Bearing deformation characteristics of large-size broken rock

In order to study the bearing deformation characteristics of large-size broken rock,a testing system used for bearing deformation test of large-size broken rock was developed. The typical sandstone from the roof was chosen as sample. In view of the particle size distribution characteristics and stress state of broken rock in caving zone,the size distribution of broken rock obeyed the law of normal distribution and the loading pattern was designed to step loading. The test results indicated that along with the increase of axial load,the axial deformation of broken rock increases grad- ually,the residual bulking coefficient and porosity decrease gradually,which are more obvious in loading stage than that in constant load stage. In the early constant load stage,the axial deformation of broken rock grows rapidly,then tends to be slow and steady gradually,the relationship between strain and time meets the logarithmic relationship. In loading stage,the D-value of axial strain decreases first and then increases,in constant load stage,which is opposite. The bearing deformation of large-size broken rock isconsisted of two parts:instantaneous compression deformation and long-term deformation,which reflects in the position adjustment of rock particles,smaller original and newborn rock particles slide and fill the gap between larger rock particles. The 15 -20 mm particle size of broken rock is constant during the process of bearing deformation test,at the end of test,the content of broken rock ( particle size is smaller than 15 mm) increases by 16. 76% and the content of broken rock (particle size is greater than 20 mm particle size) decreases by 16. 76% . The research results are of significance for forecasting and controlling the movement and de- formation of overlying strata caused by mining.