Failure characteristics of surrounding rocks along the radial direction of underground excavations: An experimental study

Abstract For the in situ surrounding rocks of underground excavations, the radial stress corresponding to the minimum principal stress σ3 varies with increasing distance away from the excavated boundary. To characterize the rock failure along the radial direction of an excavation, true triaxial compression tests with different σ3 values (0.5, 5, 10, 15, 20, 25, and 30 MPa) were performed. The characteristic stresses (including the crack initiation stress σi, crack damage stress σd, and peak stress σs) and failure mode of the tested specimen under different σ3 conditions were systematically investigated. The experimental results indicate that with increasing σ3, the characteristic stresses increase monotonically and continuously. The increasing rate of the characteristic stresses with σ3 ranging from 0 to 10 MPa is higher than that with σ3 ranging from 10 to 30 MPa, indicating that low σ3 impose a greater effect on the characteristic stresses than high σ3. In addition, the failure mode of the tested specimen changes from tensile-shear failure mode I (with vertical tensile cracks and oblique tensile-shear cracks developing along the σ2 direction) to tensile-shear failure mode II (with oblique tensile-shear cracks developing only along the σ2 direction) to tensile-shear failure mode III (with oblique tensile-shear cracks along both σ2 and σ3 directions) as σ3 increases. Then, the failure characteristic of the surrounding rocks of the underground excavation was characterized based on the obtained experimental results. When maximum radial stress σr encountered near the excavated boundary is much smaller than the axis stress σa, the surrounding rocks will exhibit a “Ternary failure characteristic”, i.e., with an increase in the distance from the excavated boundary along the radial direction, the failure mode of the surrounding rocks after excavation changes from tensile/splitting failure mode to tensile-shear failure mode I and then to tensile-shear failure mode II. When the maximum σr encountered near the excavated boundary approximates the original σa, the surrounding rock masses will exhibit a “Quaternary failure characteristic”. Specifically, the corresponding failure mode along the radial stress direction away from the excavated boundary changes from tensile failure mode to tensile-shear failure mode I then to tensile-shear failure mode II and finally to tensile-shear failure mode III.