Numerical modelling and safety factor assessment for a supported excavation under seepage condition

ABSTRACT: In soil-structure interaction problems, the numerical methods have shown their effectiveness in the analysis of the working conditions of the structure and the surrounding ground, that is for the assessment of the serviceability limit state. For the analysis of the ultimate limit state and the estimation of the safety factor, the numerical approach can still be effective provided that a suitable technique is adopted to lead the soil-structure system to a failure condition. Following this procedure and applying a selected shear strength reduction in an additional final step of a finite element analysis, both the working and ultimate conditions have been investigated of a deep excavation supported by a strutted diaphragm wall in a cohesive soil in drained conditions. The excavation stages are reproduced in subsequent steps of the analysis and a seepage condition, induced by a water table drawdown, has been considered. The results prove the potential of the finite element approach for the assessment of the safety factor. Some comments are also drawn on the effects of seepage and on the different role played by cohesive and frictional components of the soil shear strength on the failure mechanism.