This study is aimed to investigate the spalling failure mechanism of surrounding rock in the rectangular tunnel of deep hard rock. A cubic granite specimen (100 mm×100 mm×100 mm) was prepared with a precast rectangular hole (40 mm×40 mm)

and then the simulation test was carried out by using the TRW-3000 true triaxial electro-hydraulic servo mutagenesis testing machine. In the simulation test

the in-situ stress of the depth of 1 000 m was selected as the initial loading stress state

and the horizontal stresses of the rectangular hole were kept constant in the radial and axial directions. After that

the vertical loading was applied until the surrounding rock on both sidewalls of the hole was destroyed

and the whole specimen remained in a stable state. During the loading process

a real-time video surveillance system was employed to monitor the failure process of the internal structure of the rock specimen. The results showed that when the maximum principal stress was at the vertical direction and the intermediate principal stress was at the horizontal axis

the obvious spalling failure appeared on the surrounding rock of both sidewalls of the rectangular hole. Moreover

it was found that the failure zone of the surrounding rock was symmetrical

whereas the roof and the floor remained stable. Meanwhile

the failure of surrounding rock was parallel to the vertical direction

showing typical tensile cracking and spalling characteristics. The entire failure process was divided into the calm stage

the particle ejection stage on the shoulder angles of the hole

the sidewall crack propagation stage and the crack penetration spalling stage at both sidewalls. When entering the crack penetration spalling stage

the spalling failure may continue to develop regardless of the loading or load-holding process. During the test

the spalling failure of the surrounding rock on the both sidewalls of the specimen presented a static failure mode. Besides

the failure zone gradually developed towards the deep part of the wall along the horizontal direction

and finally formed a penetrating symmetric arc notch along the axial direction.