Theoretical Analysis And Experimental Study On Anomalously-low Friction Effect Of Block Rock Mass

With the deep mining,the rock burst of coal mines present new features,such as the deep rock mass tends to be broken into a block medium.The anomalously-low friction effect of the block rock mass is easily occur under the high ground stress and strong mining disturbance.So,it will induces the disaster as rock burst.Using the block rock mass as the research object,through the combination of theoretical analysis and experimental research,the anomalously-low friction effect of the deep block rock mass is studied.The research conclusion can provides an important scientific reference for understanding the deep rock burst's mechanism of coal mine.(1)Considering the characteristics of deep rock mass,the anomalously-low friction effect dynamic model of block rock mass with overburden pressure is established.The new vertical displacement difference formula and normal force formula between blocks are derived.It is the first time that the minimum normal force between blocks is proposed as a new indicator to judge the anomalously-low friction effect.The influence of block mass,the vertical impact amplitude and frequency,the structural plane stiffness and damping on the displacement difference between the blocks and the normal force are analyzed.The results show that the block motion can be divided into four stages: super extrusion,strong extrusion,weak extrusion and relative detachment stage according to its vertical displacement difference or normal force variation.The anomalously-low friction effect occurs easily when the blocks are relative detachment stage and weak extrusion stage.The block mass,stiffness and damping of structural surface changes have a significant influence on the vertical displacement difference amplitude,normal force amplitude and period.The anomalously-low friction effect is more likely to occur when the model had smaller block mass and inter-blocks damping,and larger stiffness of structural surface.The vertical impact frequency and amplitude also have a significant effect on the vertical displacement difference between blocks.The study show that the low frequency and strong impact disturbances are more likely to induce the anomalously-low friction,and then lead to dynamic disaster such as rock burst.Through the analysis of the normal force between different blocks,it is found that the contact surface position which is the most prone to the anomalously-low friction effect is the three contact faces closest to the vertical impact source.(2)The new horizontal displacement formula of working block is derived.When the vertical impact and horizontal impact amplitude are determined,the horizontal motion of the working block has a specific delay time and horizontal impact frequency range.The horizontal displacement of the working block exhibits periodic variation with the change of delay time and horizontal impact frequency.It is found that the starting time of the horizontal movement of the working block lags behind the horizontal impact action moment,there is the lag time.The work block will undergo the variable acceleration and deceleration motion in turn,and finally it is in a static state.The change of normal force between blocks is the premise of the occurrence of anomalously-low friction effect.The horizontal disturbance is the direct reason of the horizontal displacement of the working block.(3)The occurrence law of anomalously-low friction effect of block rock mass under vertical impact disturbance was analyzed experimentally.The experimental results were compared with the theoretical conclusions to verify the correctness of the theoretical model.The dangerous contact surface and blocks for the anomalously-low friction effect were confirmed.Considering the effects of breach in roof,engineering blasting and mechanical vibration,the anomalously-low friction effect experiment of block rock mass with vertical impact loading disturbance is designed.Through the experiment,the key structural planes of anomalously-low friction effect are obtained,and the influence law about the block's mass,the vertical impact loading energy and the overburden pressure on the anomalously-low friction rock burst are obtained.The results show that the higher vertical impact energy more likely to cause anomalously-low friction effect.The first,second and third contact surfaces are susceptible to the development of key structural planes that the anomalously-low friction effect easily occur.At appropriate circumstances,due to the reflection phenomenon of stress waves,the contact surface that is farther from the impact source but closer to the reflection source is also a dangerous contact surface.Experiment have shown that the block's size has a significant influence on the anomalously-low friction effect,and its influence is more obvious than the mass of the block.The block that satisfy the specific size of the deep part is more likely to induce the rock burst.