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Intersections.—At road intersections, it is always difficult to design a curve that entirely meets the requirements of safety because there is not enough room in the right-of-way, and enough additional right-of-way must be secured to permit the proper design. It is not necessary to provide an intersection that is adapted to high speed traffic, where main roads cross, but, on the contrary, a design that automatically causes traffic to slow up has distinct advantages.
Where a main route, improved with a hard surface, crosses secondary roads, it is satisfactory to continue the paved surface across the intersecting road at normal width and make no provision for the intersecting road traffic other than a properly graded approach at the intersection.
Superelevation.—On all curved sections of road, other than intersections, account is taken of the tendency of motor cars to skid toward the outside of the curve. This tendency is counteracted by designing the cross section with superelevation.
In Fig. 6, F represents the tangential force that tends to cause skidding. W represents the weight of the vehicle in pounds, THETA = the angle of superelevated surface c-d, with the horizontal c-a. R represents the radius of the curve upon which the vehicle is moving. w is the component of the weight parallel to the surface c-d, v = velocity of the vehicle in feet per second. m = mass of vehicle = W/g THETA
w = W tan THETA
mv^2 wv^2 F = ———- = ——— R gR
If F = w there will be no tendency to skid; hence the rate of superelevation necessary in any case is as follows:
Wv^2 W tan THETA = ———- gR
v^2 tan THETA = ———- gR
The amount of superelevation required, therefore, varies as the square of the velocity and inversely as the radius of the curve.
Theoretically, the amount of the superelevation should increase with a decrease in the radius of the curve and should also increase as the square of the speed of the vehicle. On account of the variation in speeds of the vehicles, the superelevation for curves on a highway can only be designed to suit the average speed. At turns approaching ninety degrees, the curve is likely to be of such short radius that it is impossible to maintain the ordinary road speed around the curve, even with the maximum superelevation permissible. It is good practice to provide the theoretical superelevation on all curves having radii greater than 300 feet for vehicle speeds of the maximum allowed by law, which is generally about 25 miles per hour. Where the radii are less than 300 feet, the theoretical superelevation for the maximum vehicle speeds gives a superelevation too great for motor trucks and horse drawn vehicles and generally no charge is made in superelevation for radii less than 300 feet, but all such curves are constructed with the same superelevation as the curve with 300 foot radius.
The diagram in Fig. 7 shows the theoretical superelevation for various curve radii.
At the intersection of important highways, the problem is complicated by the necessity for providing for through traffic in both directions and for traffic which may turn in either direction and the engineer must provide safe roadways for each class of traffic.
Tractive Resistance.—The adoption of a policy regarding the grades on a road involves an understanding of the effect of variation in the character of the surface and in rate of grade upon the energy required to transport a load over the highway. The forces that oppose the movement of a horse drawn vehicle are fairly well understood and their magnitude has been measured by several observers, but comparatively little is known about the forces opposing translation of rubber tired self-propelled vehicles.
The resistance to translation of a vehicle is made up of three elements: resistance of the road surface to the rolling wheel, resistance of the air to the movement of the vehicle and internal friction in the vehicle itself.
Rolling Resistance.—When the wheel of a vehicle rolls over a road surface, both the wheel and the surface are distorted. If the wheel has steel tires and the road surface is plastic, there will be considerable distortion of the road surface and very little of the wheel. A soft rubber tire will be distorted considerably by a brick road surface. Between these extremes there are innumerable combinations of tire and road surface encountered, but there is always a certain amount of distortion of either road surface or wheel, or of both, which has the same effect upon the force necessary for translation as a slight upward grade. When both the tire and the road surface strongly resist distortion (as steel tires on vitrified brick paving), the resistance to translation is low but the factor of impact is likely to be introduced. Where impact is present, energy is used up in the pounding and grinding of the wheels on the surface, and this factor increases as the speed of translation, and may be a considerable item. Impact is especially significant on rough roads with motor vehicles, particularly trucks, traveling at high speed. These two factors (impact and rolling resistance) combined constitute the major part of the resistance to translation for horse drawn vehicles.
Internal Resistance.—For horse drawn vehicles, the internal resistance consists of axle friction, which is small in amount. For self-propelled vehicles, the internal resistance consists of axle friction and friction in the driving mechanism, of which gear friction and the churning of oil in the gear boxes is a large item. Internal friction is of significance in all self-propelled vehicles and especially so at high speeds.
Air Resistance.—At slow speeds, the resistance of still air to translation is small, but as the speed increases, the air resistance increases rapidly and at the usual speed of the passenger automobile on the road becomes a very considerable part of the total resistance to translation. This factor has no significance in connection with horse drawn vehicles, but is to be taken into account when dealing with self-propelled vehicles at speeds in excess of five miles per hour.
Many determinations of tractive resistance with horse drawn vehicles have been made from time to time and these show values that are fairly consistent when the inevitable variations in surfaces of the same type are taken into account. Table 4 is a composite made up of values selected from various reliable sources and Table 5 is from experiments by Professor J. B. Davidson on California highways.
TABLE 4
AVERAGE TRACTIVE RESISTANCE OF ROAD SURFACES TO STEEL TIRED VEHICLES
Surface Tractive force per ton
Earth packed and dry 100 Earth dusty 106 Earth muddy 190 Sand loose 320 Gravel good 51 Gravel loose 147 Cinders well-packed 92 Oiled road—dry 61 Oiled road—wet 108 Macadam—very good 38 Macadam—average 46 Sheet asphalt 38 Asphaltic concrete 40 Vitrified brick—new 56 Wood block—good 33 Wood block—poor 42 Cobblestone 54 Granite tramway 27 Asphalt block 52 Granite block 47
TABLE 5
TRACTIVE RESISTANCES TO STEEL TIRED VEHICLES[1]
- - - - Condition Tractive Resistance Test No. Kind of Road of Road Total lb. per ton lb. - - - - 29-30-31 Concrete Good, excellent 83.0 27.6 (unsurfaced) [2]11-12 Concrete Good, excellent 90.0 30.0 (unsurfaced) 26-27-28 Concrete 3/8-in. Good, excellent 147.6 49.2 surface asphaltic oil and screenings 13-14 Concrete 3/8-in. Good, excellent 155.0 51.6 surface asphaltic oil and screenings 9-10 Macadam, Good, excellent 193.0 64.3 water-bound 22-23 Topeka on Good, excellent 205.5 68.5 concrete 8 Gravel Compact, good 225.0 75.0 condition [3]45-48 Oil macadam Good, new 234.5 78.2 [4]46-47 Oil macadam Good, new 244.0 81.3 38 Gravel Packed, in 247.0 82.3 good condition 18-19-20 Topeka on plank Good condition, 265.0 88.3 soft, wagon left marks 34 Earth road Firm, 1-1/2-in. 276.0 92.0 fine loose dust 24-25 Topeka on plank Good condition, 278.0 92.6 but soft 1-2-5 Earth road Dust 3/4 to 2 in. 298.0 99.3 3-3 Earth Mud, stiff, firm 654.0 218.0 underneath 6-7 Gravel Loose, not packed 789.0 263.0 - - - -
[1] Prof. J. B. Davidson in Engineering News-Record, August 17, 1918.
[2] Graphic record indicates that the load was being accelerated when test was started.
[3] Drawn with motor truck at 2-1/2 miles per hour.
[4] Drawn with motor truck at 5 miles per hour.
Comparatively few data are available showing the tractive resistance of motor vehicles, but the following tables are based on sufficient data to serve to illustrate the general trend.
These data on the tractive resistances of an electric truck with solid rubber tires on asphalt and bitulithic, wood, brick and granite block, water-bonded and tar macadam, cinder and gravel road surfaces were obtained by A. E. Kennelly and O. R. Schurig in the research division of the electrical engineering department of the Massachusetts Institute of Technology, and are published in Bulletin No. 10 of the division.
An electric truck was run over measured sections, ranging from 400 to 2600 feet in length, surfaced with these various materials, at certain speeds per hour, ranging from about 8 to about 15.5 miles per hour. The result of the observations of speeds, tractive resistances, conditions of surfaces, etc., were collected and studied in various combinations.
