Abstract: This study investigates the deformation characteristics of ultra-deep circular shaft excavation in soil–rock composite strata, based on a shaft project for the Shenzhen Airport–Daya Bay Intercity Railway. Monitoring data collected during excavation were statistically analyzed, and orthotropic plate elements were used in the numerical model to capture differences in the circumferential and vertical stiffness of the diaphragm wall. A comparison of measured and simulated results yielded insights into the deformation and load bearing mechanisms of large diameter shafts in complex urban environments. The results indicate that: 1) Due to the uneven distribution of composite strata, groundwater levels, and surface overloads, the excavation induced two opposite deformation modes in the diaphragm wall. The maximum deformation of the “outward bulging” type diaphragm wall was 0.50‰H_e (where H_e is the excavation depth), while that of the “inward bulging” type reached 0.55‰H_e. 2) This case study shows that shaft deformation changes as the depth-to-diameter ratio r increases. When r ≤ 1.1, the shaft deformation is in the circumferential compression stage. When the excavation depth exceeds the soil-rock interface (r > 1.1), and the shaft enters the elliptical deformation stage. 3) In the numerical simulation, the reduction coefficients for the circumferential and vertical stiffness of the diaphragm wall were set to 0.3 and 0.8, respectively. The relative difference between the calculated results and the measured data was within 13%. Finally, an elliptical deformation mode and mechanism that can explain this behavior were proposed. The research findings provide valuable references for the safety assessment of similar large-diameter circular shaft constructions in soil-rock composite strata.

Key words: deep large-diameter circular shaft, soil-rock composite stratum, deformation characteristics, site monitoring, numerical simulation