads

PAGB

Art. 24. Nature of Improvements 72

25. Grade and Cross-section 73

26. Earthwork 79

27. Earth Roads 85

28. Gravel Roads 93

29. Oiled Roads 96

30. Sand-clay Roads 107

31. Miscellaneous Roads 111

32. Width of Tires 115

CHAPTER V. Broken-Stone Roads

Art. 33. Definition 117

34. Macadam Roads 118

35. Telford Foundations 120

36. Rocks for Road Building 124

37. Methods of Testing Stone. 137

38. Road Metal 146

39. Compacting the Road 150

40. Thickness of Road Covering 153

41. Maintenance of Broken-Stone Roads.. 155

42. Bituminous Macadam . . 159

CHAPTER VI. Foundations for Pavements

Art. 43. Preparation of Road-bed 167

44. Trenches in Streets. 168

45. Purpose of Foundation , 170

46. Bases of Gravel and Broken Stone 172

47. Concrete Bases 172

48. Bituminous Foundations 175

49. Miscellaneous Foundations 176

50. Choice of Foundations. 178

CONTENTS. IX

CHAPTER X. Stone-Block Pavements

Art. 69. Stone for Pavements 278

70. Cobblestone Pavements 280

71. Belgian Blocks 281

72. Granite and Sandstone Blocks 282

73. Construction 284

74. Stone Trackways 287

X CONTENTS.

CHAPTER XL

City Streets.

PAGE

Art. 75. Arrangement of City Streets 289

76. Width and Cross-section 294

77. Street Grades 299

78. Street Intersections 302

79. Footways 304

80. Curbs and Gutters 310

81. Crossings 316

82. Street-railway Track 317

83. Trees for Streets 330

84. Selection of Pavements 331

85. Sources of Revenue for Street Improvement 338

ROADS AND PAVEMENTS.

Chapter I. Road Economics And Management

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 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 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 difficulties of drainage, the availability of various materials, the nature of the traffic 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.

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. 1), by the principle

Q

a

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 Fig. i. .... , , . .

inclination caused by its weight

upon the road surface.

Size of Wheels. — The resistance to traction 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. The experiments of Mr. Mairs (Bulletin, University of Missouri Agricultural Experiment Station, 1902) indicate that tractive resistance is somewhat less with large than with small wheels, being nearly inversely as the square root of the diameter, but as might be expected, differing somewhat for different road surfaces.

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 coefficients are somewhat affected by the sizes of the wheels.

Width of Tire. — The effect upon tractive resistance of the width of tire upon the wagon wheels depends upon the character of the surface upon which the wheel is rolling. In a series of experiments at the University of Missouri Agricultural Experiment Station in 1897, it was found that wide tires considerably diminished tractive resistance upon broken stone and gravel roads, and upon earth roads in good condition, but upon muddy roads or when a hard road is covered with deep dust the resistance is greater for wide tires. The wide tire also has considerable advantage upon plowed land or sod, not cutting in so deeply.

Speed of Travel. — Tractive resistances are somewhat greater at high than at low velocities. This difference is very slight on earth roads in good condition or on smooth pavements; but on rough pavements where concussions take place the resistance increases rapidly as the speed becomes greater.

Road Surface. — 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:

A TEXT-BOOK ON ROADS AND PAVEMENTS.

TRACTIVE RESISTANCES ON VARIOUS SURFACES.

Character of Road.

Earth Roads — in fair condition . . .

dry and hard

Macadam — very good

ordinary

poor

Granite block pavement — good

ordinary

Brick pavement

Wood block pavement

Asphalt pavement

Resistance per

Ton,

Pounds.

IOO to

60 to to to

IOO to

2 5 to to to to

So to

The resistance upon asphalt is greater at high than at low temperatures.

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 the horizontal. 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 action of the weight W, or it is the component of W parallel to the slope ac.

ac

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

Fig. 2.

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

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 1 00 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 force 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 yto = °> or tne an gl e of repose for a surface whose level resistance is 60 lbs. per ton is a 3 per cent 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.

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 250 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 footlbs, per minute.

For any other rate of speed, as 200 feet per minute, we would have 20000 -*- 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.

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

Kinds of Surface.

Earth road — good

poor

Broken-stone — good . . .

poor. . .

Stone Blocks — good . . .

poor. . . .

