The Mechanical Strength of Unsaturated Porous Granular Material

Summary The influence of pore‐water suction on the strength of a porous material is that it contributes a compressive load which increases the shear strength. When the material is unsaturated, the normal load or effective stress is due, in part to the continuous water at measured suction in unemptied pores, and in part to isolated bodies in nominally emptied pores at suctions approximating to the suction at emptying. When the material is draining from saturation, the effective stress σ is image where S is the fraction of saturation, α is the fraction of the initial water content drained at the maximum suction, P s d is the prevailing pore water suction, and P s d is a suction passed through in reaching p S d at which the reduction of S is dS . When the material is rewetting, the relationship becomes image where p s w is now the prevailing suction during wetting and f is a distribution function of the degree of saturation such that image δ S is the fractional saturation removed in the suction range δs d at s d and regained in the suction range δs w at s w . m s d is the maximum suction attained. The effective stress is revealed experimentally by unconfined compression tests on samples with imposed pore water suctions, and the dependence on this suction confirms reasonably that which is predicted by the theoretical formulas.