OCR Text |
Show FIRM FOUNDATION FOR ROADS Aim to Distribute Pressure Due to Wheels Concentrated on Surface Sur-face Look to Future. (Prepared by the United States Department Depart-ment of Agriculture.) The purposes of a foundation are to aid in distributing the pressure clue to wheel loads concentrated on the road surface, so that the intensity of pressure pres-sure on the subgrade will not exceed a safe unit-bearing power, ant) also to prevent, if possible, the crust of the road from being disturbed by the upward up-ward pressure of the subgrade due to the action of frost, ground water, or other disturbing Influence. In the case of a pavement consisting of comparatively compara-tively small blocks, such as vitrified brick, bonded to each other very slightly slight-ly or not at all, an entire wheel load might be concentrated on only a few ' Sand-Clay Road. square inches of foundation, in which case it "would be necessary for the foundation to take care'rf practically the entire distribution of the load. On the other hand, a monolithic pavement, pave-ment, in which the individual units are well bonded together, ; might distribute the wheel loads to obviate all necessity of further distribution by the foundation, founda-tion, and If adequate drainage were provided and there were no danger of heaving of subgrades, the foundation might be omitted entirely. The heaviest vehicle that ordinarily has passed over a public highway heretofore is the 15-ton traction engine. en-gine. Such engines are so designed that he rear axle carries about two-thirds two-thirds of the total weight, which arrangement ar-rangement gives a concentrated pressure pres-sure on the road surface of about five tons under each rear wheel. The use of much heavier equipment is in sight, however, and unless some regulations be passed to prohibit it, the occurrence occur-rence of ten-ton, or even heavier motortrucks, mo-tortrucks, may become common on highways adjacent to large cities or between large centers that are only a few miles apart. In assuming the maximum wheel load for any particular road a reasonable reason-able allowance should be made for future fu-ture increase. Since motortrucks have come into use there 'has been a constant con-stant tendency to increase both' their rates of speed and the loads they carry, and it is probable that this tendency will continue. Furthermore, many roads not now subjected to motortruck motor-truck traffic will attract such traffic after they are improved, and this possibility pos-sibility always should be considered. Increase In the volume of traffic also may be an Important factor . . |