Fraction of Pore Pressure and Calculation of Earth Pressure for Saturated Cohesive Soils

Earth pressure for saturated cohesive soils is closely related to the effective stress expression. For cohesionless soils, previous research indicated that the fraction of pore pressure (factor eta) in the effective stress expression is below unity at high pressures. However, for cohesive soils, reliable eta-value has not been reported. In this study, experimental data from direct shear and triaxial tests were analyzed in order to derive the eta-values of two cohesive soils. It was shown that eta-value of cohesive soils decreases significantly with increasing effective stress, and the values of eta derived from different experiments were consistent with each other. The obtained values of eta were applied to the calculations of lateral earth pressure. The lateral earth pressure based on real eta was compared with the results of traditional methods, i.e., effective stress analysis (ESA) and total stress analysis (TSA). The traditional ESA was conservative because of the overestimation of active earth pressure. Such conservatism was more significant for high plasticity clay. Contrastingly, TSA results were risky for both of the two cohesive soils.