Asphalt — clean and dry

Rate of Grade

1 in

2 in

3 in

4 in

5 in

10 in

IOO.

IOO.

IOO.

IOO.

IOO.

I IOO

4000 1600

2700 I IOO

2000 800

1600 600

1400 500

45°

700 2^0

33°°

3000 8000

2300 4000

1700 2500

1400 1800

I IOO

900 1000

45° 400

200 IOO

400 200

IO A TEXT-BOOK ON ROADS AND PAVEMENTS.

In general, the tractive effort that a horse may exert is approximately proportional to the weight of the horse and the averages above given correspond to light animals; many horses are capable of exerting double the pull mentioned.

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.

On ordinary country roads in dry weather the amount of load that can be hauled is usually determined rather by the grades than by the nature of the surface. Unless the gradients are very light the amount of load that can be carried on a broken-stone surface does not differ greatly from what may be taken on a dry and hard earth road. In improving a road by substituting a hard surface for a surface of earth the gradients and location should therefore always be carefully studied, with a view to deriving the full practical benefit from the hard surface in the light traction that it may require with easy ruling gradients.

Art. 4. Benefits Derived from Good Roads.

The condition of the public highways is a matter of the most vital interest to any rural community. Upon it depends largely the social life and enjoyment of the people living in the country as well as the ability to market the products of the farm to the best advantage.

In nearly all parts of the country the roads are fairly good during a portion of the year; but there is also usually a period when they are very bad, in very

ROAD ECONOMICS AND MANAGEMENT. II

many localities becoming practically impassable. The improvement of the road surfaces and the use of systematic maintenance would make the roads better at all times, making it possible to haul larger loads over them and rendering them more pleasant to travel; but the most important object of road improvement is to eliminate the period when roads are not in condition to use and make it possible to drive upon them and haul loads over them at all times.

The benefits of good roads may be classified as social, educational, and financial. They promote social intercourse .among the residents of a country district by making travel easy and pleasant. Where the roads become impassable during a portion of the year, the residents are practically isolated at the period of greatest leisure and lose that intercourse with their neighbors which is a most important means of enjoyment and development. Attendance at church and public meetings is facilitated by good roads. There are many localities where the condition of the roads practically closes the churches during a considerable portion of each year, and in some instances they have been so deserted on this account as to be abandoned. The rural mail delivery also depends for its efficiency upon the good condition of the roads.

The consolidation of rural schools and establishment of rural high schools, made possible by good roads, is an important advance in educational methods, and places rural communities more nearly on an equality with the cities in educational advantages offered to children.

Roads which can be traveled all the year admit of marketing the products of the farm at any time which may be most advantageous, enabling the farmer to take advantage of favorable market conditions and prices, or to transport his products at a convenient season, when he can do the work without interference with other duties of men and teams.

The condition of the highways has also considerable effect upon the business of the towns into which they may lead; where they are uniformly good throughout the year mercantile business will be better distributed between different seasons, and a larger volume of business will be transacted. The same effect is produced upon railway transportation. Congestion in railway business and scarcity of cars is frequently the result of the hurried marketing of crops to take advantage of a good condition of the wagon roads, and a much better distribution of business might be obtained through an improved condition of the highways. The area tributary to a town or a railway may also frequently be considerably extended by road improvement. *

The greatest benefits derived from good roads are in the increased comfort, convenience, and pleasure of the people living near them, and in the social and educational advantages which they make possible and which add greatly to the attractiveness and happiness of rural life.

Art. 5. Cost of Wagon Transportation.

The effect of bad roads upon the cost of wagon transportation has been the subject of much discussion and many estimates have been made which have arrived at widely different conclusions. Many of these discussions have failed to take account of all the factors entering into the problem and have arrived at wildly extravagant results. Efforts have been made by the Road Inquiry Office of the United States Department of Agriculture to collect statistics concerning the volume and cost of hauling farm produce to market for the whole United States. These statistics include estimates of the average length of haul and the cost per ton-mile, with a view to basing upon them some conclusion as to the saving to the country in general which would result from the improvement of the roads so that larger loads may be carried, and less labor be required for moving the crops. Statistics of this kind are very difficult to gather, being altogether dependent upon the judgment of the man who collects them in each locality, and representing only very approximately the average conditions. They are of value as giving information concerning the traffic to which the roads are subject in various localities and the need for road improvement, but they do not contain data upon which any reasonable estimate can be based of the actual saving which might be effected by road improvement. Such general estimates are not of any particular value other than that of showing something of the size of the problem when applied to the whole country.

