Study On Hard Overburden Weakening And Crack Arrest And Water Control Of Deep Jurassic Coal Seams

The development of water-conducting fissure zones is a core issue in preventing and controlling water damage in coal mine roof strata and protecting water resources.Numerous studies have shown that mining methods such as limiting thickness,strip mining,and filling can effectively control the development of water-conducting fissure zones,but there are problems such as low extraction rates and high costs.Therefore,actively and effectively controlling the development of water-conducting fissure zones has become the key to water conservation mining.The development height of water flowing fracture decreases significantly with the decrease of roof integrity and strength,and the pre-splitting weakening technology can effectively weaken the strength and integrity of hard roof.Based on this,a new method of ’ weakening modification ’ of hard overburden to control the development of water flowing fracture zone is proposed.In this paper,the active control theory and method of water-conducting fracture zone are innovated.The weakening and modification effect of pre-splitting on hard rock strata and the control mechanism of different pre-splitting weakening layers on waterconducting fracture zone are studied by means of laboratory test,theoretical analysis,numerical simulation and field measurement.The main research results are as follows :(1)Using a combination of laboratory experiments and numerical simulation,the influence of different pre-crack morphologies on the mechanical properties and failure characteristics of hard sandstone was quantitatively analyzed.The results showed that pre-cracks significantly reduced the mechanical parameters such as strength of hard sandstone and changed the splitting failure mode of intact samples.Different crack morphologies had different degrees of weakening on hard sandstone samples.As the length and number of cracks increased,the strength gradually decreased,and as the crack angle and spacing increased,the strength first decreased and then increased.The stress was concentrated at the tip of the pre-crack,making it easy to generate wing cracks during loading.(2)Based on the theory of damage mechanics and fracture mechanics,the damage variables under the coupling condition of rock stress and cracks were calculated,and the "weakening modification" effect of pre-cracking on hard rock was studied.Precracking can transform hard rock towards lower strength rock types.Based on the "weakening modification" effect of hard overlying rocks,combined with mining response,the main characteristics of roof damage and the control mechanism of waterconducting fissure zone height in three different pre-cracking weakening layers(high,medium,and low)were explained.(3)Using the UDEC numerical simulation software,the crack-stopping effect of different weakening layers was numerically verified.The simulation results of roof damage morphology and characteristics,evolution of mining crack number,and dynamic evolution of roof damage height were comprehensively analyzed.The results showed that the control effect of different weakening layers on the development of water-conducting fissure zones from strong to weak was: low weakening layer,medium weakening layer,and high weakening layer.(4)The "two zones" development height under the roof cracking conditions of Mengcun Mine was measured on-site.The relationship between the crack-to-mining ratio under weakening and non-weakening conditions and the evolution law of the maximum water inflow of the working face with advancing distance were compared and analyzed.It was found that after weakening,the crack-to-mining ratio and the maximum water inflow of the working face decreased,and the growth rate was slow.The inhibitory effect of pre-cracking weakening on the development of waterconducting fissure zones was verified by engineering cases.