Compression-shear failure properties and acoustic emission (AE) characteristics of rocks in variable angle shear and direct shear tests

The surrounding rocks of underground engineering are usually subjected to the coupling of compressive and shear stresses. The variable angle shear tests (VASTs) and direct shear tests (DSTs) are two types of lab tests that can easily achieve different compression-shear stress states. The VASTs and DSTs on cubic rock specimens were conducted to analyze the shear strength parameters and acoustic emission (AE) characteristics during rock failure process. The test results show that it is more reasonable to use the test data in VASTs with shear angles greater than 45° for calculating the shear strength parameters of rocks. The essence of changing the shear angles in VASTs or the normal loads in DSTs is to change the micro-crack component produced in the compression-shear failure mode, in which the shear cracking mainly trigger the AE signals with low peak frequency around 100 kHz, and the tensile cracking mainly induce the AE signals with relatively high peak frequency around 300 kHz. Although the main type of micro-crack produced by shear failure and compression failure are both shear micro-crack, the micro-cracks generated by shear failure have higher efficiency in generating AE energy compared with compression failure, which means that the large-scale micro-crack expansion activities are more intense in shear failure than that in compression failure.