occur. In the case of long piles driven into soft material, it is almost entirely the side friction which gives them supporting power. Again, someone may question the correctness of loading sand apparently as high as nine tons per square foot at a depth of 250 feet below low water level, when the depth of water is eighty feet; but it must be remembered that the net weight of 170 feet of earth loads the soil some five tons per square foot, and that before any settlement can occur, the material adjacent to the caisson has to be raised. The reason for this is that the sand at such a great depth is practically incompressible, so that for any settlement to occur it must flow. It cannot flow downward or laterally, because there is no vacant space for it to fill; consequently, if flow it must, it will have to pass upward; and, in order to do so, it must lift a large column of the adjacent solid material. In the author's opinion, it would take an excessively large unit loading on the base of a filled caisson resting on coarse sand at a depth of two hundred and fifty feet to cause the slightest settlement.

The permissible loading for long piles has been taken at forty tons per pile, this being in accordance with the author's practice for a quarter of a century; and he has never yet found any settlement to occur under such loading.

Unit Prices of Materials in Place

The following table gives the unit prices for materials in place assumed for the purpose of this investigation:

Table 18b