TABLE 6
+ -+ -+ Tractive Tractive Resistance Resistance Type of Surface Condition of Surface in lbs. in lbs. per ton per ton 10 miles 12.4 miles per hr. per hr. + -+ -+ Asphalt Good 20.4 Asphalt Poor 22.6 25.5 Wood block Good 24.2 25.3 Brick block Good 24.6 26.6 Granite block Good 40.3 45.75 Brick block Slightly worn 25.1 28.0 Granite block with cement joints Good 25.5 30.2 Macadam, water bonded Dry and hard 23.3 25.8 Macadam, water bonded Fair, heavily oiled 35.9 38.7 Macadam, water bonded Poor, damp, some holes 36.3 41.6 Tar macadam Good 25.7 28.0 Tar macadam Very soft 36.8 38.7 Tar macadam Many holes, soft, extremely poor 52.4 60.6 Cinder Fair, hard 27.5 30.6 Gravel Fair, dusty 30.4 33.0 + -+ -+
Effect of Grades.—Grades increase or decrease the resistance to translation due to the fact that there is a component of the weight of the vehicles parallel to the road surface and opposite in direction to the motion when the load is ascending the hill and in the same direction when the vehicle is descending. In Fig. 8 _W_ represents the weight of the vehicle, acting vertically downward, _w_ is the component of the weight perpendicular to the road surface and _W_{2}_ is the component parallel to the road surface.
_W_{2}_ = _W_ tan _THETA_.
tan THETA = 0.01 x per cent of grade.
_W_{2}_ = 0.01 _W_ x per cent grade.
_W_{2}_ = 0.01 x 2000 x per cent of grade, for each ton of weight of vehicle.
Hence _W_{2}_ = 20 lbs. per ton of load for each one per cent of grade.
The gravity force acting upon a vehicle parallel to the surface on a grade is therefore 20 lbs. per ton for each one per cent of grade and this force tends either to retard or to accelerate the movement of the vehicle.
Let F = the sum of all forces opposing the translation of a vehicle.
_F = f_{r} + f_{i} + f_{p} + f_{a} + f_{g}_ (1)
where
_f_{r}_ = rolling resistance of road surface. _f_{i}_ = resistance due to internal friction in the vehicle. _f_{p}_ = resistance due to impact of the road surface. _f_{a}_ = resistance due to air. _f_{g}_ = resistance due to grade, which is positive when ascending and negative when descending.
All of the above in pounds per ton of 2000 lbs.
Let T = the tractive effort applied to the vehicle by any means.
T >= must be greater than F in order to move the vehicle.
By an inspection of (1), it will be seen that for a given vehicle and any type of road surface, all terms are constant except f{a} and f{g}. f{a} varies as the speed of the vehicle and the driver can materially decrease f{a} by reducing speed. f{g} varies with the rate of grade. For any vehicle loaded for satisfactory operation on a level road with the power available, the limiting condition is the factor f{g}. If the load is such as barely to permit motion on a level road, any hill will stall the vehicle. Therefore, in practice the load is always so adjusted that there is an excess of power on a level road. If draft animals are employed the load is usually about one fourth of that which the animals could actually move by their maximum effort for a short period. With motor vehicles, the excess power is provided for by gearing.
If it be assured a load of convenient size is being moved on a level road by draft animals, there is a limit to the rate of grade up which the load can be drawn by the maximum effort of the animals.
Tests indicate that the horse can pull at a speed of 2-1/2 miles per hour, an amount equal to 1/8 to 1/10 of its weight, and for short intervals can pull 3/4 of its weight. The maximum effort possible is therefore six times the average pull, but this is possible for only short intervals. A very short steep hill would afford a condition where such effort would be utilized. But for hills of any length, that is, one hundred feet or more but not to exceed five hundred feet, it is safe to count on the draft animal pulling three times his normal pulling power for sustained effort.
The limiting grade for the horse drawn vehicle is therefore one requiring, to overcome the effect of grade, or _f_{g}_, a pull in excess of three times that exerted on the level.
A team of draft animals weighing 1800 lbs. each could exert a continuous pull of about 1/10 of their weight or 360 lbs. If it be assumed that the character of the vehicle and the road surface is such that _f_{r}_ + _f_{i}_ + _f_{p}_ + _f_{a}_ = 100 lbs. per gross ton on a level section of road, then the gross load for the team would be 3.6 tons. The same team could for a short time exert an additional pull of three times 360 lbs. or 1080 lbs. For each 1 per cent of grade a pull of 20 lbs. per ton would be required or _f_{g}_ for the 3.6 tons load would be 72 lbs. for each per cent of grade. At that rate, the limiting grade for the team would be fifteen per cent.
If, however, the character of the vehicle and the road surface were such that f{r} + f{i} + f{p} + f{a} = 60 lbs. per gross ton on a level section of road, the gross load for the team on the level would be 6 tons, and the limiting grade 9 per cent.
The above discussion serves to illustrate the desirability of adopting a low ruling or limiting grade for roads to be surfaced with a material having low tractive resistance and the poor economy of adopting a low ruling grade for earth roads or roads to be surfaced with material of high tractive resistance.
It may be questioned whether horse drawn traffic should be the limiting consideration for main trunk line highways, but it is certain that for a number of years horse drawn traffic will be a factor on secondary roads.
In the case of motor vehicles, excess power is provided by means of gears and no difficulty is encountered in moving vehicles over grades up to 12 or 15 per cent, so that any grade that would ordinarily be tolerated on a main highway will present no obstacle to motor vehicles, but the economy of such design is yet to be investigated.
Energy Loss on Account of Grades.—Whether a vehicle is horse drawn or motor driven, energy has been expended in moving it up a hill. A part of this energy has been required to overcome the various resistances other than grade, and that has been dissipated, but the energy required to translate the vehicle against the resistance due to grade has been transformed into potential energy and can be partially or wholly recovered when the vehicle descends a grade, provided the physical conditions permit its utilization. If the grade is so steep as to cause the vehicle to accelerate rapidly, the brakes must be applied and loss of energy results. The coasting grade is dependent upon the character of the surface and the nature of the vehicle. In the cases discussed in the preceding paragraph, the coasting grades would be five per cent and three per cent respectively. For horse drawn vehicles then the economical grades would be three and five per cent, which again emphasizes the necessity of lower grades on roads that are surfaced than on roads with no wearing surface other than the natural soil.
The theory of grades is somewhat different when motor vehicles are considered, since it is allowable to permit considerably higher speed than with horse drawn vehicles before applying the brakes and the effect of grade can be utilized not only in translating the vehicle down the grade, but also in overcoming resistances due to mechanical friction and the air. On long grades, a speed might be attained that would require the use of the brake or the same condition might apply on very steep short grades. There is at present insufficient data on the tractive resistance and air resistance with motor vehicles to permit the establishing of rules relative to grade, but experience indicates a few general principles that may be accepted.
If a hill is of such rate of grade and of such length that it is not necessary to use the brake it may be assumed that no energy loss results so far as motor vehicles are concerned. Where there is no turn at the bottom of the hill and the physical condition of the road permits speeds up to thirty-five or forty miles per hour grades of five per cent are permissible if the length does not exceed five hundred feet and grades of three per cent one thousand feet long are allowable. It is a rather settled conviction among highway engineers that on trunk line highways the maximum grade should be six per cent, unless a very large amount of grading is necessary to reach that grade.
Undulating Roads.—Many hills exist upon highways, the grade of which is much below the maximum permissible. If there are grades ranging from 0 to 4 per cent, with a few hills upon which it is impracticable to reach a grade of less than six per cent, it is questionable economy to reduce the grades that are already lower than the allowable maximum. It is especially unjustifiable to incur expense in reducing a grade from two per cent to one and one-half per cent on a road upon which there are also grades in excess of that amount. The undulating road is not uneconomical unless the grades are above the allowable maximum or are exceptionally long or the alignment follows short radius curves.
Safety Considerations.—On hills it is especially desirable to provide for safety and curves on hills are always more dangerous than on level sections of road. Therefore, it is desirable to provide as flat grades as possible at the curves and to cut away the berm at the side of the road so as to give a view ahead for about three hundred feet. Whether a road be level or on a hill, safety should always be considered and the most important safety precaution is to provide a clear view ahead for a sufficient distance to enable motor vehicle drivers to avoid accidents.
