Physical Derivation of Effective Stress from Balance Law and Experimental Evidence

Abstract

It has long been discussed whether Terzaghi’s expression for effective stress holds true for all saturated soils. To examine the physical basis of the effective stress expression, a new mechanical model is proposed. According to that model, the fraction of pore pressure depends on the porosity and the proportion of the particle surfaces to which the pore pressure is applied. The new expression is then tested against the experimental evidence for varied soils. For cohesionless soils, values of η (the faction of pore pressure) are close to unity, because the real contact area is extremely small. Significant deviation from Terzaghi’s expression occurs only at very high stresses. For cohesive soils, a method is developed to calculate the η values based on the data from shear strength tests. A variation of factor η with effective stress is obtained for five different clays. Significant reduction in η is observed for clays with a high plastic limit. Without consideration of this reduction, effective strength parameters may be overestimated for the consolidated undrained triaxial tests.