According to the stress concentration path near an underground excavation-"Unloading the minimum principal stress (σ3), keeping the intermediate principal stress (σ2) increasing the maximum principal stress (σ1)"experimental studies were carried out on granite and red sandstone cubic specimens (100 mm × 100 mm × 100 mm) under true triaxial unloading condition. The tests were completed by using a self-developed true triaxial testing system. The testing results showed that the strength of the two rocks increased with the increase of a2• By comparing the strength between the tested and the computed results based on the classic Mohr-Coulomb criterion, it was clearly showed that the strength of rock specimens with σ2 was less than the computed strength by Mohr-Coulomb criterion. The difference value increased with the increase of a2• Based on Mohr-Coulomb criterion was put forward on the condition of the shear failure, the failure modes of granite and red sandstone specimen were changed from shear to slabbing with certain intermediate principal stress, for example, larger than 20 MPa.
With the increase of the crust depth, the vertical in-situ stress was reported to increase about 27 MPa every 1 km. The lateral stress coefficient λ (the ratio of horizontal stress to vertical stress) was quite scattered under shallow crust depth. However, as the depth increases, the λ value gradually reached to 1.0, which indicated that the horizontal stress was approximately equal to the vertical stress (Brown & Brandy 1978). Recently, the mining depths at a number of underground metal mines have been closed to or exceeded 1 000 m in China (Gu & Li 2006). Due to unloading by excavation, the stress condition of surrounding rock mass is correspondingly changed. For example, it may be changed from triaxial stress condition to biaxial or even one dimensional stress. Afterwards, the rock mass may not resist the adjusted stress after excavation which can lead to rock failure in mining engineering.
