Surface settlement analysis of deep pit in soft plastic loess under fluid–solid coupling

A tunnel shaft in the soft plastic loess stratum of Xi'an was analysed in terms of changes in groundwater level and surface settlement around the shaft during excavation and dewatering. This was done by using field monitoring and fluid–solid coupling numerical simulations. The results showed that the dewatering effect on saturated soft loess was significant. However, the macropore characteristics were not obviously improved. The dewatering effect on saturated Q2 soft plastic loess was limited, which caused large settlements during dewatering. The settlement curve of the ground surface outside the shaft, under the combined impact of excavation and dewatering, showed a triangular distribution. The maximum settlement appeared near the maintenance structure. The settlement was in the range of 0.021–0.114% of the excavation depth (H). Numerical simulations showed that there were clear maximum settlement areas and transition zones on the surface after dewatering and excavation. The maximum settlement occurs within 0.6H outside the pit, while the total affected range extended up to 1.5H.