Rankine Active Earth Pressure of Unsaturated Soil Based on the Envelope Shell Model

In this study, a novel approach for the active earth pressure on retaining wall with unsaturated backfill is proposed based on the nonlinear shear strength envelope shell model and classical Rankine theory. This method is able to reflect the nonlinear characteristics of unsaturated shear strength in dimensions of net normal stress and matric suction. According to the critical Mohr circle in active Rankine state, the shear strength parameters (equivalent cohesion and equivalent friction angle) can be obtained by adopting optimization techniques. Specific formulations of the active earth pressure intensity, the cracking depth, as well as the magnitude and action height of resultant force are presented. Besides, the proposed method, which is of good adaptability, is theoretically compared and experimentally verified by four cases. The calculation results show that the active earth pressure intensity based on the envelope shell model exhibits remarkable nonlinear characteristics along the wall-back height. As the matric suction goes up, both the cohesion and the cracking depth increase, while the friction angle and resultant pressure show decrease. Compared with the conventional envelope plane, the envelope shell model attributes the effect of matric suction on the unsaturated shear strength to both cohesive and frictional factors. It further reveals that the unsaturated soil exhibits a self-compensating mechanism on shear strength. In other words, the unsaturated soil attempts to maintain a cohesion-to-friction balance upon fluctuating load or suction conditions. For the retaining structures with high filling and matric suction, the envelope shell model can provide a better estimation of Rankine active earth pressure.