关键词: 软土地层, 超大直径装配式竖井, 施工全过程, 桩体力学响应, 现场监测, 流固耦合数值模拟

Abstract: To elucidate the soil-deformation evolution and cooperative mechanical characteristics of the pile-soil-shaft lining system for ultra-large-diameter prefabricated shafts in soft soil strata using the suspension-assembly mechanized excavation method and to clarify the influence of key design and construction parameters, a systematic study was conducted on the vertical-excavation underground intelligent garage project in Jing’an District, Shanghai. This project was the world’s largest completed prefabricated shaft, with an excavation diameter and depth of 22.8 and 51.0 m, respectively. On-site automatic monitoring and FLAC3D fluid-solid coupling numerical simulation methods were adopted to examine the evolution of ground-surface displacement, pile mechanical responses, and soil-deformation patterns during dynamic shaft sinking. Sensitivity analysis was conducted for key parameters, including pile spacing, pile diameter, pile length, soil reinforcement strength, and over-excavation depth. Results confirm the following: (1) The ground settlement due to shaft construction presents a nonlinear attenuation trend along the radial direction, with 22.5 m as the substantial influence boundary of settlement. The maximum recorded settlement is 12.72 mm, confirming the proposed method’s excellent control of environmental disturbance. (2) The uplift piles and shaft show a synchronous slight upward displacement during construction, with all vertical displacements controlled within 3 mm. (3) The horizontal displacement of the piles follows a distribution pattern of “large in the middle and small at both ends”, with the maximum value occurring at a depth of 38－41 m. A significant abrupt change in pile bending moment is observed at the soft stratum-hard stratum interface. (4) The pile-soil system exhibits remarkable cooperative deformation behavior. Excavation speed and over-excavation depth exert dominant control on excavation face stability; thus, differentiated parameter control must be implemented inside and outside the reinforcement area. Based on the engineering case and parameter-sensitivity analysis, the following optimized design and construction parameters are proposed: (1) pile spacing = 4.0 m, pile diameter = 1.2 m, and pile length = 70 m (for uplift piles); (2) unconfined compressive strength = 1.0 MPa (for the soil in the reinforcement area); (3) over-excavation depth ≤ 2 m with an excavation speed = 2.5－3.0 m/d (inside the reinforcement area) and over-excavation depth ≤ 1 m with an excavation speed = 1.0－1.5 m/d (outside the reinforcement area).

Key words: soft soil stratum, ultra-large-diameter prefabricated shaft, entire construction process, pile mechanical response, field monitoring, fluid-solid coupling numerical simulation