An improved theoretical method for assessing tunnel response to pre-excavation dewatering: Time-dependent deflection, internal force, joint opening, and dislocation

Abstract

Adjacent construction activities can cause significant deformation of the existing shield tunnel, yet the time-dependent development of these deformations is rarely documented, hindering the accurate prediction of potential hazards. This study presents a theoretical model based on the displacement input method to characterize the time-dependent response of tunnel induced by pre-excavation dewatering in an unconfined aquifer. The tunnel and subgrade are modelled as a Timoshenko beam and Pasternak foundation, respectively. The greenfield soil displacement is derived by consolidation theory, incorporating dynamic changes in the phreatic surface and the embedding depth of the waterproof curtain. The proposed solution is evaluated by well documented results of model testing on drawdown and finite element analyses on deformation of both soil and tunnel. Parametric assessments of tunnel deformation are conducted, analyzing the time-dependent response and influences of factors including the tunnel’s relative position to the dewatering zone, soil modulus, specific yield, and the embedding depth of the waterproof curtain. Results indicate that accounting for time-dependent effects significantly reduces the overestimation of tunnel deformation prior to excavation. Additionally, higher soil modulus and greater curtain embedding depth decrease final tunnel deformation, while tunnel position and specific yield primarily influence deformation distribution without altering maximum deformation. The findings of the study provide a more accurate basis for designing dewatering strategies and offer improved prediction for existing tunnel deformation caused by adjacent foundation pit projects.