On Methods of Estimating the Base Stability of Excavations in Clay

Abstract

This study investigated three conventional methods in estimating stability of wide excavations in clay, including the load factor method, the strength factor method, and the slip circle method. Estimates of the factor of safety and required wall embedded depth were compared with those from the finite element method (FEM) using strength reduction technique, which were herein treated as the standard values. Results showed that wall friction had a pronounced effect on the factors of safety by the load factor method and the strength factor method but the slip circle method. For constant s_u/σ_v′ (normalized undrained shear strength) case, the H_p/H_e (wall embedded depth to excavation depth) ratios determined by the three hand-calculation methods were smaller than those by the FEM as N_b (the stability number) < 7.0 but larger as N_b ≥ 7.0. For constant s_u (undrained shear strength) case, as increasing s_u, the H_p/H_e ratios determined by the load factor method were illogical. Estimates by the strength factor method and the slip circle method were larger than those by the FEM as N_b < 4.5 but smaller as N_b ≥ 4.5.