Jointed Coal Mass Model Reconstruction And Its Mechanical Response Characteristics

Coal mass roadway instability is the macroscopic mechanical response of coal mass in one or more stress states.The mechanical response characteristics of coal mass as a combination of discontinuities and coal matrix are significantly affected by the discontinuities.At present,the research on coal is mainly focused on coal block,while the study on the mechanical response characteristics of coal mass is less.In order to obtain the mechanical response characteristics of coal mass,the distribution characteristics of coal mass discontinuities were extracted,the synthetic coal mass model was constructed,by the method of field measurement,probability statistics,laboratory test and numerical simulation,based on which,numerical tests of different loading forms were carried out to analyze the characteristics of strength deformation and fracture development of coal mass and discuss the influence mechanism of discontinuities to coal mass mechanics response.The main contents and conclusions are as follows:(1)By observing the spatial relationship between bedding planes and cleats of different coal mas samples,the distribution characteristics of coal mass discontinuities were summarized.The geometrical parameters of coal mass discontinuities were measured by method of sampling line and sampling window,and the probability distribution of the geometrical parameters was calculated,based on which the coal mass DFN(Discrete Fracture Network)was constructed and checked by trace distribution and RQD(Rock Quality Destigation).The REV(Representative Elementary Volume)of coal mass was determined by discontinuities intensity index.The study shows:(1)There are five typical characteristics of coal mass discontinuities distribution: i The shape of the coal mass discontinuity is approximately rectangular.ii The coal is cut into layers of different thickness by parallel bedding planes.iii Face cleats and butt cleats are approximately vertical in the coal mass layer,which are both vertical to bedding planes approximately.iv Face cleats are through one or more layers in the height direction,ending at bedding planes.Butt cleats ends at bedding planes in the height direction,while does not exceed face cleats generally in the length direction.v Face cleats are not strictly parallel to each other,but rarely crossed.Butt cleats are the same.(2)The coal mass DFN are close to the actual at both sides of trace distribution and RQD,which indicate that the construction method of coal mass DFN is feasible and the coal DFN is reasonable.(3)The REV of the studied coal is 1.0×1.0×2.0 m.(2)Introducing the principle of SRM(Synthetic Rock Mass)method,and determining the contact models in coal mass SRM model and the calibration method of microscopic parameters.The microscopic parameters of the coal mass SRM model are calibrated by matching the coal block uniaxial compression and the coal mass discontinuity direct shear laboratory test and the corresponding numerical test.Based on which,the coal mass SRM model with REV size was constructed and the damage description indexes and fracture morphology description method of the coal mass SRM model were put forward.(3)The numerical model of uniaxial compression was constructed base on the coal mass SRM model,by which the numerical analysis of the mechanical response of coal mass under different loading azimuth conditions was carried out,and the influence of the discontinuities on the mechanical response characteristics of uniaxial compression was discussed.The study shows:(1)The discontinuities would weaken the uniaxial compressive strength and elastic modulus and increase the deformation degree of coal mass,and promote the mechanical properties of coal mass to show anisotropy.(2)The discontinuities control the failure modes of coal mass in uniaxial compression condition.The coal mass show split damage in loding direction.In case of single-axis loading verticle to bedding planes,the number of splitting fracture is small.In case of single-axis loading verticle to face cleats,the coal mass show plate damage,while in case of singleaxis loading verticle to butt cleats,the coal mass show strip damage.(3)In condition of uniaxial compression,there is a positive correlation between the coal mass strength and the failure proportion of coal matrix,and there is a noncritical positive correlation between coal mas deformation and the failure proportion of coal mass DFN.(4)The numerical model of triaxial compression was constructed based on the coal mass SRM model,by which the mechanical response characteristics of coal mass under different loading conditions and different confining pressures,and the sensitivity of coal mass residual strength to confining pressure were studied,based on which,the effect of discontinuities and confining pressure on the mechanical response characteristics of triaxial compression of coal mass were summarized.The study shows:(1)Under condition of triaxial compression and low confine pressure(1 MPa),the discontinuities weaken the strength parameters and increase the degree of deformation of coal mass.The discontinuities orientation only affects the shape of fracture and does not affect the mechanical properties of coal mass.The confine pressure strengthen the strength parameters and weaken the anisotropy of mechanical properties of coal mass.