| Hard roof controlling has been a key and difficult issue for coal mine strata control in China.With the increase of depth and intensity of coal mining,the problems of hard roof accidents and disasters become more and more prominent associated with efficient coal mining.The high disturbance level of overlying strata in thick coal seam mining aggravates the disturbance range and degree of roof fracture,which makes strong pressure behavior of the stope exhabit unique characteristics.Under these conditions,it is an important guarantee for safe and efficient mining of hard roof coal seam to carry out the research on the efficient roof control technology and mechanism.Based on these,a new fracturing technology of confined pressure blasting with water-filled in borehole surrounding rock is proposed.By virtue of the superior load transfer performance of water medium in borehole and infiltration-modified rock mass,the full utilization of high explosion energy is realized.Various methods are employed for researching the law of wave load transfer and the process of detonation gas expansion in multi-medium layer and reveal the high-efficiency rock fracturing mechanism of confined pressure blasting with water-filled in boreholes,which provides a scientific basis for determining reasonable blasting pre-splitting parameters of hard roof blasting in coal mines.The results have been achieved in the paper as follows:(1)Under the condition of high strain rate loading,the stress-strain curves of natural and infiltrated rock mass are similar.With the increase of infiltration time and water pressure,the strength of rock increases exponentially with the rise of strain rate.Under the condition of low strain rate,the relationship between compressive/tensile strength and strain rate of natural and infiltrated rock satisfies exponential and logarithmic relationship respectively,and meanwhile the strength of rock decreases with the rise of infiltration time and water pressure.A comprehensive study points out that the brittleness and strength of infiltrated rock under high strain rate increase,but the strength tends to weaken under low strain rate.(2)Based on the analysis on detonation parameters of condensate explosives,the wave load transfer law in water medium layer and rock fracturing mechanism are discussed,and the mechanical model of wave load transfer and fracturing of confined pressure water layer is established,revealing the mechanism of high-efficiency rock fracturing under the condition of confined pressure with water-filled in borehole.On the basis of the analysis on rock fracturing by explosive wave,the mechanism of high efficiency fracturing enhancement of detonation gas combined with water medium is further expounded.A comprehensive study points out that the existence of water medium in boreholes is favorable for the full utilization of wave load transfer efficiency.By properly increasing the pressure of the water medium in boreholes and improving the dynamic mechanical properties of the infiltration rock mass,it is beneficial to the realization of the high-efficiency fracturing effect of the rock mass.(3)The evolution law of stress and fracture in surrounding rock of large-scale engineering rock mass under confined pressure blasting is studied.The influence of factors such as the thickness of confined water layer in borehole,charge quantity,confined pressure of water medium and existing condition of empty hole on blasting fracturing of infiltration rock mass is discussed,obtaining the main controlling factors affecting the high-efficiency blasting fracturing effect of rock mass.Meanwhile,combined with the specific mining conditions of thick coal seam,reasonable parameters of confined pressure blasting for the hard roof are determined.The field implementation of deep-hole confined pressure blasting with water-filled technology has achieved remarkable effect on the hard roof pre-splitting and stope pressure relief. |
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