Analysis of excavation deformation and bearing characteristics of ultra deep circular shafts for shield tunneling

To investigate the excavation deformation and bearing characteristics of ultra-deep circular shafts for shield tunnels in soil-rock composite strata, this study focuses on the circular shaft foundation pits of a shield tunnel section along the Shenzhen Airport-Daya Bay Intercity Railway. By analyzing field measurement data and numerical calculation results from three adjacent shafts—one main shaft with an inner diameter of 36 m and two service shafts with inner diameters of 12.6 m—an in-depth examination of the excavation deformation characteristics and internal force responses of the ultra-deep circular shafts was conducted. The results indicate that: (1) Under the influence of groundwater level fluctuations, soil and rock excavation, and adjacent slopes, the diaphragm walls of the main shaft exhibit a composite deformation pattern characterized by “cantilever” and “bulging,” with a maximum lateral displacement of 0.52‰He (where He denotes the excavation depth). The diaphragm walls of the service shafts demonstrate overall tilting deformation, with a maximum lateral displacement of 0.18‰He, while the maximum surface settlement reached 0.37‰He. (2) During asynchronous excavation of the shafts, the ratios of the maximum circumferential normal stress to the maximum vertical normal stress for the main and service shafts are 4.8 and 4.4, respectively. In addition, the ratios of the maximum vertical bending moment to the maximum circumferential bending moment are 6.9 and 1.9, respectively. Furthermore, the diaphragm walls exhibit compressive bending per unit depth. The axial forces in the ring beam supports of each shaft are relatively high within the groundwater fluctuation zone or near the soil-rock interface due to various factors, including local overloading and uneven soil-water pressure. (3) A simplified calculation method for determining the vertical bending moment of the diaphragm wall based on inclinometer data was proposed and successfully applied in the engineering case. The calculated maximum vertical bending moment of the main shaft is 78% of the design value. This method can effectively and quantitatively assess the bearing capacity of diaphragm walls during shaft construction, particularly in cases where reinforcement meters are not installed. The findings of this research provide valuable insights for similar circular foundation pit engineering in soil-rock composite strata.