Re-profiling of a squeezing tunnel considering the post-peak behavior of rock mass

Abstract

In problems involving large deformation, one of main concerns in rock engineering is the occurrence of a heavily reduction in tunnel section and even shield jamming. In this paper, a rigorous theoretical model is established to characterize the process of tunnel re-profiling works in heavily squeezing ground to re-construct the desired profile, based on the finite strain approach and Lagrangian material description. The post-peak behavior of rock mass and elasto-plastic strains in plastic zone during each excavation process are taken into account. Therefore, the proposed method enables to quantify the repetitive disturbances to a plastified surrounding rock and accumulated degradation of material properties. A numerical procedure is presented, in which the history-dependent behavior and path of material point in strain-softening rock mass during the entire excavation process are determined via our previous work, in combination with incremental algorithm and spatial interpolation. The discrepancy from the accurate solution induced by the negligence of elastic strains in all plastic zones is investigated by comparing the present results with the existing study. An extensive computations are then carried out to clarify some practical questions, including the effects of post-peak behavior, ground condition and old temporary support on the repetitive re-profiling, the variation of rock pressure developing on a rigid support after excavation, and the required minimum over-excavation size to achieve the desired profile. Also the effectiveness of advancing a pilot tunnel is investigated under different post-peak and geometrical conditions, in order to illustrate the applicability of the construction method in squeezing ground.