Strength characteristics of brittle rocks under multi-stage intermediate principal stress loading

A series of single-stage true triaxial compression (TTC) tests and multi-stage intermediate principal stress (sigma (2)) loading tests were carried out on three types of brittle rock specimens in this study to examine the influence of sigma (2) and the validity of obtaining strength parameters by using a multi-stage loading method under true triaxial stress conditions. There exists a critical sigma (2) at which the rock strength attains its ultimate value under both loading conditions. Under the same sigma (3) level, the critical sigma (2) is much smaller under the multi-stage sigma (2) loading compared with under the single-stage TTC. The test results also show that the strength determined under multi-stage sigma (2) loading is much smaller than that determined under single-stage TTC. The strength difference can attain 39%. A lower strength is measured under multi-stage sigma (2) loading because the local fracture plane induced by true triaxial stress is parallel to the sigma (2), and the strength of the rock specimen containing this type of local fracture plane is not affected by sigma (2). On the contrary, rock damage accumulates during multiple loading and unloading cycles. The current testing results indicate that the multi-stage loading test is not suitable for determining the strength envelope of brittle rock under true triaxial conditions.