(2)The mechanical response characteristics of the triaxial compression of coal mass has confining pressure effect: With the increase of confining pressure,the compressive strength and residual strength of coal mass increase obviously.The elastic limit of the coal mass increases obviously and the elastic modulus is basically stable.The post-peak stress decrease of the coal mass is reduced and even disappeared.The mechanical properties of coal appear brittle-ductile transition.The macroscopic failure mode of the coal mass is changed from tension failure to shear failure.The proportion of coal matrix damage increases,while the proportion of DFN damage decreases.The impact of the discontinuities to mechanical properties of coal mass would weaken,and even disappear.(3)The brittle-ductile transition pressure is 10 MPa for the studied coal mass.(4)The residual strength of coal mass is extremely sensitive to confining pressure.(5)The numerical model of direct shear was constructed base on the coal mass SRM model,by which the shear mechanical response characteristics of coal mass under different shear azimuth and different normal stress were studied respectively,and the influence of discontiuities on shear mechanical response characteristics of coal mass was summarized.The study shows:(1)Under condition of low confine pressure(1 MPa),the discontinuities weakens the shear strength parameters,reduces the shear dilation deformation,controls the shearing failure mode of the coal mass,and promotes the shear mechanical properties of the coal mass to show anisotropy.(2)Under certain condition of normal stress,there is a positive correlation between the shear strength,the shear dilation deformation and the proportion of coal matrix damage,and a negative correlation with the proportion of coal mass DFN damage.(3)The shear mechanical response characteristics of coal mass have significant normal stress effect: With the increase of normal stress,the peak shear strength and residual shear strength of coal mass are obviously improved.The shear stiffness of coal mass is basically stable.The macroscopic failure mode of the coal matrix changes from tension cracking to shear cracking.The proportion of coal matrix damage increases,while the proportion of DFN damage decreases.The leading capacity of the discontinuities for shear failure would weaken.(6)By contrasting the mechanical response characteristics of coal mass under different stress conditions and analyzing the failure mode and fracture formation of coal mass,the influence mechanism of discontinuities on uniaxial compression,triaxial compression and direct shear mechanics response of coal mass was obtained.(1)The influence mechanism of discontinuities on uniaxial compression mechanical response of coal massUnder the effect of compressive stress,the discontinuities parallel to the loading direction would slides and expand,transforming to DFN fracture.The matrix cracks which grow and develop at the end of DFN fracture,form compression-induced tension fractures with the DFN fracture.For coal mass containing parallel loading direction and non-penetrating dicontinuities,a large number of compression-induced tension fractures are formed,which are connected to each other to form splitting fractures.For coal mass that contains parallel loading direction and through dicontinuities,the splitting fractures consist essentially of the through DFN fractures.(2)The influence mechanism of discontinuities on triaxial compression mechanical response of coal massUnder condition of low confine pressure,the formation mechanism of DFN fracture is still the same as that of uniaxial compression.However,the existence of confining pressure increases the difficulty of converting the discontinuities into DFN fractures,so the discontinuities in the parallel loading direction under vertical confining pressure can only be transformed into small-scale and small-sized DFN fractures.The number and distribution of small-sized DFN fracutres will change the shear stress distribution in coal mass,which will affect the fracture direction and shape.However,due to the small number and size of DFN fractures,not enough to dominate the damage of coal mass,coal mass damage is still dominated by the coal matrix damage caused by the shear stress.Under condition of triaxial compression,the confining pressure and the discontinuities control the fracture formation in coal mass jointly,but with the change of the confining pressure,the dominant ability of the frature formation is shifting.When there is no confining pressure,the fracture formation is dominated by the the discontinuities.When there is low confining pressure,fracture formation is dominated by shear stress and discontinuities jointly.When there is middle and high confining pressure,the fracture formation is dominated by the shear stress.(3)The influence mechanism of discontinuities on shear mechanical response of coal massUnder the effect of non-linear and opponent thrust force,the discontinuities near the designed shear plane and parallel to the shear direction is more easier to slide and expand,so it has stronger DFN fracutre forming ability than cases of uniaxial compression and triaxial compression.The matrix cracks which gorw and develop at the end of the DFN fractures,would connecte to the adjacent discontinuities or go through the coal mass,forming discontinuity-oriented shear failure surface.With the increase of normal stress,the difficulty of discontinuities converting into DFN fractures increases,so the dominant ability of discontinuities for forming shear failure surface would weaken.