On the system stiffness of deep excavation in soft clay

In urban area with soft clay deposit, it is required that the wall deformation of a deep excavation be limited to a very low value in order to minimize damages to adjacent buildings. Major factors that control the magnitude of wall deformation includes excavation depth, shear strength of soft clay, stiffness of bracing system, and flexural rigidity of retaining wall. The combined effect of bracing stiffness and flexural rigidity of retaining wall are often referred to as the system stiffness of an excavation. However, if low deformation is mandated in the design, the effects of auxiliary measures, such as soil improvement, buttress or cross walls, may also have to be incorporated as part of the system stiffness. In essence, the presence of auxiliary measures is equivalent to an increase of undrained shear strength of soft clay within the excavation zone, that in turn leads to an increase of passive resistance and a lower magnitude of wall deformation. The factor of safety against basal heave is also tied to the magnitude of undrained shear strength, therefore, reassessing the relationship between wall deformation and factor of safety against basal heave allows the authors to incorporate the effect of auxiliary measures in the framework of system stiffness. Four deep excavation cases in soft clay were presented in this paper, the effects of auxiliary measures were evaluated, and attempts were made to extend the system stiffness design curves into low deformation area. It is also noted that the 3-dimensional effect or preloading effect of soil mass within the excavation may overshadow the effect of system stiffness on wall deformation, provided that the factor of safety against basal heave is higher than 3.