Guard Railing.—When a section of road is on an embankment, guard rails are provided at the top of the side slope to serve as warnings of danger, and to prevent vehicles from actually going over the embankment in case of skidding, or if for any reason the driver loses control. These are usually strongly built, but would hardly restrain a vehicle which struck at high speed. But they are adequate for the protection of a driver who uses reasonable care. A typical guard rail is shown in Fig. 9, but many other designs of similar nature are employed. At very dangerous turns a solid plank wall six or eight feet high is sometimes built of such substantial construction as to withstand the severest shock without being displaced.
Trees, shrubs and the berms at the side of the road in cuts are particularly likely to obstruct the view and should be cleared or cut back so far as is necessary to provide the proper sight distance.
Width of Roadway.—For roads carrying mixed traffic, 9 feet of width is needed for a single line of vehicles and 18 feet for 2 lines of vehicles. In accordance with the above, secondary roads, carrying perhaps 25 to 50 vehicles per day, may have an available traveled way 18 feet wide. Those more heavily traveled may require room for three vehicles to pass at any place and therefore have an available traveled way 30 feet wide. Greater width is seldom required on rural highways, and 20 feet is the prevailing width for main highways.
Cross Section.—The cross section of the road is designed to give the required width of traveled way, and, in addition, provide the drainage channels that may be needed. In regions of small rainfall the side ditches will be of small capacity or may be entirely omitted, but usually some ditch is provided. The transition from the traveled way to ditch should be a gradual slope so as to avoid the danger incident to abrupt change in the shape of the cross section. The depth of ditch may be varied without changing to width or slope of the traveled part of the road as shown in Fig. 10.
Control of Erosion.—The construction of a highway may be utilized to control general erosion to some extent, particularly when public highways exist every mile or two and are laid out on a gridiron system, as is the case in many of the prairie states. The streams cross the highways at frequent intervals and the culverts can be placed so as effectually to prevent an increase in depth of the stream. This will to some extent limit the erosion above the culvert and if such culverts are built every mile or two along the stream, considerable effect is produced.
Where small streams have their origin a short distance from a culvert under which they pass, it is sometimes advisable to provide tile for carrying the water under the road, instead of the culvert, and, by continuing the tile into the drainage area of the culvert, eliminate the flow of surface water and reclaim considerable areas of land.
Erosion in the ditches along a highway can be prevented by constructing weirs across the ditch at frequent intervals, thus effectually preventing an increase in the depth of the ditch.
Wherever water flows at a velocity sufficient to produce erosion or where the drainage channel changes abruptly from a higher to a lower level, paved gutters, tile or pipe channels should be employed to prevent erosion.
Private Entrances.—Entrance to private property along the highway is by means of driveways leading off the main road. These should always be provided for in the design so as to insure easy and convenient access to the property. The driveways will usually cross the side ditch along the road and culverts will be required to carry the water under the driveway. Driveways that cross a gutter by means of a pavement in the gutter are usually unsatisfactory, and to cross the gutter without providing a pavement is to insure stoppage of the flow at the crossing. The culvert at a driveway entrance must be large enough to take the ditch water readily or it will divert the water to the roadway itself. Generally end walls on such culverts are not required as in the case of culverts across a highway.
Aesthetics.—Much of the traffic on the public highways is for pleasure and relaxation and anything that tends to increase the attractiveness of the highways is to be encouraged. Usually the roadside is a mass of bloom in the fall, goldenrod, asters and other hardy annuals being especially beautiful. In some states wild roses and other low bushes are planted to serve the two-fold purpose of assisting to prevent erosion and to beautify the roadside. In humid areas trees of any considerable size shade the road surface and are a distinct disadvantage to roads surfaced with the less durable materials such as sand-clay or gravel. It is doubtful if the same is true of paved surfaces, but the trees should be far enough back from the traveled way to afford a clear view ahead. Shrubs are not objectionable from any view-point and are to be encouraged for their beauty, so long as they do not obstruct the view at turns.
CHAPTER V
EARTH ROADS
Highways constructed without the addition of surfacing material to the natural soil of the right-of-way are usually called earth roads. But if the natural soil exhibits peculiar characteristics or is of a distinct type, the road may be referred to by some distinctive name indicating that fact. Hence, roads are referred to as clay, gumbo, sandy or caliche roads as local custom may elect. In each case, however, the wearing surface consists of the natural soil, which may have been shaped and smoothed for traffic or may be in its natural state except for a trackway formed by the vehicles that have used it.
Variations in Soils.—The nature of the existing soil will obviously determine the serviceability and physical characteristics of the road surface it affords. That is to say that even under the most favorable conditions some earth roads will be much more serviceable than others, due to the better stability of the natural soil. Some soils are dense and somewhat tough when dry and therefore resist to a degree the tendency of vehicles to grind away the particles and dissipate them in the form of dust. Such soils retain a reasonably smooth trackway in dry weather even when subjected to considerable traffic. Other soils do not possess the inherent tenacity and stability to enable them to resist the action of wheels and consequently grind away rapidly. Roads on such soils become very dusty. These are the extremes and between them are many types of soils or mixtures of soils possessing varying degrees of stability, and, in consequence, differing rates of wear. Similarly the various soils exhibit different degrees of stability when wet.
It is to be expected that soils will differ with the geographical location, for it is well known that there is a great variation in soils in the various parts of the world. But wide differences are also encountered in the soil on roads very near each other and even on successive stretches of the same road. It is for this reason that earth roads often exhibit great differences in serviceability even in a restricted area.
Variation in Rainfall.—The stability of a soil and its ability to support the weight of vehicles varies greatly with the amount of water in the soil. A certain small amount of moisture in the soil is beneficial in that practically every soil compacts more readily when moist than when dry because the moisture aids in binding together the particles. But most soils also become unstable when the amount of water present is in excess of that small amount referred to above and the stability decreases very rapidly as the amount of water in the soil increases.
The serviceability of an earth road will change continually as the moisture content of the soil changes and consequently the general utility of the earth road system in any locality is dependent to a considerable extent upon the amount and seasonal distribution of precipitation. The methods of maintaining earth roads appropriate to any locality must of necessity be adapted to the climatic conditions, and the amount of work required to give the highest possible degree of serviceability will be exceedingly variable from season to season and from place to place. In regions of great humidity, earth roads may be expected to have a low average of serviceability, while in arid regions they may possess sufficient durability for a considerable volume of traffic. The design adopted for earth roads and the methods of maintenance followed should therefore be carefully evolved to meet the soil and climate conditions where the roads are located. These will differ greatly throughout a state or even a county.
Cross Sections.—The general principles of road design were set forth in Chapter IV. In Fig. 11 are shown typical cross sections for earth roads adapted to various conditions as indicated. It is not apparent that one form of ditch is particularly preferable to the other and since some engineers prefer the V section and others the trapezoidal section both are shown. It would appear that the V shaped ditch is somewhat the easier to construct with the blade grader while the trapezoidal is readily excavated with the slip or fresno scraper. The ditch capacity required and consequently the dimensions will depend upon the drainage requirements, as was pointed out in Chapter III.
EARTH ROADS IN REGIONS OF CONSIDERABLE RAINFALL
In the zones where the annual precipitation exceeds 30 inches distributed over several months, earth roads will be unserviceable for a considerable period each year unless they are constructed so as to minimize the effect of water. This is done by providing for the best possible drainage and by adopting a method of maintenance that will restore the surface to a smooth condition as quickly as possible after a period of rainy weather or after the "frost comes out" in the spring.
Before the construction of the desired cross section is undertaken, all of the grade reduction should be completed, except for minor cuts which can be handled with the elevating grader in the manner that will be described presently.
Where any considerable change in grade is to be effected, the earth can be moved in several ways and of these the most economical cannot be readily determined. Ordinarily a contractor or a county will use the equipment that happens to be at hand even though some other might be more advantageous.
