Abstract: In existing norms, the method for basal heave stability analysis of excavations is mainly the circular sliding method. This method bases on the limit equilibrium theory and has no rigorous theoretical basis. The safety factors derived by the circular sliding method are generally overestimated, and the derived safety factors are highly deviant when the wall embedment depth ratios are relatively small. The limit analysis method has a rigorous theoretical basis, based on which some failure mechanisms have already been proposed. However, there is a lack of research on the applicability of these mechanisms. In this study, the theoretical framework of the upper bound analysis for basal heave stability analysis is introduced, and the concept of mechanism optimization as well as the combined mechanism of rigid blocks and continuous velocity field are discussed comprehensively. The upper bound mechanisms are compared with the elastoplastic finite element method in uniform clay ground and the circular sliding method in non-uniform clay ground respectively. The applicable conditions of these mechanisms are concluded. The results show that the stiffness of the wall should be treated as infinity when the stiffness is large and the embedded depth ratio is relatively small, while the influence of the stiffness should be considered when the stiffness is small and the embedded depth ratio is relatively large. The proposed circular upper bound mechanism is verified by a field case study.

Key words: braced excavation, limit analysis method, basal heave stability, wall stiffness, circular sliding method