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.
[Ill.u.s.tration: Fig. 16.--Road Planer]
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 a.s.sumption 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 a.s.sured 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 pa.s.senger automobile traffic or motor truck traffic. If surfaced with a bituminous material it will carry up to 1200 pa.s.senger 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 cla.s.sed 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 cla.s.sed 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 sh.e.l.ls, burnt shale, and slag are most common.
Sh.e.l.ls 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 pa.s.s a 2-1/2-inch screen. Stone of excellent wearing qualities may be somewhat smaller, but never less than that which will just pa.s.s 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 pa.s.s 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 pa.s.s a one-fourth inch screen, or may be so coa.r.s.e as to just pa.s.s 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.
[Ill.u.s.tration: Fig. 17.--Cross Section for Macadam]
=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 s.p.a.ces 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 s.p.a.ced 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.
CHAPTER VIII
CEMENT CONCRETE ROADS
The cement concrete road is one of the later developments in highway construction, but the type has had sufficient use to show that it is one of the satisfactory types for heavy mixed traffic, and, where the proper materials are available, it is one of the economical types of construction.