Abstract: Based on Clough system stiffness model, this paper introduces a novel synthetic stiffness expression of strut system for excavation. This expression considered the wall stiffness, strut stiffness and the soil stiffness behind the wall, and thus it had substantial advantage to predict the deformation induced by excavation. Clough system stiffness, Bolton system stiffness and the synthetic system stiffness proposed in this paper were applied to measure the tilt of 469 retaining walls in 22 deep excavations of Tianjin subway line 6 engineering. The maximum deformation of the retaining structure in foundation pit decreased with the increase of the stiffness of the dimensionless composite support system. The deformation value tended to be constant, until increased to a certain value. When the mobilized undrained shear strength ratio of soil was further analyzed, it is found that the ratio of the soil behind the wall along Tianjin subway line 6 was less than 0.6. The upper limit value of this ration is 0.8, which indicates that the excavation engineering of Tianjin subway line 6 has sufficient safety reserves. Therefore, research results can provide significant reference for the economic and safety design of similar engineering.

Key words: wall deflection, support system stiffness, undrained shear strength, shear strain