Deformation And Breaking Characteristics Of Compound Hard Roof And Its Influence On Pressure Behavior

Hard roof is a major disaster factor causing strong pressure behaviors and the working face with overlying compound hard roof is even more serious,which has an influence on the safety production.In recent years,some work on hard roof control by hydraulic fracturing have been reported.However,there are still many problems for hydraulic fracturing used in underground coal mine,such as affecting production,uncertain fracturing stratum for multi-layer hard roof,limited control area of single fracturing borehole,great difficulty in fracturing high hard roof,which influence the control effects of hard roof.Therefore,it is necessary to carry on systematic research on judgment method of compound hard roof,deformation and breaking laws and its influence on pressure behaviors,which is of great theoretical significance and research value for solving the problem of strong pressure behaviors caused by compound hard roof.Based on the occurrence characteristics of compound hard roof,theoretical analysis,numerical simulation,laboratory and field experiment method were used.Firstly,judgment method of compound hard roof based on elastic mechanics was established.Combining with similar model experiment,the influencing factors and deformation and breaking laws of compound hard roof were analyzed.On the basis of secondary development of FLAC,numerical simulations of influence of single stratified hard rock,multilayer stratified hard rock and different stratified thickness on the pressure behaviors were carried out.Unstability and breaking characteristics and advance bracing stress of working face with stratified compound hard roof were studied by similar model experiment.Finally,field test of compound hard roof control by hydraulic fracturing of surface borehole was carried out.The main conclusions are as follows:(1)Judgment method of compound hard roof was established and breaking span expression was obtained.According to the neutral axis position,the compound hard roof could be divided into tensile and compressive zone.The mechanical model of compound hard roof was established by elastic mechanics and stress field distribution equations of tensile and compressive zone were obtained.Then the judging criterion of compound hard roof was established,which provides evidence for target stratum determination of hydraulic fracturing.Based on the analysis of failure modes of compound hard roof,failure critical condition of compound hard roof was established and the breaking span expression was obtained.(2)Influence laws of span and stratum thickness of compound hard roof on the deformation and breaking characteristics were revealed.With the increase of span and decrease of stratum thickness,the shearing stress in the compound hard roof showed a gradually increasing trend basically,breaking span decreased and layered movement was more easily happen because of shear failure.The hard rock thickness had greater impact on the deformation and breaking characteristics than soft layer thickness and thickness decrease of two-layer hard rock was more effective than thickness decrease of single hard rock.(3)The influence laws of stratified compound hard roof on pressure behaviors were revealed.The pressure behaviors weakened after the compound hard roof was stratified and the support pressure decreased obviously.Two-layer stratified hard rock was more effective than single stratified hard rock and the decrease extent of support pressure was inversely proportional to stratified thickness.After two-layer hard rock was stratified,shearing layered movement would take palce,the breaking span shortened and the advance bracing stress of working face decreased.(4)Taking the 8101 working face in Tashan coal mine of Datong mining area as the research object,field test of compound hard roof control by hydraulic fracturing of surface borehole was carried out and the fracturing target stratum determination method,the influence laws of stratified compound hard roof on pressure behaviors were verified.The results showed that radius of fracturing influence area along the strike direction of working face was 107.8~137.2 m,which could be the basis for the distance determination of borehole location ahead of the working face.The hydraulic fracture had an obvious stratified effect on the compound hard roof.During the mining period of the fracturing influence area,there was no strong pressure behavior in the working face.The support pressure in the working face,the roadway deformation of the advance supporting section and the area with large roadway deformation were significantly reduced.