Elevating Grader.—Where the topography is such as to permit its use, the elevating grader is employed in grade reduction to load the earth into dump wagons in which it is hauled to the fill or waste bank. The elevating grader consists essentially of a heavy shear plow or disc plow which loosens the earth and deposits it on a moving canvas apron. The apron carries the material up an incline and deposits it into a wagon which is driven along under the end of the apron. When the wagon is loaded, the grader is stopped while the loaded wagon is hauled out and an empty one drawn into position. The motive power for the elevating grader is either a tractor or five or six teams of mules. For many kinds of work, particularly where frequent turning is necessary or where the ground is yielding, mules are preferable to a tractor. The apron is operated by gearing from the rear wheels of the grader. Generally four mules are hitched to a pusher in the rear of the grader and six or eight in the lead. This method of grade reduction is particularly advantageous when the material must be hauled a distance of 500 yards or more, because wagon hauling in such cases is the most economical method to employ. A tractor may be used to draw the elevating grader and one having a commercial rating of 30 to 45 horsepower is required.
Maney Grader.—If the haul is long and the nature of the cut will not permit the use of the elevating grader because of excessive grades or lack of room for turning, a grader of the Maney type may be used. This consists of a scoop of about one cubic yard capacity, suspended from a four-wheel wagon gear. When loading, the scoop is let down and filled in the same manner as a two-wheeled scraper or "wheeler." The pull required to fill a Maney grader is so great that a tractor is ordinarily employed in place of a "snap" team. The tractor is hitched at the end of the tongue, without interfering with the team drawing the grader. One team readily handles the grader after it is loaded. For this service a tractor having a commercial rationing of 25 to 30 horsepower is required.
Wheel Scraper.—For moving earth for distances between 150 and 500 yards, the wheel scraper of a capacity of about 1-1/2 yards is quite generally employed. The soil must be loosened with a plow before it can conveniently be loaded into the wheeler and a heavy plow is ordinarily employed for that purpose. Two furrows with the plow will loosen a strip of earth about as wide as the scoop of the scraper and if more is loosened it will be packed down by the scrapers wheeling in place to load. A helper or "snap" team is employed to assist in loading, after which the wheel scraper is handled by one team.
Slip Scraper.—The slip scraper differs from the wheel scraper in that the scoop is not suspended from wheels but is dragged along the ground. It is drawn by one team and the capacity is two to five cubic feet, but the material spills out to some extent as the scraper is dragged along and the method is not suitable for long hauls, 100 feet being about the economical limit.
Fresno Scraper.—The Fresno scraper is one form of slip scraper requiring four horses or mules for efficient work. It differs somewhat from the ordinary slip scraper in shape and is of larger capacity, but is a drag type of scraper much favored in the western states.
SHAPING TO PROPER CROSS SECTION
If a road has been graded so that the profile is satisfactory or if the existing profile of the location is satisfactory, and the surface is to be shaped to a prescribed cross section, either the elevating grader or the blade grader may be employed.
Elevating Grader Work.—If the elevating grader is used in shaping the earth road, the apron will be lowered and the material will be excavated at the sides of the road and deposited on the middle portion. If slight changes in grade are desired, wagons will accompany the grader and catch under the apron at the high places and haul the material to the low places. After the earth has been deposited it must be worked over to secure the correct cross section and be made passable for vehicles. This requires that clods be broken, weeds and grass that are mixed with the earth be removed by harrowing and forking and that the surface be carefully smoothed with a blade grader. This latter operation will have to be repeated several times before a satisfactory surface is secured. But this miscellaneous work is highly important and under no circumstances ought to be neglected. Nothing so detracts from an otherwise creditable piece of work as failure to provide a smooth surface for the use of vehicles. It is especially uncomfortable for the users of a highway if sods and weeds in quantity are left in the road after it has been graded. The humus that will be left in the soil as the vegetable matter decays increases the porosity of the road surface making it more absorbent than soil without humus. This increases the susceptibility to softening from storm water or ground water.
The tractor can advantageously be used to draw the elevating grader on this class of work, but will be greatly handicapped if there are wet sections along the road, through which the tractor must be driven. In many cases its use is prohibited by such conditions and for all-round service of this character, mules are preferred for motive power.
Use of Blade Grader.—Heavy blade graders designed to be drawn by a tractor are suitable for shaping the earth road. Some of these have blades 12 feet long and excellent control for regulating the depth of cutting. Often two such graders are operated tandem. These machines have a device which permits the operator to steer the grader independently of the tractor. Thus the grader can be steered off to the side to cut out the ditches, while the tractor continues to travel on the firm part of the road. Earth moved with the blade grader is usually fairly free from large lumps and can readily be smoothed to a satisfactory surface for the use of traffic. The sods and weeds will be drawn into the road along with the earth just as they are when the elevating grader is employed. Precaution must therefore be taken to eliminate them before the vegetable matter decays, and to smooth the surface for the use of traffic.
Costs.—The cost of shaping an earth road in the manner described above will vary through rather wide limits because the nature and amount of work to be done varies so greatly. Some roads can be graded satisfactorily for $300.00 per mile, while others will cost $700.00. But $425.00 per mile may be taken as an average for blade or elevating grader work plus a moderate amount of grade reduction in the way of removing slight knolls. For the amount of grade reduction necessary in rolling country, followed by grader shaping, $1000.00 to $1800.00 per mile will be required. The method is not adapted to rolling country where the roads are undulating and require some grade reduction on every hill. For hilly roads one of the methods described for grade reduction will be required and the cost will obviously depend upon the amount of earth moved. Averages of cost figures mean nothing in such cases as the cost may reach $10,000.00 per mile, or may be as low as $2000.00 per mile.
Maintenance.—Regardless of the care with which an earth road has been graded, it will be yielding and will readily absorb water for a long time after the completion of the work. The condition of the surface will naturally deteriorate rapidly during the first season it is used unless the road receives the constant maintenance that is a prerequisite to satisfactory serviceability. The road drag is generally recommended for this purpose, and if a drag is properly used it will serve to restore the shape of the surface as fast as it is destroyed by traffic.
Good results with the drag depend upon choosing the proper time to drag and upon doing the work in the right way when using the drag. The best time to drag is as soon after a rain as the road has dried out enough to pack under traffic. If the work is done while the road is too wet, the first vehicles traveling the road after it has been dragged will make ruts and to a considerable extent offset the good done by the drag. If the road is too dry, the drag will not smooth the irregularities. A little observation will be required to determine the proper time for dragging on any particular soil, but usually after a rain or thaw there is a period lasting a day or two when conditions are about right.
The drag is used merely to restore the shape of the surface and to do so a small amount of material is drawn toward the middle of the road. But there must not be a ridge of loose material left in the middle after the work is completed. Some patrolmen start at one side of the road and gradually work across the road on successive trips, finally finishing up at the side opposite that at which the start was made. The next dragging should start on the opposite side from the first if that method is followed.
By shifting his weight on the drag, the operator can adjust the cutting edge so that very little loose material is moved crosswise of the road and that is the proper method to pursue. In that case no ridge will remain at the middle of the road. If a slight one is left it should be removed by a final trip with the drag.
In addition to the dragging, weeds must be cut along the road about twice a year, the ditches must be kept cleaned out and culverts open.
All of the maintenance for 10 miles of earth road can be accomplished by one man giving his entire time to the work, and that is the only method that has proven adequate to the problem.
EARTH ROADS IN ARID REGIONS
In areas where the rainfall is less than 18 inches per year, and especially where it is 10 inches or less, an entirely different road problem exists. The effect of precipitation is of significance primarily from the standpoint of erosion, and the design of cross section and ditches and the culvert provisions are entirely different from those necessary in humid regions.
Frequently the rainfall in semi-arid regions will be seasonal and provision must be made to care for a large volume of water during the rainy season, but, in general, road design is adapted to prevention of erosion rather than to elimination of ground water effects, or the softening effects of surface water. Generally the rainy period does not last long enough to warrant expensive construction to eliminate its general effects. In fact, the saturation of the soil is more likely to be a benefit than otherwise.
Earth roads are likely to be satisfactory except where the traffic is sufficient to grind the surface into dust to such an extent that an excessive dust layer is produced. In such locations the problem is one of providing a durable surface unaffected by long continued dry weather.
