Study On Flow And Slip Instability Mechanism In Rough Rock Fractures Under Hydro-mechanical Coupling Effects

In numerous underground resource mining engineering,such as mining engineering,fracturing engineering,oil&gas engineering and so on,the rock mass around the exploitation space occur in the stable geostatic stress and groundwater environment.However,as the mining proceeds,the mining-induced stress redistribution will cause the micro-cracks initiation,propagation,coalescence and interaction,and hence bring about the macro-fractures in the rock mass that will experience the shear-slip and change the permeability of rock mass significantly due to the stress perturbation caused by repeated mining.Meanwhile,the varying permeability also affects the water pressure and result in the effective stress alteration in the fractured rock mass,then causing the unbalance of the geostatic stress and groundwater environment.Besides,it should be noted,the intrinsic roughness characteristics of the rock fracture deeply determine its mechanical and hydraulic behaviors and will make the above hydro-mechanical(HM)coupling process more complicated.Thus,the knowledge of the HM behaviors in mining-induced rough fractures is of great significance to the controlling of the imbalance of the geostatic stress and groundwater environment due to the large-scale resource mining.Taking the "Flow and slip instability mechanism in rough rock fractures under hydro-mechanical coupling effects" as the subject,a large amount of experimental research were conducted in this paper.Based on these test results,a coupled hydr-mechanical model of the rough fracture was built and further employed in the simulations of the coal mining-caused underground water damage and rock mass instability around the excavation.The main contents are as follows;1)On the basis of the 3D scanned morphology data,the geometric characteristics of the rough fracture were characterized by using the newly proposed algorithm of calculating the three-dimensional fracture apertures.Research results show that the surface and inner rough characteristics provide a full description of the geometric characteristics of the rough fracture.Apertures and asperities are complementary aspects of the same fracture geometry and both of them are intrinsically heterogeneous.The isolated void spaces existing between two rough surfaces have little contribution to the flow seepage,and the effective percolation network is largely comprised of interconnected void spaces in the fracture.The discrete points of contact are connected through the rock matrix as a separate stress network that controls the mechanical deformation of a fracture.These two networks combine to provide a full description of the fracture geometry,which is the nexus between the hydraulic and mechanical properties of a fracture.2)A series of high-precision hydraulic tests for rough single fractured sandstone samples with three types of grain sizes were conducted in a tri-axial cell under a variety of confining stresses.Research results show that the permeabilities of all fractured samples decrease nonlinearly with the increase of the confining stress,and the decrease tendency is dependent upon the geometric characteristics of the rough fracture.Three FGC(fracture geometric characteristics)representation parameters,including the effective mechanical aperture e'm(average apertures of interconnected void spaces),the contact ratio ?(the ratio of contact area over total area of fracture surface),and the relative fractal dimension D?*(representation of the heterogeneity level of the interconnected void spaces),were proposed and their evolution with increasing confining stresses were calculated.A so-called FGC model was established to reflect the quantitative relationship between FGC representation parameters and fracture permeability k in the deformable fracture:K=e'm2/12(1-1.1?)8(1+2/D*?)6/5In addition,an empirical model relating the interior geometric characteristics of fracture,which is represented by the fractal dimension D of the heterogeneous distribution of the interconnected void areas,with the fracture apertures(initial hydraulic aperture e0 and hydraulic aperture eh,),is established based on above experimental results:D=p(e0/eh)qThe model relating the non-Darcy coefficient ? with the fracture geometries is modified to incorporate the effect of the interior geometric characteristics of fractures.On this basis,the variation of non-linear coefficient B in the Forchheimer's equation and the critical Reynolds number Re,can also be associated with fracture geometric characteristics.The proposed models are validated by the experimental data and would be helpful to characterize and predict the non-Darcy flow behavior in rough fractures under various confining stresses;B=mp/e3hW2(e0/eh)n(logc?/eh)d Re=12/m(1-E)(e0/eh)n(logc?/eh)d3)On the basis of the established asperities-voids-water pressure conceptual model of the rough fracture,an effective stress coefficient a is introduced through the stress balance analysis to describe the effective stress characteristics of rough rock fractures.The quantitative relationship between the coefficient a and the confining stress ?3 and water pressure p is investigated by using the hydraulic test results of two single fractured sandstones.The results show that the ratio of interconnected voids in the fracture,been assumed as the effective stress coefficient a,is capable of describing the variation of hydro-mechanical coupling behaviors of rough fractures well.The empirical model?