Fully hydro-mechanical coupled analyses of the deep excavation above a multi-aquifer-aquitard system

Deep work shaft excavations have become increasingly common as a preliminary step of tunnel construction. Deep excavation above a multi-aquifer-aquitard system (MAAS) complicates the flow field and stress field surrounding the excavation, inevitably affecting the performance of excavations. However, previous studies with drained or undrained analyses ignored the effects of dewatering-induced groundwater flow on the performance of deep excavations. To better understand this aspect, this study performs fully hydro-mechanical coupled analyses to investigate the performance of a 39.5 m-deep excavation above a multi-aquifer-aquitard system, which considers dewatering in two confined aquifers. A sophisticated three-dimensional (3D) numerical model is developed to simulate the detailed construction processes of drainage, excavation, dewatering, and strut installation. The study presents a comparison of two scenarios (with and without dewatering in a confined aquifer) with respect to wall deflection, stratum deformation, pore pressure, and effective stress path. The analysis results indicate that the effective stress paths are highly dependent on stratum permeability and construction activities. The dewatering behavior significantly reduces the excavation-induced deformation, primarily due to increased effective level and consequently increased resistance on the excavated side. As a result, ground settlements decrease due to less wall deflection.