Grade reduction will have the same importance as in humid areas and will be carried out in the same way.
Maintenance will consist in repairing the damage from occasional floods and in removing or preventing accumulations of drifting sand or dust. Crude petroleum oils have been satisfactory for maintenance in such locations when used on stable soils.
Value of Earth Roads.—The serviceability of the earth road depends to a large extent upon the care exercised in its maintenance. The only part of earth road construction that is permanent is the grade reduction. The cross section that is so carefully shaped at considerable cost may flatten out in one or two years, especially if the road goes through unusually wet periods. Traffic will continually seek a new track during the period when the road is muddy and is as likely to cross the ditch to the sod near the fence as to use any other part of the road. Continual and persistent maintenance is therefore essential to even reasonable serviceability. At best the earth road will be a poor facility for a considerable period each year in the regions of year-around rainfall. In most localities, roads of distinctly minor importance are of necessity only earth roads and for the comparatively small territory they serve and the small amount of traffic, they probably serve the purpose. For roads of any importance in the humid areas of the United States, the earth road cannot carry satisfactorily the traffic of a prosperous and busy community.
CHAPTER VI
SAND-CLAY AND GRAVEL ROADS
In Chapter IV, mention was made of the variation in serviceability of road surfaces composed of the natural soil existing on the right-of-way of the road. It has been found that soils of a clayey nature in which there is a considerable percentage of sand usually afford a serviceable road surface for light or moderate traffic, especially in areas where climatic conditions are favorable. A study of these soils, together with the construction of experimental roads of various mixtures of sand and clay, has led to a fairly comprehensive understanding of the principles of construction and range of capacity of this type of road surface, which is known as the sand-clay road.
The sand-clay road surface consists of a natural or artificial mixture of sand and clay, in which the amount of clay is somewhat greater than sufficient to fill the voids in the dry sand. It may be assumed that the sand contains 40 per cent of voids and that at least 45 per cent of clay is required to fill the voids and bind the sand grains together, because the clay spreads the sand grains apart during the mixing, thus having the effect of increasing the voids. As a matter of experiment, it is found to be impractical to secure by available construction methods mixtures of sufficient uniformity to render it necessary to exercise great exactness in proportioning the components, but reasonable care in proportioning the materials is desirable.
Successful utilization of this type of surface requires considerable study of available materials and investigations of their behavior when combined. Extensive and exhaustive experiments have been conducted with sand-clay mixtures in various places where they are widely used for road surfaces and the following general principles have been deduced.
The Binder.—In the sand-clay road, stability is obtained by utilizing the bonding properties possessed to some degree by all soils. Naturally this characteristic may be expected to vary widely with the several types of soil. It is generally considered to be a common property of clay, but the term clay is a general one that is often applied to soils differing greatly in physical characteristics and the term therefore loses its significance in this connection. Those soils that are properly and technically called clay are decidedly sticky when wet and are the best materials for sand-clay construction. Of the clays, those that produce a tough sticky mud are best. This can be tested by mixing a small quantity into a stiff mud and molding it into a ball and immersing in water. If the ball retains its shape for some little time, it is likely to prove a very satisfactory binder, but, if it becomes plastic and loses its shape, it will be an inferior binder, as a general rule. The ball clay, as the former is called, may be of any color common to soils, not necessarily yellow or reddish as is sometimes supposed. Likewise, balls of mixtures containing varying percentages of sand and the binder to be used may be made up and immersed in water. The mixture that holds its shape longest is of course the best combination of the materials and indicates the mixture to use in the construction.
An ideal, or even a fairly satisfactory soil for a binder may not exist in the vicinity of a proposed improvement, and consequently an inferior binder is frequently the only material available.
Sometimes deposits of clay or gravel contain a considerable percentage of gypsum which serves as a binder and is particularly effective when used in combination with clay and sand or gravel.
In many places a soil of the type used for adobe and called "caliche" may be found and this is an excellent binder for sand or gravel.
Top-Soil or Natural Mixtures.—Deposits consisting of a natural mixture of sand and clay in which the ingredients happen to exist in about the correct relative proportions for sand-clay road surfaces are found in many localities. These mixtures are commonly referred to as top-soil. If the deposits are somewhat deficient either in sand or clay, they can be utilized if the proper corrections in the proportions are made during construction. Very satisfactory road surfaces are sometimes constructed with mixtures that appear to be far from ideal in composition, but experience and frequent trials are needed to determine the best way in which to handle these mixtures.
Sand-Clay Surfaces on Sandy Roads.—Sand-clay surfaces may be constructed on naturally sandy roads either by adding clay and mixing it with the sand to secure the desired composition, or a layer of a natural sand-clay mixture, caliche or sand-clay-gypsum may be placed on top of the sand.
The most widely used method is to mix clay or other binder with the sand. Since there is no need to provide for ditches to carry storm water on a deep sand soil, the sand is graded off nearly flat across the road and no ditches are provided. The clay is dumped on the road in a layer about 8 inches thick and is then mixed into the sand. It is desired to mix enough sand with the clay to produce a mixture composed of approximately 1/3 clay and 2/3 sand. The mixing is accomplished in various ways, the most common being to use a heavy plow at first and to follow this with a heavy disc harrow. The mixing is a tedious and disagreeable process, but its thorough accomplishment is indispensable. The mixing is most readily done when the materials are saturated with water and in practice it is customary to depend upon rain for the water, although in the final stages water may be hauled and sprinkled on the road to facilitate final completion of the mixing. After the mixing has been completed, the surface is smoothed with the blade grader and is kept smooth until it dries out. Repeated dragging will be required, during the first year especially, and to some extent each year in order to keep the surface smooth, but the dragging can be successfully accomplished only when the road is wet.
In regions where several months of continued hot, dry weather is to be expected each year, the sand-clay mixture is likely to break through unless it is of considerable thickness and generally the surface layer is made much thicker than for regions where the annual rainfall is fairly well distributed. This is especially necessary when the binder is of inferior quality. It is not uncommon in such cases to make the sand-clay surface as much as two feet thick.
As the mixing progresses it may appear that patches here and there are deficient in either clay or sand and the mixture in these places is corrected by the addition of a little sand or clay as may be required.
If the top-soil is used it is deposited on the sand in the required quantity and is remixed in place to insure uniformity. If either sand or clay is needed to give a satisfactory mixture, the proper material is added and mixed in as the work progresses. The surface is finally smoothed by means of the grader and drag.
Sand-Clay on Clay or Loam.—If the existing road is of clay or loam, ample drainage will be required as discussed in Chapter IV. The surface may be constructed of a natural sand-clay mixture or of a sand mixed with the natural soil. If the former, the surface of the existing road is prepared by grading so as to insure good drainage and the natural mixture is then deposited and the surface completed as described in the preceding section.
If the surface is formed by mixing sand with the existing soil, the sands may be deposited in a layer about six inches thick which will gradually mix with the soil as the road is used. A second application of sand may follow in a year or two if it is needed. Such a road surface will lack uniformity of composition and it seems preferable to mix the sand with the soil by plowing and discing as previously described.
Characteristics.—Sand-clay road surfaces do not have sufficient durability for heavily traveled highways, but will be satisfactory for a moderate amount of traffic. These surfaces have maximum serviceability when moist, not wet, and consequently are not as durable in dry climates as in humid areas. They are likely to become sticky and unstable in continued wet weather and to become friable and wear into chuck holes in long continued dry weather. At their best, they are dustless, somewhat resilient and of low tractive resistance.
GRAVEL ROAD SURFACES
Natural Gravel.—Gravel is the name given to a material consisting of a mixture of more or less rounded stones, sand and earthy material, which is found in natural deposits. These deposits exist in almost every part of North America, being especially numerous in the glaciated areas, but by no means confined to them. Gravel deposits consist of pieces of rock varying in size from those of a cubic yard or more in volume to the finest stone dust, but with pieces ranging in size from that which will pass a 3-inch ring down to fine sand predominating. The larger pieces are usually more or less rounded and the finer particles may be rounded or may be angular. Many varieties of rocks are to be found among the gravel pebbles, but the rocks of igneous origin and possessing a considerable degree of hardness generally predominate. Intermixed with the pieces of rock there is likely to be clay or other soil, the quantity varying greatly in different deposits and even in various places in the same deposits.
