Study on the failure mechanism of a tunnel reinforced by a heterogeneous grouting layer based on upper bound method

Abstract

During shield tunnel construction, the proper application of backfill grout can effectively prevent tunnel instability and excessive ground settlement. Studying the failure mechanisms of grouting-reinforced tunnels can ensure engineering safety. However, few scholars have investigated the stability of tunnels reinforced by grouting layers. In this work, the upper bound finite element method (UBFEM) is employed to investigate tunnel stability by considering homogeneous grouting and eccentric grouting layers, and the impacts of different parameters on the grouting layer properties are evaluated. To reduce the difficulty of using the UBFEM, a simplified rigid block upper bound method is proposed. This method can effectively reflect the instability mechanism of a tunnel reinforced by grouting, and it is convenient for practitioners to use. The results indicate that, compared with homogeneous grouting layers, tunnels reinforced with eccentric grouting experience a 30.8% smaller plastic failure area, a 16.8% greater ultimate support force coefficient, and an approximately 9.0% greater maximum vertical settlement coefficient due to tunnel instability caused by eccentricity. The proposed method provides a quantitative evaluation index for tunnel stability with eccentric grouting layers. The findings are synthesized into nondimensional charts and tables for practical application by industry professionals.