Undrained stability of deep excavations with the strut-free retaining wall system

The strut-free excavation system is typically arranged by a combination of diaphragm walls, capping slabs, buttress walls, and cross walls. This system has a good advantage in optimizing the working area, shortening the construction time, and effectively reducing excessive wall deformation, which has been validated through comprehensive studies and case histories. However, no study has been conducted on the stability of excavation with a strut-free system, which is the main concern for deep excavation in undrained clays. This study evaluates the undrained stability of deep excavations with a strut-free system through a series of three-dimensional finite element analyses. The numerical models are initially validated by the experimental test and then parametric studies are performed to evaluate the stability resistance mechanism of the strut-free excavation system. The results reveal that the friction and bearing resistance acting on the buttress wall could contribute to the overall stability resistance. Thus, enlarging the buttress wall dimension could enhance the factor of safety. In addition, the combination of the buttress and cross wall (so-called U-shape wall system) could significantly increase the excavation stability, in which the required diaphragm wall penetration depth can be minimized and still satisfy the factor of safety. Furthermore, new schematics of the strut-free system by combining buttress wall and ground improvement are introduced, which can be recommended as an innovative and alternative solution. Finally, new simplified methods are proposed to estimate the factor of safety for excavation stability with a strut-free system, which is verified by the three-dimensional finite element results.