Comprehensive Evaluation And Classification Of Rock Mass Mechanical Strength Of Coal-measure Strata

The Longtan Formation in the Dahebian block of the Shuicheng mining area in Guizhou Province has thick coal seams and abundant coal-measure gas resources,and has good development prospects.In this thesis,taking the Dahebian block as a research example,the uniaxial compressive strength,elastic modulus,cohesion and tensile strength of rocks in different layers were studied through coring of parametric wells and physical and mechanical tests of columnar rocks.In conjunction with coalbed methane geological exploration,comprehensive geophysical logging interpretation,and indoor physical and mechanical tests,we have carried out research on the rock mechanical characteristics and rock mass mechanical strength of the Longtan Formation coal measure strata,and conducted a comprehensive evaluation of rock mass mechanical strength.It provides the basis for the research on the mechanism of coal-measure gas co-production and co-production controlled by framework construction and rock mechanics.The thesis research has achieved the following main results and understandings:1)The failure characteristics of sandstone,siltstone,mudstone and basalt of the Longtan Formation coal measure strata under confining pressure of 0MPa,5MPa,10 MPa,15MPa,20 MPa and 25 MPa were studied.When the confining pressure(<10MPa),the failure fracture is a small-angle vertical fracture,and some mudstones undergo shear failure,which is related to the development of natural fractures;when the confining pressure is high(>10MPa),the failure is shear failure,and the fracture angle increases with the confining pressure.increasing trend.1)The mean uniaxial compressive strengths of fine sandstone,siltstone,silty mudstone,mudstone and basalt are 83.01,118.93,70.32,68.14 and 175.29 MPa,respectively,and the mean elastic moduli are 10.21,11.19,8.15,10.21 and 17.53,respectively GPa.The mechanical properties of rocks in different layers are quite different,and the uniaxial compressive strength and elastic modulus of the same layer of the same lithology(mudstone)are also different to some extent.The confining pressure was set according to the variation range of rock in-situ stress of Longtan Formation coal measure strata,and triaxial compression tests of rocks under different confining pressures were carried out.The increasing range of the influence of the rock itself gradually decreases after the confining pressure of 15 MPa.3)The uniaxial linear and multivariate linear prediction models of uniaxial compressive strength and elastic modulus and logging parameters are established.Comparing the actual value with the predicted value,it is found that the prediction effect of the multiple linear regression model is significantly better than that of the univariate linear prediction model.It is feasible and highly accurate to quantitatively predict the physical and mechanical properties of coal-measure formation rocks by using logging interpretation parameters combined with multiple linear regression models.4)The rock mechanical parameters such as uniaxial compressive strength,modulus of elasticity,angle of internal friction and tensile strength,and rock formation parameters such as integrity coefficient and rock thickness are selected as evaluation indexes.In this way,the evaluation system of rock mechanical strength indexes of coal strata is established.Based on the theory of fuzzy mathematics,a comprehensive index evaluation model is established,and a mechanical strength classification scheme suitable for coal-measure strata rock mass is constructed.The coal measure strata of the Longtan Formation are divided into grades I～V.Specifically,basalt is grade I,and the rock strength is relatively large;fine sandstone is divided into grades I～III,siltstone is divided into grades II～III,and the rock mass is relatively compact and complete;while the mudstone rock mass is complex in structure and has many fractures.Covers grades Ⅰ to Ⅳ.