Abstract: The softening and then disintegration of granite residual soil encountering water often pose great difficulties to construction and security threats to foundation pit and surrounding environment. With the deep excavation of a metro station in granite residual soil zone as case study, we probed into the influence of granite residual soil softening at the bottom of excavation encountering water through PLAXIS finite element simulation. Results illustrated that the maximum horizontal displacement of support structure, the deformation of skirting, and the maximum ground settlement all raised with the deepening of softened soil. When the depth of softened soil was small, the above deformations were small too; when the depth of softened soil was large, the above values surged correspondingly. Moreover, the location of maximum horizontal displacement and the maximum bending moment also increased with the deepening of softened soil. The inner bracing around excavation face undertook more shift pressure as the depth of softened soil increased. Therefore, the depth of softened soil in passive zone should be reduced in the deep excavation in granite residual soil zone. The research finding serves as an experience for similar construction.

Key words: deep excavation, passive zone, granite residual soil, softening and disintegration, depth of softened soil, deformation behaviour