Mechanism And Control Of Strong Ground Pressure Induced By Instability Of High-position Hard Roof

With the increase of mining depth and development intensity,the substantial migration of overburden increases sharply after fully mechanized top-coal caving mining in extra-thick coal seam.The strong ground pressure behavior(SGPB)induced b instability of high-position hard roof(HHR)above the basic roof is highlighted,which serious threat to mine safety and efficient production.Traditional methods of controlling hard roofs have a limited scope of action and equipment performance,and the control range is limited.What’s more,the selection of control strata is blind,so it is difficult to effectively solve SGPB induced by HHR.Focusing on the difficult problem of controlling SGPB induced by HHR,the overburden structure of HHR and the mechanism of SGPB induced by HHR were studied with a series of comprehensive methods.Next,the mechanism of weakening HHR to release SGPB by ground hydraulic was discussed.Then,the selection of key fracturing parameters and fracturing effect are analyzed in detail.Main research achievements are as follows:(1)Expounding the mechanism of ground pressure behavior induced by instability of overlying strata structure with “HHR-support”.Based on studying the evolution of overlying strata fracture and SGPB characteristics,it was clear that the HHRs was the main control strata of SGPB.A numerical simulation method for the whole process of fully mechanized top-coal caving mining in extra-thick coal seam was developed,through which the influences of mining height,compressive strength,horizon and thickness of HHR on the characteristics of ground pressure behavior(GPB)were analyzed.The evolution characteristics of overlying strata structure with HHR and its pressure effects were analyzed by simulation study.The overlying strata structure with“HHR-support” was established.The criteria of GPB induced by instability of overlying strata structure with “HHR-support” were discussed.(2)Mechanism of ground hydraulic releaseing the SGPB induced by HHR in large-space stope was revealed.Large-scale ground hydraulic test of original rock specimen with hard roof(1420×530×420 mm)was carried out by using the developed large-scale 3-D physical simulation test system,through which the spatial expansion pattern and distribution characteristics of hydraulic cracks in hard roof were analyzed.What’s more,the formation mechanism of large-scale hydraulic cracks in hard roof was studied.The mechanical characteristics and strength deterioration mechanism of fractured rock mass under the stress path of fully mechanized top-coal caving mining in extra-thick coal seam were revealed.The influence of strength deterioration of HHR on SGPB was analyzed by simulation.A mechanical model of the “coal wall-hydraulic support-gangue” support system after fracturing HHR was established,which revealed the influence mechanism of fracturing HHR on GPB.These provided theoretical basis for fracturing HHR by ground fracture.(3)The determination criteria of fracturing horizon and fracturing thickness for weakening hard roof by surface drilling fracturing were established.The technical parameters of weakening hard roof by surface drilling hydraulic fracturing were optimized.Then,combined with microseismic monitoring technology of hydraulic fracture,an innovative method of fracturing HHRs by vertical-well separated fracturing to release SGPB was constructed.Field engineering test was carried out in Tashan Coal Mine,which effectively solved the SGPB induced by HHR.The control range of ground fracture was wide,with hydraulic fractures as long as 218-250 m and as wide as 20-50 m.The average support load,the average periodic weighting step,the opening rate of support safety valve,the average deformation of roadway height and width were reduced by 16.9%,15%,37%,46.5% and 48.9%,respectively.