SUPPORTING POWER or Sonzw AND DISH PIES. The sup porting power depends upon the nature of the soil and the depth to which the pile is sunk. A screw pile "in soft mud above clay and sand" supported 1.8 tons per sq. ft. of blade.* A disk pile in "quick sand" stood 5 tons per sq. ft. under vibrations.t Charles Macdonald, in constructing the iron ocean-pier at Coney Island, assumed that the safe load upon the flanges of the iron disks sunk into the sand, was 5 tons per sq. ft.; but "many of them really support as much as 6.3 tons per sq. ft. continually and are subject to occasional loads of 8 tons per sq. ft., without causing any settlement that can be detected by the eye."$ On account of the many uncertainties in connection with piles, a wide margin of safety is recommended by all authorities. The factor of safety ranges from 2 to 12 according to the importance of the structure and according to the faith in the formula employed or the experiment taken as a guide. At best, the formulas can give only the supporting power at the time when the driving ceases. If the resistance is derived mainly from friction, the supporting power generally increases for a time after the driving ceases, since the coefficient of friction is usually greater after a period of rest. If the supporting power is derived mainly from the resistance
to penetration of a stiff substratum, the bearing power for a steady load will probably be smaller than the force required to drive it, as most materials require a less force to change their form slowly than rapidly. If the soil adjoining the piles becomes wet, the supporting power will be decreased; and vibrations of the structure will have a like effect.
The factor to be employed should vary with the nature of the structure. For example, the abutments of a stone arch should be constructed so that they will not settle at all; but if a railroad pile trestle settles no serious damage is done, since the track can be shimmed up occasionally.
In conclusion, it should not be overlooked that the bearing power of a piled area is not necessarily equal to the bearing power of one pile multiplied by the number of piles in that area. If the stratum below the piles is at all yielding, the supporting capacity of the foundation is the bearing power of the area on the soft stratum below plus the friction on the outer side surface of the entire mass surrounding the piles.*