Optimization of Support System for Deep Foundation Pit with Special-shaped Soil-rock Combination Strata

The foundation pit of a certain open-cut tunnel is located in a soil-rock combination layer, with a large depth of about 17.1 m and a deep excavation area of about 27.9 m. The original design of the overall foundation pit used underground continuous walls and 4 reinforced concrete supports, and the inner wall of the pit used drilled cast-in-place piles as vertical support structures. Two steel pipe supports were added to the deep pit area. To save construction period and project cost, a series of optimizations have been made to the original design scheme, taking into account the distribution of strata and geological conditions. Numerical simulation was used to compare and analyze the deformation characteristics of the foundation pit support system and surrounding environment before and after optimization. The results showed that the optimized support scheme met the design and regulatory requirements. The foundation pit was constructed according to the optimized plan and on-site monitoring work was carried out. The monitoring results of the foundation pit showed that: (1) The horizontal displacement of the continuous wall of the foundation pit, surface settlement, support axial force, deformation of surrounding buildings, and uplift at the bottom of the pit were all below the control requirements; (2) The maximum horizontal displacement of the continuous wall occurs at a certain height above the soil rock interface, with a maximum value of approximately 0.025% of the shallow foundation pit depth; (3) The deformation on both sides of the irregular foundation pit is asymmetric, and the maximum surface settlement occurs on the shallow foundation pit side. The impact range of the outer surface of the foundation pit is about 1.0H; (4) The lateral deformation of the foundation pit excavation in the lower bedrock section is small, and it has little impact on the range of surface settlement grooves. The optimization plan saves time and cost and can provide a reference for similar foundation pit projects.