Deformation analysis of tunnel excavation in gravelly formation using the anisotropic degradation model

Abstract

This study adopts a shear-induced anisotropic degradation model to analyze the deformation of excavation in gravelly formations. The adopted model is a variable moduli model with the following characteristics: the stress–strain relationship originates from degradation of the bulk modulus and shear modulus subjected to different loadings. Under hydrostatic loading, gravelly soil may behave as an isotropic material. However, when gravelly soil is subjected to shear loading, the material becomes anisotropic and degrades before ultimate strength is attained. Therefore, this study introduces an anisotropic factor to reflect the tendency to shear-induced volumetric deformation. To analyze the deformation of the Pakuashan tunnel, which passes through a gravelly formation in Taiwan, the model was first validated by comparing the drained triaxial test results of gravelly materials sampled from the tunnel. The proposed model is implemented with a finite element code to predict the tunnel deformation under construction. A comparison between the monitoring data and numerical analysis shows that the proposed model can reasonably simulate the behavior of a gravelly formation under excavation. Numerical analysis shows that the main deformation of the tunnel is the result of significant degradation of the moduli around the whole section, especially at the crown and invert of the tunnel.