A series of single-stage true triaxial compression (TTC) tests and multi-stage intermediate principal stress (σ₂) loading tests were carried out on three types of brittle rock specimens in this study to examine the influence of σ₂ and the validity of obtaining strength parameters by using a multi-stage loading method under true triaxial stress conditions. There exists a critical σ₂ at which the rock strength attains its ultimate value under both loading conditions. Under the same σ₃ level, the critical σ₂ is much smaller under the multi-stage σ₂ loading compared with under the single-stage TTC. The test results also show that the strength determined under multi-stage σ₂ 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 σ₂ loading because the local fracture plane induced by true triaxial stress is parallel to the σ₂, and the strength of the rock specimen containing this type of local fracture plane is not affected by σ₂. 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.

中文翻译：

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多级中间主应力加载下脆性岩石的强度特征

本研究对三类脆性岩石试件进行了一系列单级真三轴压缩（TTC）试验和多级中间主应力（_σ2 ）_{加载试验，以检验}σ2的影响和试验的有效性。在真三轴应力条件下，采用多级加载法获得强度参数。存在临界σ 2 ，在该临界σ ₂处，岩石强度在两种加载条件下均达到其极限值。在相同的σ ₃水平下，临界σ _2在多级σ ₂下要小得多与单级TTC下的负载相比。试验结果还表明，多级σ ₂载荷下测定的强度远小于单级TTC下测定的强度。强度差可达39%。由于真三轴应力诱发的局部断裂面平行于σ ₂ ，在多级σ ₂加载下测得的强度较低，含有这种局部断裂面的岩石试样的强度不受σ _2的影响. 相反，岩石损伤会在多次装载和卸载循环中累积。目前的试验结果表明，多级加载试验不适合确定真三轴条件下脆性岩石的强度包络线。