=a+bexp(-c?3+dp)shows a good representation of the changes of the coefficient a with the confining stress and the water pressure.4)Based on a realistic tensile rock fracture,a series of flow simulations under increasing compression stress was conducted through solving the Navier-Stokes equations.Modelling results show that the flow velocity,water pressure and streamlines distribute non-uniformly in the rough fracture and such heterogeneities will become more pronounced with the increase of the compression stress,which is largely ascribed to the increase in the heterogeneity of the distributions of contact areas and void spaces under high stress.Besides,the decrease of the water pressure along the flow direction shows larger nonlinearity with the increase of the applied stress,mainly due to the high resistance of the contact regions to the water pressure.Besides,the streamlines will become much channelized with obvious tortuosity,indicating that the preferential paths for fluid flow easily form within the large aperture and the locations of the contact areas are the main cause of the deviation of linear streamlines and may bring the nonlinear flow behaviors in fractures.5)A new laboratory test method for the coupled shear-flow behaviors of rough-walled rock fractures was developed and applied to a series of single tensile sandstone fractured samples under different confining stresses and injection water pressures.Research results show that both the peak shear strength and residual shear strength increase with the confining stress,while the variations of normal deformation were on the opposite.With the increase of shear displacement,the fracture permeability presents the four-stage change of "decreasing-increasing-decreasing-stabilizing",which is largely related to the mechanical behaviors of the fracture asperities.The decrease of permeability at the residual shear stage is attributed to the asperities degradation and gouge production resulted from the relative sliding of the rough fractures,and such effect can be represented by the plastic work of the shear stresses.Besides,at each post-peak shear stage,the increasing water pressure will bring the sudden instability of the rough fracture and result in the non-linear increase of the sliding displacement.In addition,the roughness characteristics of fracture surfaces decreased significantly after the shear-flow test,mainly ascribed to the shear-caused surface asperity degradations,which can be predicted by using the AE numbers.6)The complete stress-strain tests under different combinations of confining stresses and injection water pressures were conducted to investigate the damage-induced permeability variations and instability of the rock mass under the coupled HM effects.Research results show that the water pressure will decrease the deformation and strength parameters of the rock significantly,and also weaken the residual strength and accelerate the sliding of the fractured rock mass at post-peak stages.With the axial stress increases,the permeability of the rock goes through four stages-"decreasing-increasing-decreasing—stabilizing",which is similar to the variations in the shear-flow test of the rough fracture.The permeability k increases exponentially with the damage variable D in the pre-peak stage,and is largely related to the rough characteristics and shear-caused degradation of the asperities in the post-peak stages.The sliding instability at the post-peak stage dependents upon the confining stress,water pressure,rough characteristics and fracture inclination,which can be expressed as the relation F=f(?1,?3,Pf,?,?).7)Based on the above experimental results,a coupled hydro-mechanical model for the rough fracture was built and verified with the specific example built with the discrete element software UDEC.The simulation results show that the newly proposed model can characterize the permeability evolution and instability characteristics of the rough fracture under different test conditions well,including different normal stress,the dynamic coupled HM conditions,the non-dynamic coupled HM conditions and different scales of the fractured samples.8)The coal mining-caused underground water damage and rock mass instability around the excavation were simulated by use of the newly proposed hydro-mechanical model.The results show that the mining-induced stress transition from the compressive stress to tensile stress is easy to cause the instability of fractured rock mass.The vertical permeability in the fractured rock mass increases while the horizontal permeability decreases,which are largely attributed to the shear dilation of the longitudinal fractures with high dip and the closure of the horizontal fractures due to the increase of the vertical stress and the decrease of the horizontal stress,respectively.The sliding instability of fractured rock mass is related to the development of the external stress,hydraulic pressure and the fracture inclination.The fractured rock mass with more high-dip fractures is prone to slip than that with more horizontal fractures under the mining-induced stress alterations.