Experimental study of face stability for shield tunnels in sandy cobble strata of different densities

This paper aims to study the influence of relative density on the working face stability of shield tunnel in sandy cobble stratum. A series of geomechanical model tests were conducted for shield tunnels excavating through loose sample (LS), medium dense sample (MS) and dense sample (DS), respectively. Relative densities of LS, MS and DS are 0.35, 0.55, and 0.75. The method of continuous rotation of spiral excavators while shield excavators stop tunneling is adopted to simulate the instability of tunnel face. Experimental results effectively reveal the variation characteristic of vertical and horizontal earth pressure and lateral pressure coefficient of shield tunneling. The evolution of the failure zone and soil arch height of strata with different densities was explored. The failure area and collapse range obtained from the model test were compared with the existing method. The effect of relative density on the horizontal earth pressure in the instability stage of the tunnel face in the sandy cobble strata is greater than that of the vertical earth pressure. In addition, increasing the relative density can effectively increase the lateral pressure coefficient as well as the stability of the tunnel face. The existing methods have deviations in analyzing the instability area of the tunnel face in the sandy cobble strata with different relative densities. The research in this paper provides a reference for the construction and theoretical analysis of sandy cobble strata shield tunnel engineering.

Automated summary

This study investigates the influence of relative density on the stability of shield tunnel working face in sandy cobble stratum. Geomechanical model tests were conducted for shield tunnels excavating through loose, medium dense, and dense samples. The results reveal the variation of earth pressure and lateral pressure coefficient during tunneling and explore the failure zone and soil arch height. It is found that relative density has a greater effect on horizontal earth pressure and increasing it can enhance the stability of the tunnel face. The existing methods for analyzing the instability area in different relative densities have deviations.