Experimental simulation and investigation of spalling failure of rectangular tunnel under different three-dimensional stress states

Spalling (or slabbing) in the sidewalls of deep rectangular tunnels is a type of rock failure phenomenon under the three-dimensional stress environment, which often occurs on the surrounding rocks after the tunnel face excavation is completed and has a significant impact on the safety of construction and support. In order to investigate the spalling process in deep rectangular excavated tunnels under three-dimensional stress, a granite material was firstly processed into cubic specimens (100 mm x 100 mm x 100 mm) with prefabricated rectangular holes (40 mm x 40 mm). Simulation experiments were performed using a rock true-triaxial electrohydraulic servo mutagenesis testing system under four different initial stress states. Meanwhile, a wireless microcamera was used to record and monitor the damage process on the sidewalls of the rectangular tunnel in real time. The results showed that under the four stress conditions, an evident spalling failure occurred on the entire sidewalls of the rectangular hole. It was observed that the spalling-damaged zone gradually developed toward the deep part of the hole in a horizontal direction. Finally, it formed a penetrating symmetric arc-shaped groove along the axial direction, whereas the roof and bottom slab remained stable. The spalling failure process of the sidewalls had four stages: the calm period, particle ejection at the upper and lower shoulders, sidewall crack propagation, and crack penetration spalling failure. Compared with the dynamic failure (rockburst) of a circular tunnel under the same three-dimensional high stress conditions, the spalling failure of the rectangular tunnel sidewalls was of the static failure mode. Under the three-dimensional stress condition, the radial stress had a greater effect on the spalling failure than the axial stress, by increasing the radial stress can be significantly reduced the degree of spalling failure and effectively improved the stability of the surroundings.