Study on unified brittle-ductility index based on burst tendency and large deformation of coal-rock

Burst tendency and ductility are the essential attributes of hard and brittle rocks respectively. It is of great significance to carry out lithology characterization and brittle-ductile transformation research for safe mining in deep engineering. In order to propose a comprehensive evaluation index to express the burst tendency and ductility characteristics, the existing burst tendency indices were summarized. A unified brittle-ductile index (Bu)was established based on the complete stress-strain curve. The new index was analyzed and discussed with tests. The results show that the unified brittle-ductility index reflects the deformation characteristics and damage degree of rocks. Bu less than 0 corresponds to the post-peak strain-hardening. Bu equal to 0 corresponds to the ideal elastic-plastic deformation. Bu greater than 0 expresses the post-peakstrain-softening. Bu approaching infinite corresponds to the ideal brittle failure.Secondly, based on unified brittleness and ductility index, the burst tendency is divided into three levels. When Bu is less than 4.0, there is no impact tendency, when Bu is between 4.0 and 28.0, there is weak impact tendency, and when Bu is more than 28.0, it indicates a strong impact tendency. At last, the unified brittle-ductile index is a dimensionless index, which can reflect the brittle failure characteristics under uniaxial conditions and different stress levels, as well as the evaluation of post-peak strain-hardening characteristics of soft rocks or rocks under weak or high stress levels, especially highlighting the brittleness drop of different coal rocks under uniaxial conditions. The difference is very suitable for the impact tendency of coal rock falling sharply after the test peak.