Many palpably erroneous and exaggerated estimates of the saving in cost of transportation by road improvement have been published and have often seriously injured the cause of good roads. They aim to show the large saving which may be effected by the farmer through reducing the cost of moving his crops to market, but their fallacies are evident to the farmer who reads them and applies them to his own conditions, and in many instances lead him to doubt the good faith of the whole movement for good roads. These estimates commonly treat the subject as though the whole of the crops were hauled to market in full loads by teams kept by the farmers for that purpose alone, and which could be dispensed with if the roads were so improved as to require a less number of loads, and consequently less teams to transport the crops, which is clearly not the case.

The effect of the condition of the highways upon the cost of wagon transportation depends upon the character of the traffic. Where this consists of the transportation of some product which is hauled in full loads, with teams which are employed for this purpose only, the cost of transportation is readily ascertained, and the saving due to any improvement which increases the load carried by each team may be found by estimating the cost of the decreased number of teams required. If an earth road in poor condition be replaced by a good macadam surface, the load which can be taken over the road may easily be doubled if upon light gradients, while where a traffic of this character must be taken over an earth road in bad or muddy condition, the construction of an improved road surface may result in loads four or five times as heavy as before. In this case the number of teams is inversely proportional to the maximum load which may be hauled and the cost is proportional to the number of teams. This condition soemtimes, though rarely, occurs upon wagon roads, the traffic usually being of a mixed character, with varying percentages carried in full loads, and with teams kept for other purposes and only incidentally used for transportation upon the roads.

For the purpose of estimating the cost of transportation upon ordinary country roads it is necessary to separate the traffic into classes and determine what portion of it is carried in full loads. This is always a matter of difficulty where the traffic is varied and can only be done in a very roughly approximate manner. The light portion of the traffic will, of course, be benefited by improved roads, but the saving in cost of conducting the traffic, while existent, is usually comparatively small and practically indeterminate. It consists in saving time of men and teams through greater speed of travel, and in less wear upon teams and vehicles. When such traffic must be conducted over muddy and bad roads these items may be of considerable importance, although they can not be evaluated, but commonly they are of slight importance.

The heavy portion of the traffic is more directly affected by the character of the roads over which it passes. This traffic is carried in full loads, which are limited in amount by the condition and gradients of the roads. In some localities this constitutes the main portion of the traffic; in others it is a comparatively small part of the whole. No generalization concerning the value of road improvement as reducing the cost of transportation can therefore be made with any approach to accuracy; but in a particular instance where data is obtainable concerning the heavy traffic, it is possible to roughly estimate the saving in labor of transportation through road improvement. If we can determine the cost of using teams for this purpose, an approximate estimate may also be made of the saving in cost of transportation through such improvement. The cost of using teams for highway transportation is often difficult to obtain on account of the fact that such teams are commonly kept for other purposes and only incidentally used for road work, and in some instances it is possible that the transportation is done when there is no other work which could be done by the team. In general, however, it is fair to assume that the cost of the work is proportional to its amount, and that if the teams were not employed in transportation on the highway, they would be otherwise usefully engaged. , In estimating the cost of work of teams, the actual cost to the farmer of keeping the team should be used and not the rental value of teams in the vicinity.

A careful examination of the local conditions surrounding the traffic is essential to any reasonable estimate of saving to be effected in cost of transportation upon highways. Such estimates are not of much value at best as giving actual amount of savings, but studies of this kind may be of value in giving a better conception of the economics of the good roads problem.

Art. 6. Economic Value of Road Improvement.

The value of a road improvement to a community and the amount of money that may reasonably and profitably be expended in the construction and maintenance of common roads is a subject the discussion of which leads different persons to widely different conclusions, depending upon the point of view and the data assumed. Any improvement, either in position or surface, that has the effect of increasing the loads that may be taken over a road by a given power lessens the number of loads necessary to carry the traffic, and effects a saving in time and labor of men and teams, which may reasonably be considered to have the same money value as the time used in the work. This has been discussed in Art. 5 and is the most direct and obvious financial gain which may result from road improvement.

Saving in cost of transportation is not, however, the most important advantage to be gained by road improvement, and if it were the only one, in many instances, the expenditure of money necessary to secure better roads could not be justified as economically profitable.