Deformation analysis of a deep excavation with a servo control system adjacent to an existing foundation pit: A case study in Shenzhen

During the sequential excavation of closely spaced foundation pits, the deformation behavior of the retaining structures significantly differs from that of conventional pits due to asymmetric loading. As excavation proceeds, both adjacent retaining systems tend to shift toward the newly excavated pit. This study investigates the deformation response of a case in Shenzhen involving closely adjacent but non-simultaneously constructed foundation pits, using a combination of field monitoring and numerical simulation. The results reveal that staged excavation leads to pronounced asymmetrical deformation. By introducing an axial-force servo-control system, the deformation pattern can be effectively controlled, resulting in a reduction of the maximum lateral displacement by 23.4 mm and the maximum ground settlement by 50 mm. The servo system compensates for axial force loss and stabilizes internal forces, thereby reducing peak support loads and wall bending moments. Numerical analyses further show that the diaphragm wall adjacent to the existing pit has minimal impact on deformation and settlement, allowing its thickness to be optimized from the original 800 mm to a minimum structural thickness of 400 mm while ensuring strength requirements are met. In contrast, the wall on the opposite side plays a more critical role; however, with the servo system, its thickness can be optimized from the original 1000 mm to 800 mm. Thus, effective deformation control and cost efficiency can be achieved by combining servo-control systems with an optimised design of the diaphrams walls.