Often there are found deposits of material which are by the layman termed gravel, which are really clayey sand or sand containing a few pebbles, but which are of value to the road builder for the sand clay type of surfacing. The term gravel is exceedingly general and unless specifically defined, gives little indication of the exact nature of to which it is applied.
TABLE 7
SHOWING CEMENTING PROPERTIES OF SEVERAL SAMPLES OF GRAVEL
-+ Cementing Value Per Cent Clay by + -+ Weight As Received Washed -+ -+ 4.4 276 43 6.4 105 285 5.1 241 70 14.5 500 279 8.5 500 112 10.1 300 267 14.8 500 107 7.5 184 198 16.5 500 428 2.0 185 239 1.5 500 500 4.5 212 204 2.5 116 363 -+ -+
The value of any gravel for road surfacing depends upon the degree to which it possesses the properties of an ideal gravel for road surfacing. Ideal gravel is seldom encountered, but a consideration of its characteristics serves to establish a measure by which to estimate the probable value of any deposit.
The Ideal Road Gravel.—The ideal road gravel is a mixture of pebbles, sand and earthy material, the pieces varying from coarse to fine in such a manner that when the gravel is compacted into a road surface the spaces between the larger pebbles are filled with the finer material. The pebbles are of a variety of rock that is highly resistant to wear so that the road surface made from the gravel will have the quality of durability. The gravel possesses good cementing properties, insuring that the pieces will hold together in the road surface. The cementing property may be due to the rock powder in the deposit or to earthy material mixed with the rock particles, or to both. Table 7 shows the results of a number of tests made upon gravels and indicates that the cementing property of the gravel does not always depend upon the clay content.
Permissible Size of Pebbles.—The larger pebbles in the gravel are less likely to crush under loads than smaller pebbles of the same sort of rock, but if the rock is of some of the tougher varieties such as trap, there is very little likelihood of even the smaller pebbles crushing. If the pebbles are of rock of medium toughness, the smaller pebbles might be crushed under the heavier loads. It is the usual practice to permit gravel to be used for the foundation course in which the pebbles are as large as will pass a 3-1/2-inch circular screen opening, and for the wearing course, as large as will pass a 2-1/2-inch circular screen opening. If larger pebbles are allowed in the wearing course, the surface is certain to become rough after a time. If the gravel is to be placed in a single course as is a very common practice, then the maximum size should not exceed that which will pass a 2-1/2-inch circular screen opening.
The Wisconsin Highway Commission has constructed a very large mileage of excellent gravel roads and the sizes specified for their roads are as follows:
"Bottom Course Gravel.—Bottom course shall consist of a mixture of gravel, sand and clay with the proportions and various sizes as follows:
"All to pass a two-inch screen and to have at least sixty and not more than seventy-five per cent retained on a quarter-inch screen; at least twenty-five and not more than seventy-five per cent of the total coarse aggregate to be retained on a one-inch screen; at least sixty-five and not more than eighty-five per cent of the total fine aggregate to be retained on a two hundred-mesh sieve."
"Top Course Gravel.—Top course shall consist of a mixture of gravel, sand and clay with the proportions of the various sizes as follows:
"All to pass a one-inch screen and to have at least fifty and not more than seventy-five per cent retained on a quarter-inch screen; at least twenty-five and not more than seventy-five per cent of the total coarse aggregate (material over one-fourth inch in size) to be retained on a one-half-inch screen; at least sixty-five and not more than eighty-five per cent of the total fine aggregate (material under one-fourth inch in size) to be retained on a two hundred-mesh sieve."
"Screened Gravel and Sand Mixtures.—Where it is impossible to obtain run of bank gravel containing the necessary binder in its natural state, screened gravel shall be used and the necessary sand and clay binder added as directed by the engineer. Gravel and sand shall be delivered on the work separately. Clay binder shall be obtained from approved pits and added as directed by the engineer."
"Run of Bank Gravel.—When run of bank gravel is permitted either for one course or two course work, the size shall not exceed that specified for bottom or top course. If necessary, the contractor shall pass all the material through a two-inch screen for the bottom course, and through a one-inch screen for the top course. When the work consists of only one course, the material shall be of the sizes as specified for the top course. The necessary binder shall be contained in the material in its natural state, excepting that a small percentage of clay binder may be added as directed by the Engineer."
Wearing Properties.—A certain amount of grinding action takes place on the road surface under the direct action of wheels, especially those with steel tires. Where rubber tired traffic predominates, this action is much less severe than where steel tired vehicles predominate, but the tendency exists on all roads. In addition, there is distortion of the layer of gravel under heavy loads which causes the pieces of stone in the surface to rub against each other and to wear away slowly.
The gravel road in the very best condition is slightly uneven but there is comparatively little jar imparted to vehicles, and, consequently, little impact on the surface. When somewhat worn, the impact becomes a factor of some importance and the pounding of vehicles has a very destructive action on the surface. Soft pebbles will be reduced to dust in a comparatively short time.
The degree to which any gravel resists the destructive action of traffic depends upon the varieties of rock represented by the pebbles in the gravel. If the pebbles are mostly from rocks of good wearing properties, that quality will be imparted to the road surface. If mostly from rocks of little durability, the same characteristic will be imparted to the road surface. A very good general notion of the probable durability of gravel can therefore be obtained by a careful visual examination of the material and classification of the rock varieties represented by the pebbles.
Utilizing Natural Gravels.—Gravel road construction is advantageous only when it can be accomplished at low first cost. This usually presupposes a local supply of gravel that can be utilized, or at any rate a supply that need not be shipped a long distance. In the nature of things, such deposits are likely to be deficient in some of the desirable characteristics, and may be deficient in most of them. By various means, the defects in the materials can be partially corrected while constructing the road.
If the gravel deposit consists of layers of varying composition as regards size and clay content, the material may be loosened from the exposed face and allowed to fall to the bottom of the pit thereby becoming mixed to a sufficient extent to produce a reasonably uniform product. If deficient in clay, it often proves feasible to add a small part of the clay over-burden, thereby insuring enough binder. Sometimes adjoining deposits will consist one of relatively fine material, the other of relatively coarse. These may be mixed on the work by first placing the coarse material in a layer about 5 inches thick and adding the finer material in a similar layer. The two will mix very rapidly during the operations of spreading and shaping.
When deposits contain pebbles larger than will pass a 3-1/2-inch ring, these larger stones will prove to be undesirable if placed on the road, as they are almost sure to work to the surface of the gravel layer and become a source of annoyance to the users of the road. Oversize stone can be removed while loading the gravel or while spreading it, if care is exercised and not too large a proportion is oversize. It is preferable however to remove the oversize by means of screens at the pit. Usually on large jobs the oversize is crushed and mixed with the supply so as to utilize what is really the best part of the material.
Gravels deficient in bonding material are often encountered in deposits where there is insufficient overburden to give enough additional binder or where the overburden is of a material unsuitable for binder. Such materials may be utilized by adding binder in the form of clay after the gravel has been placed on the road.
Almost any gravel deposit can be utilized in some way if the material is of a durable nature, regardless of other characteristics. The serviceability of a gravel road will depend largely on how nearly the gravel approaches the ideal, but variations in the manipulations will do much to overcome deficiencies in materials.
Thickness of Layer.—The thickness of the layer of gravel required depends both upon the type of soil upon which it is placed and the nature of the traffic to which the road will be subjected. Gravel surfaces should not ordinarily be constructed on highways carrying heavy truck traffic, but if gross loads of three or four tons are the heaviest anticipated, the gravel will be reasonably stable. On such roads, a layer of well compacted gravel ten inches thick will support the loads if a well drained earth foundation is provided. If but little truck traffic is anticipated and loads up to three tons on steel tires are the average, a layer 8 inches thick will be sufficient. In dry climates, a layer six inches thick will be adequate if it can be kept from raveling.
On secondary roads, carrying principally farm-to-market traffic, and not a great volume of that, the above thicknesses may be reduced about one-fourth.
