Abstract: The block set mechanism is used to conduct upper bound analysis of the basal stability of braced excavations in undrained clay. Applications with consideration of different conditions are given to study the characteristics of stability factor and failure surface for excavations. The proposed mechanism is validated by case studies in which the factors of hard stratum, embedment depth of retaining wall and external load are taken into consideration. Improvement of the failure surface obtained from the Block Set Mechanism over that obtained from the multi-rigid-block mechanism (Qin et al., 2010) is achieved, since in the latter a partially assumed failure surface is adopted. Especially, for the cases of large embedment depth of retaining wall, the accuracy of solution obtained from the block set mechanism is better that that obtained from the multi-rigid-block mechanism (Qin et al., 2010) and the strength reduction finite element method (Faheem et al., 2003).