Research On Theroy And Application Of Hydraulic Fracture Weakening For Coal-Rock Mass

Hydraulic fracture weakening technology for coal-rock mass is an efficient means to improve hard thickness top coal(high gas) caving property,deal with hard roof,prevent rook burst,increase coal seam permeability and avoid coal-gas outburst,which is safeguard technology for safety and high efficiency mining of coal mine. Based on the engineering background of coal-rock mass strength weakening (stucture reform) , research methods including experiment research,theory analysis,numerical simulation and test on site have been used to study hydraulic fracture weakening theory and technology of coal-rock mass systematically,which has important theoretic meaning and widespread engineering application prospect.True triaxial hydraulic fracture experiment system with 4000kN anchorage force was developed,which can perform the stress field simulation in-situ with the greatest size 500×500×500mm in true triaxial test. Designed boil-water loading converter has increasing pressure function. Coal mechanical property and influence of water content have been analyzed. For similar form with complete stress-strain curve of (coal) rock mass,material mixture ratio of analogue coal(coal: cement: plaster) and sanded cement grout has been obtained.Crack propagation law of hydraulic fracture in coal-rock mass has been studied. Hydraulic fracture simulation experiment with greatest size in true triaxial test indicates that front hydraulic fracture shape is elliptic and its center is the part of drillhole fracture. Ratio a/b of major axis to minor axis in hydraulic fracture front elliptic curve equation (x²)/(a²)+ (y²)/(b²) = 1 keeps unchanged. Hydraulic fracture crack is expands vertically to the direction of minimum principal stress. The closer medium principal stress is to minimum principal stress,the more prone drill hole bottom hydraulic fracture is to deflect vertically to the direction of medium principal stress;its propagating direction is parallel to vector surface ofσ₂ andσ₃. Bifurcation phenomenon has been found in the hydraulic fracture of drillhole bottom. Strike of hydraulic fracture failure surface is vertical of bothσ₂ andσ₃ after bifurcation.Besides hydraulic fracture technology for coal-rock mass leads to main hydraulic crack to produce main failure surface, the seepage water pressure forms seepage water wedge in coal both sides of main hydraulic crack. Then, fissure and bedding plane will open and propagate. Meanwhile, because of shortcoming of fissure in coal mass, hydraulic fracture failure surface roughness of coal mass is larger than common rock material obviously. Consideration of pore water pressure (hydraulic gradient)caused by seepage filtration from water to rock in the process of hydraulic fracture, expansion net water pressure in crack-tip is equal to crack-tip water pressure subtract filtration water pressure and original rock pore water pressure. Hydraulic decay laws between drillhole and hydraulic crack have been analyzed by fluid mechanics.When the angle between initial fissure and minimum principal stress is more than some value and rock bridge between hydraulic wing crack and hydraulic main crack is shorter,hydraulic crack along the direction of initial fissure and wing crack will keep on propagating constantly.Research on microscopic structure destroy of fissured rock mass has been made under solid and liquid coupling condition. According to coal seam crack distribution laws, minimum fissure water pressure and hydraulic wing crack propagation length of initial fissure in the rock mass have been gained by fracture mechanics. Structure reformation form of hydraulic fracture weakening has been simulated by RFPA software under the condition of a group of initial fissure. According to energy dissipation damage theory,relation between microscopic structure failure of fissured rock mass and macroscopic mechanical property weaken has been obtained under the precondition of rock block isotropy.Control technology of rock mass hydraulic fracture weakening has been made in this paper. With the advantages both hydraulic slotting and hydraulic fracture, guide orientation technology of beforehand hydraulic slotting has been put forward. Because of superposed influence of drillhole and its water pressure, propagating direction of hydraulic fracture is controlled by combination multi drillholes with its liquid injection mode. Combining the advantage of hydraulic blasting and hydraulic fracture,structure reforming technology of hydraulic blasting fracture for coal-rock mass has been presented. The experiment result shows that crack coal-rock mass will generate multiple water pressure main fracture and fracture zone along the water pressure failure surface. Under pressure of explosion,coal joint fissure will open and propagate so that structure plane cohesion stress decreases. As a result, coal cutting becomes serious and whole mechanical property of rock mass has been weakened. It has been proved to be a useful method to increase water pressure crack numbers and coal seam permeability.Research has been made on temporal spatial relation of hydraulic fracture weakening for mining coal-rock mass. Tests have been carried out to study the relation between coal natural water absorption and time,and to analyze the water absorption difference between dry coal sample and natural one. By seepage initial hydraulic slope degree and radial hydraulic fracture slope degree decay laws, seepage radius of injecting water has been acquired by hydraulic fracture weakening. Base on moving abutment pressure distribution and failure character of surrounding rock, rational distance advancing wall of hydraulic fracture weakening has been obtained.Weakening tests of hard thickness top coal with high gas in situ have shown that,hydraulic fracture weakens the whole mechanical property of coal mass and reduces the size of top coal broken and increases recovery ratio of top coal by 23%. It can release gas in advance , reduce gas concentration, reduce 21 % of dust concentration,and suppress coal spontaneous combustion.