Theoretical analysis of anomalously low friction effect of block rock media based on vertical displacement difference

Due to the strong impact disturbance,the relative compaction of deep block rock media changes with time and results in anomalously low friction effect. The vertical displacement difference between blocks is a visual indicator of the relative compression between the blocks. A dynamic model of block rock media considering the overburden pres- sure was established in this study. The new formula of vertical displacement difference between rock blocks subjected to vertical impact loading was derived and compared with the experimental results. It verified the correctness of the theoretical formula. The influence of various factors,including the mass of the block,the structural surface damping, the frequency and amplitude of vertical impact loading,on the regularity of the displacement difference between the blocks was analyzed. The occurrence of the anomalously low friction effect was assessed by the change of the minimum vertical displacement difference. The anomalously low friction effect occurs easily with the smaller minimum vertical displacement difference. The results show that when the block model is subjected to vertical impact disturbance,the block will go through forced vibration and free vibration. Its vertical movement can be divided into four stages:super strong extrusion,strong extrusion,weak extrusion and mutual separation according to the change of the vertical dis- placement difference. The phase of mutual separation and the weak extrusion are prone to induce anomalously low fric- tion effects. The block mass and structural surface damping changes have a significant effect on the vertical displace- ment difference amplitude and period. The minimum value of the vertical displacement difference between blocks in- creases with the logarithm of the mass of the block,and increases linearly with the increase of the damping of the struc- tural surface between the blocks. The smaller the block mass and structural surface damping,the more significant the anomalous low friction effect. The vertical impact loading frequency and amplitude make the vertical displacement difference between blocks change significantly. The minimum value of the displacement difference decreases linearly with the increase of the vertical impact loading amplitude,and increases in logarithm with the increase of the vertical impact frequency. The study concludes that low frequency and strong impact disturbance are more prone to induce anomalously low friction typels rock burst.