STUDY ON TEST METHOD AND FORMATION MECHANISM OF CLASS Ⅱ ROCK BEHAVIOR

摘要

Class Ⅱ curve is the manifestation of rock sample' unstable failure, which is important for the prediction of geotechnical engineering calamities such as rock burst etc.. In this paper, several typical sedimentary rocks' class Ⅱ curves are obtained by combined loading control mode of axial-displacement and circumferential-displacement on MTS test system. The formation mechanism of class Ⅱ curve is analyzed based on test results. The occurrence of class Ⅱ curve needs two conditions. The first one is appropriate loading mode. The circumferential displacement should be used as the loading control variable in sample failure process. The second condition is that the rock presents brittleness property and can amass some elastic energy. Only when the variable with maximum change rate is used as the loading control parameter in different stages of the whole stress-stain curve, the class Ⅱ curve may be obtained. For the post-peak phase of class II curve, the elastic energy amassed in the rock can cause the sample collapse, when the circumferential displacement increases rapidly without loading. In the meantime, if the loading rate of circumferential displacement is less than the change rate caused by its broken down, the MTS will unload. At the same time, the sample failure caused by elastic energy released makes the circumferential strain increasing and the axial strain reducing. Rock samples with the same mechanical properties are characterized as Class Ⅰ or Ⅱ, which are controlled by different loading control mode.