Pile bearing capacity in crushable soils

The existence of large magnitude stresses at the tip of a free floating bearing pile is a well known phenomenon which in turn leads to crushing of soil grains thus affecting the pile load displacement characteristics. Classical foundation design methods can be safely used where stress levels or particle strengths prevent crushing, however in the case of weak grains or high foundation stresses, consideration should be given to the effects of grain crushing. All materials are crushable at high stress levels but there exists a large class of crushable materials with weak grains such as carbonate sands, volcanic soils, decomposed granites and mica rich sands. Model pile tests have been carried out on two types of carbonate sand and a standard silica sand for comparison purposes in order to assess the influence of crushability and degree of penetration on the end bearing capacity of deep foundations. Results are presented to show the extent and degree of crushing around a pile. It is shown that the use of Prandtl's bearing capacity factor N-q provides an optimistic over-prediction of bearing capacity for piles in crushable sands which is shown to be a function of normalized pile penetration and a compressibility factor C-p'.