Deformation characteristics and excavation strategy optimization of two adjacent deep shafts: Insights from field monitoring and numerical simulation

• A novel staggered excavation method for adjacent shaft pits is proposed. • Multiple sequences are compared using 3D numerical simulations. • Field monitoring validates results and reveals deformation laws. • The optimal sequence is identified for deformation control and safety. Recent years have seen the emergence of complex excavation of two or more adjacent shafts in dense urban environments, implying higher level technical challenges for geotechnical engineers. Due to a lack of documented case histories and field measurements, the deformation characteristics of twin-shaft excavations remain insufficiently understood. This study therefore investigates the optimal time-staggered excavation strategy and further demonstrates the ground disturbance characteristics based on a detailed case study. A comprehensive field-monitoring program records retaining wall displacements, ground settlement and adjacent building responses throughout the twin pits excavation and the subsequent construction of internal structures. Further, a three-dimensional finite-element model is developed to simulate ground and structural behavior under alternative excavation sequences, while the computed results are validated against the monitoring data. The findings confirm that: (1) the commonly adopted “deep-before-shallow” excavation sequence is not always optimal for adjacent twin shafts, as numerical results indicate that a “shallow-before-deep” sequence reduces overall deformation, while simultaneous excavation produces the largest response; (2) concurrent staggered excavation of two adjacent shafts in soft ground is technically feasible and can significantly shorten the construction schedule while keeping deformations of both shafts and the surrounding ground within allowable limits; (3) most deformation occurs during excavation up to base slab completion for both pits. While the timing of shallow pit excavation has limited impact on global deformation, it significantly affects deformation in the inter-pit zone and its vicinity, which should be prioritized for monitoring and protection during scheme selection. These results provide practical guidance for implementing complex adjacent shafts excavation where construction duration and environmental disturbance must be balanced.