Engineering and Contracting . subgrade can be de-pended on 6 in. of metalling is sufficient and over sand4 in. of metalling has often given satisfaction. On theother hand places aie occasionally found where even 10m. of metalling is none too much. In every case the de-termining factor is the sub-grade, and not the metalling. It has been said, and this point should be remembered,that the roads which have been built in the PhilippineIslands are can roads. Heavy truck traffic would re-quire very much heavier metalling or the construction ofsubgrades that could be depended on to support moreweight
Engineering and Contracting . subgrade can be de-pended on 6 in. of metalling is sufficient and over sand4 in. of metalling has often given satisfaction. On theother hand places aie occasionally found where even 10m. of metalling is none too much. In every case the de-termining factor is the sub-grade, and not the metalling. It has been said, and this point should be remembered,that the roads which have been built in the PhilippineIslands are can roads. Heavy truck traffic would re-quire very much heavier metalling or the construction ofsubgrades that could be depended on to support moreweight that a wet Philippine subgrade will support whenprotected by an ordinary thickness of metalling. Thussuppose 5-ton trucks (truck and load 8 tons) had to beprovided for under the above assumptions. A 5-ton truckunder a very slight overload will throw 3 tons of weightonto each of the rear wheels. Running at a speed of 10miles an hour, not an uncommon speed, and equippedwith steel tires, the loads delivered to the metalling by. Fig. -IVIanlla North Road at Km. 50. Low subgrades are used even in the rice region. During at leastfour months of every year water stands in these rice fields. such a machine cannot be considered as simple deadloads, but must be dealt with as impact loads. This israther an e.\treme assumption, but any one who caresto observe the pound of a 5-ton truck as the wheelsdrop off of the slight projections which exist on the sur-face of every gravel or macadam road, will soon con-clude that even though it should ultimately be provedthat full impact loads are not developed, there can be,no doubt that the loads actually delivered to the metallingpartake very largely of the nature of an impact being the case, the rear wheels of a 5-ton truckmust be constantly delivering to the metalling, not 3-ton dead levels, but blows which approach 12 tons. Ifthe truck is traveling over a 6-in. pavement, the sub-grade must, therefore, be receiving shocks that reach anin
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