s of a layer of broken stone, bonded or cemented together by means of stone dus

he gravel road surface of equal thickness, and since this is probably the consensus of opinion of engin

factors that were considered in connection with the thickness of the gravel surfac

ns, 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 highway

city 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

cadam 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

y distributed areas and vary in physical characteristics from very soft material of no use to the road builder to materials possessing considerable durability. It i

f trap rock afford an excellent material for broken stone roads where the severest conditions of traffic

ting properties and a binder of some kind must be added to cement the pieces together. F

construction of macadam surfaces. Of these, oyster or

possess good cementing properties. Shale is a mak

tones 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 siz

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?-inch screen is used for the lower

terial may be so finely crushed as to pass a one-fourth inch screen, or may be so coarse as to just pass a o

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 the

ore 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 f

s, 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

e 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 dra

t may be observed that this type of surface is not suitable for such climates unless some sort of bituminous binder is emp

oss Section

he 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 wi

e 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 ro

road. The weight varies from eight to fifteen tons, but for most conditions the ten or twelve ton size seems to be prefera

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. Th

then placed and rolled in t

o 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 m

obviate the tendency of broken stone surfaces to loosen in dry weather, there has been developed a method of coverin

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 de

will eventually peel off in places and these

ning the entire surface to a depth of 3 or 4 inches and then adding a new layer of broken stone. The loosening is sometimes acc

original construction, but the layer may not be as thick as the original wearing course. A new c

way for steel tired vehicles, but cannot carry any considerable amount of motor traffic. It is easily repaired. When finished with a bituminous surface its durab