Abstract
Geo-materials may present varying mechanical properties under different stress paths, especially for tunnel excavation, which is typically characterized by the decreased radial stress and increased axial stress during the complex loading and unloading process. This study carried out a comparative analysis between the loading and unloading model testing, which was then combined with PFC2D simulation, aiming to reveal the fracture propagation pattern, microscopic stress and force chain distribution of the rock mass surrounding the tunnel. Comparisons of extents and development of tensile strain between loading and unloading testing results were made. The overall stability, the integrity of rock mass, and the failure pattern transition under loading and unloading processes were systematically examined. In addition, for the two unloading cases with different vertical stresses imposed, the failure patterns were both identified as the collapse of the V − shaped extruded sidewall, due to the coupling of the shear failure and the vertical tensile failure in the sidewall wedge.
Highlights
Geo-materials may present varying mechanical properties under different stress paths, especially for tunnel excavation, which is typically characterized by the decreased radial stress and increased axial stress during the complex loading and unloading process
The failure pattern transformed from the tensile fracture of the arch bottom with minor load into collapse of V − shaped damaged and extruded sidewall led by joint tensile and compressive effects
This paper studies the failure mechanism of the tunnel during the loading and unloading processes, on the basis of the comparison between physical model testing and discrete element method − based simulation
Summary
Based on these analytical methods, so many deformation control and stability analysis approaches of surrounding rock mass have been proposed 29–37. The loading stage is more commonly used to study the failure mode[3,4,5,41] In such cases, the stress state and displacement field variation during the failure propagation are different from those in the actual unloading process of tunnel excavation, and no effective reflection upon the influence of unloading tunnel excavation on surrounding rocks can be anticipated. The study on tunnel excavation by loading method is to some extent limited, especially when it comes to deeply understand the regularities behind the surrounding rock deformation and failure led by excavation disturbance. To systematically analyze the tunnel stability during excavation, this paper conducts a comparative loading/unloading experiment during tunnel excavation along with the discrete element method (DEM) to shed light upon the failure plane propagation and failure mechanism during the loading and unloading processes. New insights into the tunnel deformation and failure caused by excavation were obtained, which can provide vital guidance on tunnel design and construction
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