Under the condition of deep mining, anomalously low friction effect in deep block rock mass is related with dynamic disaster in deep rock mass. Based on practical situation of deep mines in China, the model of deep block rock mass considering overlying rock mass pressure was established. The analytical expression of relationship between block kinetic function and displacement-time function was derived with rock depth h parameter. The maximum rate of load drop is 1.06× 105 N/s when anomalously low friction effect generate at -800m coal and rock mass, and the wave periodicity of normal load in rock mass is 0.1 s. The results show that with the wave of normal load in rock mass, relatively blocks compacting drive to weaken. The anomalously low friction effect is easy to trigger. The rate of load drop sharply increase, and it can also be one of the characteristic index of anomalously low friction effect in rock mass.
At present, with the rapid development of national economy, more and more coal and metal mines are necessary to deep mining when shallow resources dried up gradually. With the increase of exploit depth, the geological condition of deep becomes more complicated comparison to the upper part and has the noticeable features of high geo-stress, high geo-thermal and high porewater pressure, as the result, engineering geologic hazards such as rock burst, coal and gas outburst and water burst lead to more serious and frequent[1–8].
The anomalously low friction effect of rock mass is the key scientific problem of deep mining[4]. Due to the critical characteristic of structure, deformation and high stress state, the physical and mechanics properties of deep rock mass are significantly different from shallow rock mass, and the deep rock mass has specific dynamic reaction phenomenon under the dynamic load[9] and the anomalously low friction effect of deep rock mass is the mechanics performance of the phenomenon. When the impulse power acting on the block rock mass system, the relative compaction degree between block rocks varies with time owing to the vibration of block mass, at some point, when contact surface between rocks relatively loose, the friction will reduce sharply, even reduce several times, so this is the effect of anomalously low friction in block media. Mines in China will enter deep mine in next twenty years, deep rock mass will be more broken, significant stratification and transformation of brittle to plasticity and ductility, and it will occur shear and tensile deformation, then shear fracture and tensile failure with the high geo-stress, high geo-thermal and high pore pressure and men mining activities, structural plane tends to be broken and intermittent and the rock mass gradually transform to block media, the friction of contact surface is made weaken and even extremely weaken, thus the anomalously low friction effect is generated, and balance constraints is destroyed, the instability of rock mass friction sliding occurs, finally the dynamic disasters such as rock burst, coal and gas burst and earthquake will be induced.