The exact thickness needed for any particular road is a matter for special study on account of the variations in the gravels and in the supporting power of the soil upon which they are placed.
PLACING GRAVEL
Preparation of the Road.—The roadway that is to be surfaced with gravel is first brought to the desired grade and cross section. It would be advantageous if this could be done a year before the gravel is placed so that no settlement of the earth foundation would occur after the gravel surface is completed. But if that is impractical, the grading may be done just prior to placing the gravel, providing appropriate methods are adopted for securing compacted fills.
Trench Method.—Two distinct methods of placing the gravel are in general use, known as the trench method and the surface or feather edge method respectively. The method to adopt for any particular road will depend largely on certain conditions that will be explained later.
In the trench method, a trench of the proper width and depth for receiving the gravel is excavated in the earth road surface and the gravel is placed therein.
The trench is formed by plowing a few furrows and scraping out the loosened earth with a blade grader. The loose material is generally moved out laterally to build up earth berms or "shoulders" alongside the gravel. Into this trench the gravel is dumped in the proper quantity to give the required thickness after being compacted.
The greatest care must be exercised in spreading the gravel to eliminate unevenness where the loads were deposited. An ordinary blade grader is one of the best and most economical implements to use for spreading the gravel. When the gravel has been deposited in the trench for a distance of a thousand feet or more, the spreading is accomplished by dragging the surface repeatedly with the blade grader, the work being continued until all waviness disappears. The gravel is then thoroughly and repeatedly harrowed with a heavy stiff tooth harrow to mix thoroughly the fine and coarse gravel so as to produce as nearly a uniform mixture as may be. The gravel is then finally smoothed with the blade grader.
The gravel may be compacted by rolling or may be allowed to pack from the action of traffic. The former is greatly to be preferred where practicable. The rolling is performed with a three-wheeled self-propelled roller weighing about 8 tons and must be done while the gravel is wet. Generally a sprinkling wagon is used to wet down the gravel, but advantage is always taken of rains to facilitate the work. The gravel must be spread in layers not over 5 or 6 inches thick to get the desired results, which means that for an ordinary gravel road about 10 inches thick, the gravel will be placed in two layers of about equal thickness, each of which will be rolled.
The gravel will compact slowly even if it is not rolled, but generally does not become stable until the material is thoroughly soaked by rains. Then it will begin to pack, but will become badly rutted and uneven during the process. During this period the surface must be kept smooth by means of the blade grader. The drag does not suffice for this purpose, tending to accentuate the unevenness rather than to correct it.
If gravel is placed in a trench in dense soil and rainy weather ensues, sufficient water will be held in the trench to cause unevenness from foundation settlement and the gravel will become mixed with the soil to some extent and be thereby wasted. Trenches cut from the road bed upon which the gravel is placed, to the side ditches, will relieve this condition by affording an outlet for the surplus water. Nevertheless some difficulty may be expected if the trench method is used and wet weather prevails. If it is possible to close the road against traffic until the road is dry the method is applicable. Moreover, in long-continued dry weather, the dispersion and loss of considerable gravel from the action of automobile traffic is avoided because the gravel is held between substantial earth berms and the gravel will pack better and hold its shape longer when constructed by the trench method than otherwise.
Surface Method.—The surface method is one in which the gravel is placed on the graded earth road surface without earth shoulders to hold the gravel in place. It is also sometimes called the feather-edge method. Except for the manner of placing as just mentioned, the several operations are conducted in the same general manner as for the trench method. The gravel does not compact as quickly as in the trench method and a considerable loss of material is likely to result from the effect of automobile traffic while the gravel is loose. But it has the advantage of being free from difficulties in wet weather and in some locations is therefore preferable to the trench method. It is particularly applicable to those projects on which the placing of gravel continues throughout the winter, the gravel being dumped and spread, to be finally smoothed and finished in the early summer.
Bonding.—Where gravels deficient in binder are utilized, clay for binder is sometimes added as the gravel is placed on the road. This may be done by spreading the clay on top of the lower course of gravel, placing the upper layer and sprinkling and rolling until the clay squeezes up through the surface layer. It may also be accomplished by spreading dry clay on the upper course before it is harrowed and then harrowing to mix it with the gravel. Both methods are practiced, but the former is believed to be preferable. A third method is to separate the sand and pebbles and to mix the clay binder with the sand and then spread the sand on top of the pebbles and mix by harrowing.
Maintenance.—Gravel surfaces require careful maintenance, especially during the first season the road is used. The gravel will compact slowly and during the process will be rutted and otherwise disturbed by traffic. It is important during this period to restore the shape once a week or at least twice a month. The light blade grader is usually employed for the purpose so long as the gravel is somewhat loose. Later a drag of the type known as the planer will prove to be the most effective. Figure 16 shows a type of drag that is very satisfactory for use on gravel roads.
CHAPTER VII
BROKEN STONE ROAD SURFACES
The broken stone road surface, or macadam road as it is usually termed, consists of a layer of broken stone, bonded or cemented together by means of stone dust and water. The surface may or may not be coated with some bituminous material.
Design.—It has been an accepted assumption that the macadam road surface is somewhat more stable than the gravel road surface of equal thickness, and since this is probably the consensus of opinion of engineers familiar with both types, it may be accepted until experimental data are available on the subject.
The thickness of the layer of macadam required for a road will depend upon the same factors that were considered in connection with the thickness of the gravel surface, i.e., kind of stone used, character of earth foundation and nature of the traffic.
The standard macadam surface where good earth foundation is to be had and where the loads do not exceed about four tons has for years been eight inches thick. For heavier loads or inferior foundation, a somewhat greater thickness would be employed, but the best practice would probably provide a foundation course of the Telford type for doubtful foundation conditions, especially for the extremely uncertain cases. For soils of very good supporting strength such as very sandy loam or deep sand or for arid regions where stable foundation is always assured the thickness of the macadam might be reduced to six inches. It should be borne in mind that the broken stone road is not adapted to the traffic carried by trunk line highways in populous districts, but is rather a type permissible on secondary roads and usually adequate for local roads. It should never be employed for roads carrying any considerable volume of passenger automobile traffic or motor truck traffic. If surfaced with a bituminous material it will carry up to 1200 passenger automobiles per day, but not to exceed fifty trucks.
Properties of the Stone.—The stone employed for the broken stone road should possess the qualities of hardness and toughness and should be capable of resisting abrasion sufficiently well to have reasonable life under the traffic to which it is subjected. Since the traffic may vary from very light on some roads to far beyond the limit of the economical capacity of this type of pavement on others, it follows that any particular deposit of stone might be durable enough for some roads, while for others it might be entirely inadequate. As a general rule it has been found that stone that wears away at a moderate rate will, when used for water-bound macadam surface, result in a smoother trackway than one that will wear very slowly. It is not therefore altogether certain that the most durable stone to be had should be selected for a particular road. This is especially true now that the water-bound macadam surface has been largely superseded for trunk line highways and other heavily traveled roads, and is employed in locations where service conditions are not severe.
The stone employed for the water-bound macadam surface must possess good cementing properties, because the surface depends for stability primarily upon the bonding action of the dust from the broken stone. This is in contrast to the gravel road, where little dependence is placed upon the bonding effect of the rock dust. In preparing the stone for macadam surfaces, the ledge rock is crushed and screened, and in that way a supply of the finer particles, which are a part of the output of the crusher, is obtained for use in bonding the surface. This finely broken material, usually called screenings, is essential to the construction of the water-bound type of surface. Rocks vary considerably in the cementing properties of the dust, but usually the rocks classed as "trap," such as andesite, gabbro and rhyolite, and schist and basalt possess good cementing properties. Limestones usually possess good cementing properties, but some of the dolomitic limestones are of low cementing value. Quartz, sandstone and the granites are of low cementing value.
Kinds of Rocks Used for Macadam.—Limestone and chert are the two sedimentary rocks, employed most extensively for broken stone roads. These rocks are found in widely distributed areas and vary in physical characteristics from very soft material of no use to the road builder to materials possessing considerable durability. It is desirable to carefully test out the deposits of these materials before using to ascertain the probable value of the rock, for the construction of the road surface.
