Macro and micro failure mechanism of surrounding rock of small span tunnel under different stress paths

Abstract

According to the different stress paths, similar model test and PFC simulation test of tunnel surrounding rock are designed to compare the failure mechanisms at macroscopic and mesoscopic scales. The following conclusions are drawn. 1) Excavation unloading will disturb the surrounding rock to form a certain excavation damaged zone. 2) Under the loading path, the stress of surrounding rock failure is 1.500 MPa; under the unloading path with initial stress of 60% σ_Zmax and 100% σ_Zmax, the failure stress is 1.583 and 1.833 MPa respectively in the model test. 3) In terms of the failure mode of rocks under different stress paths, tensile fractures first appear in two sides of the vertical walls; thereafter, the spandrel and arch foot are loosened due to the stress concentration. The fractures gradually coalesce with those occurring in the vertical walls. 4) In the process of excavation unloading, the proportion of shear cracks is 35.3%, and the rock is subject to strong shear effect. The final failure surface is approximately V-shaped. 5) The tangential peak stress on the vertical walls at the free face is the lowest; the vertical walls at the free face show the poorest bearing capacity and are easily subjected to tensile failure.

Keywords

stress paths / similarity test / PFC simulation / excavation unloading / macroscopic and mesoscopic scales

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