Abstract: The soft soil is characterized by high moisture content, low shear strength and high sensitivity. These characterizations make the foundation excavations prone to collapse and deform excessively, and therefore it poses significant safety hazards. Using a deep and large soft soil foundation pit project in Hangzhou as a case study, this paper examines the key issues and challenges in the design of supporting structures. The design solution is achieved through a sophisticated combination of bored pile/diaphragm wall + steel composite support + support replacement + servosystem. This design also considers the construction sequence, space-time effect, and combines with the surrounding environmental conditions and numerical simulation optimize the excavation division and internal support of the large deep excavations in soft soils. The active deformation control and design methods of deep excavations in large excavation depth and soft soils in sensitive environment based on space-time effect have been accomplished. The simulation results show that the maximum horizontal convergence displacement of the subway structure caused by the excavation is 5.2mm, and the maximum settlement is 5.8mm, which meets the deformation control requirements of the subway structure. The results provide valuable reference and guidance for similar engineering projects in the future.

Key words: Deep Excavations, Hot Rolled Steel Supports, Servosystem, Numerical Analysis