Of the igneous rocks, those classed as trap are best known to the road builder and many of the deposits of trap rock afford an excellent material for broken stone roads where the severest conditions of traffic are encountered. The trap rocks are tough and durable and generally possess excellent cementing properties.
Granite and sandstone are seldom used for water-bound macadam as they possess poor cementing properties and a binder of some kind must be added to cement the pieces together. For this purpose clay or the screenings from some other variety of stone may be utilized.
Some other materials are occasionally employed for the construction of macadam surfaces. Of these, oyster or marine shells, burnt shale, and slag are most common.
Shells and slag are of rather low durability but possess good cementing properties. Shale is a makeshift suitable only for very light traffic roads.
Sizes of Stone.—The stone for the wearing course of a macadam road should be as large as practicable, because the larger the pieces the more durable the surface. If the individual stones are too large it is difficult to secure a smooth surface, and large stones will be readily loosened by tipping as the wheels roll over them. These considerations limit the size to a maximum of that which will pass a 2-1/2-inch screen. Stone of excellent wearing qualities may be somewhat smaller, but never less than that which will just pass a 1-1/2-inch screen.
For the lower course, the size is not particularly important except where the earth foundation is such as to require special construction. It is not uncommon to use the same size of stone for both upper and lower course and yet in many instances stone up to that which will just pass a 3-1/2-inch screen is used for the lower course. Stone much smaller in size may also be used successfully, but if the stone is broken to a smaller size than is required, unnecessary expense is incurred.
The bonding material is the finer portion of the product of the crusher, which is called screenings. This material may be so finely crushed as to pass a one-fourth inch screen, or may be so coarse as to just pass a one-half inch screen, but in any case must contain all of the dust and fine material produced by the crusher.
Where the soil and drainage conditions demand an especially stable foundation course, the Telford type is used. The Telford foundation consists of a layer of stones of various dimensions that can be laid so as to give a thickness of 8 inches. These large stones are placed by hand and therefore the size requirements are not rigid. Stones having one dimension about 8 inches and the others not over 10 or 12 inches are satisfactory.
Earth Work.—A thoroughly drained and stable earth foundation is essential to success with the macadam type of surface. Before placing the stone, the road must be shaped to the proper cross section and all grade reduction work completed. Preferably heavy fills should have a year to settle before the macadam surface is placed. Side ditches, necessary culverts and tile drains should be constructed as required for drainage. The earth work is often carried out in connection with the construction of the macadam surface, being completed just ahead of the surfacing. In that case, the fills must be carefully rolled as they are placed. The road bed may be shaped in connection with the other earthwork. If the road has been brought to a satisfactory grade some time prior to placing the macadam, the road bed for the broken stone will be prepared as needed for placing the stone.
Foundation for the Macadam.—Macadam surfaces are quite generally placed in a trench as described in the trench method for placing gravel. It is an almost universal practice to compact the layer of stone by rolling with an 8- or 10-ton power roller, and if the stone is not held between substantial earth berms or shoulders, the rolling merely serves to spread the stone out over the road bed instead of compacting it. If an attempt is made to roll broken stone which has been placed on a yielding foundation, no benefit results, but on the contrary the stone is likely to be forced down into the soil. To insure that the layer of broken stone can be compacted by rolling, it is first necessary to roll the earth foundation until it becomes hard and unyielding. If soft or yielding places appear during the rolling these should be corrected by tile drains or by removing the earth from the spongy place and back-filling with material that will compact when rolled.
It is not always easy to determine why these soft places exist in what appears to be a well drained roadway, especially since they are as likely to be found on fills as anywhere else. Apparently they are due to local pockets of porous soil held by denser soil so that the water does not readily drain away. It is usually true that such places are observed during the season of frequent precipitation more often than during other seasons of the year.
In dry climates, the difficulties of securing suitable foundations for the broken stone road are largely eliminated, but it may be observed that this type of surface is not suitable for such climates unless some sort of bituminous binder is employed to hold the stones in place. The cementing power of the stone dust is inadequate when the surface is continually dry.
Telford Foundation.—When the Telford type of foundation is employed, the earth subgrade is prepared and then the Telford stone placed carefully by hand. The spaces between the large stones are filled with the spalls broken from the larger stones in fitting them in place. When completed the base is rolled with a heavy roller to secure a firm unyielding layer. The thickness is generally about eight inches. Any fairly sound stone may be used for the Telford base.
Placing the Broken Stone.—It has been found impracticable properly to roll a greater thickness than about 5 or 6 inches of loose stone, therefore, the stone for the macadam surface is usually placed in two layers, the first or lower layer being rolled before the next layer is placed. The stone is hauled in dump wagons, trucks or dump cars, dumped on the road bed and spread by hand rakes or by means of a blade grader and is then rolled. To insure the proper thickness the loads are accurately spaced to spread to the proper thickness.
Rolling.—A three-wheeled or "macadam" type of roller, of the self-propelled type, is best for compacting the broken stone road. The weight varies from eight to fifteen tons, but for most conditions the ten or twelve ton size seems to be preferable. On Telford base construction, a heavier machine is desirable and for very hard stone it may be successfully employed.
The first trip with the roller is made along the edge of the stone and each successive trip is made a little nearer the middle until finally one half of the strip of stone has been rolled. The roller is then taken to the opposite side of the roadway and the operation repeated on the other half. The rolling is continued until the stone is thoroughly compacted, which is evidenced by the fact that the roller makes but a slight track in the surface.
The second layer of stone is then placed and rolled in the same manner as the first.
Spreading Screenings.—After the upper course has been rolled, the screenings are spread on it from piles alongside the road, enough being used to fill the voids in the layer of stone and furnish a slight excess. As the screenings are spread they are rolled to work them into the voids. When these are filled, the surface is sprinkled thoroughly by means of an ordinary street sprinkling cart and again rolled. In this way the dust and water are mixed into a mortar which fills the crevices between the stones. This mortar hardens in a few days, giving a bond that is weak, but sufficient for the purpose if the traffic is not too heavy. A broken stone road finished in this way is called a water-bound macadam, and is ready for traffic in three or four days after completion.
Bituminous Surfaces.—On account of the inadequacy of the water-bound macadam when subjected to motor traffic and to obviate the tendency of broken stone surfaces to loosen in dry weather, there has been developed a method of covering the surface with a bituminous material such as tar or asphalt. This will be described in detail in a later chapter.
Maintenance.—Even under favorable conditions as regards kind and amount of traffic the macadam road requires constant maintenance. The first effect of traffic will be to brush away the fine materials used for bonding the surface, thus exposing the larger stones in such a way that they are rather easily loosened and removed from the surface by wheels and the hoofs of animals. This finer material must be replaced as fast as it is removed so as to protect the surface. Either stone dust or clayey sand may be used, but clay if used alone is likely to be sticky when wet and prove to be worse than the condition it was expected to correct. In time, ruts and depressions will appear, either as the gradual effect of wear, which will inevitably effect some portions of the surface more than others, or on account of subsidence of the foundation. Uneven places are repaired by first loosening the stone, then restoring the cross section by adding new material and tamping or rolling it in place.
If a bituminous coating has been applied, it will eventually peel off in places and these places must be recoated as soon as practicable.
Eventually the surface will be worn to such an extent that an entirely new wearing surface must be added. This is done by loosening the entire surface to a depth of 3 or 4 inches and then adding a new layer of broken stone. The loosening is sometimes accomplished by means of heavy spikes inserted in the roller wheels, and at others by means of a special tool known as a scarifier.
The new surface is placed and rolled in precisely the same manner as the wearing surface of the original construction, but the layer may not be as thick as the original wearing course. A new course will not bond to the old surface unless the old macadam has been thoroughly broken up first.
Characteristics.—The water-bound macadam is a dusty, somewhat rough surface of low durability for rubber tired vehicles. It has long been the standard rural highway for steel tired vehicles, but cannot carry any considerable amount of motor traffic. It is easily repaired. When finished with a bituminous surface its durability is greatly increased and the dust is eliminated. It does not seem to be sufficiently rigid for truck traffic, unless placed on exceptionally good foundation. |
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