Study On Viscoplastic Large Deformation Mechanism Of Surrounding Rock Of Deep Mining Roadway

Deep mining roadways are located in high in-situ stress environment and mostly arranged in a coal seam.The surrounding rock is weak,the mining disturbance is strong,causing the viscous flow related to time and plastic flow related to stress adjustment to occur in a large range of the surrounding rock,which seriously restricts the safe and efficient mining of deep coal mine.To reveal the viscoplastic large deformation mechanism of the surrounding rock of deep mining roadways,relying on the“Surrounding Rock Control and Intelligent Mining Technology of 1 000 m Deep Coal Mine”National Key Research and Development Program of“the 13th Five-Year Plan”of China,taking a mining roadway in No.13-1 coal seam of Kouzidong coal mine,Xinji mining area,China as the engineering background,theoretical analysis,laboratory tests,numerical simulations and on-site measurements were conducted to establish a visco-elastic-plastic constitutive model considering strain softening,dilatancy,rheological damage and directional difference of the surrounding rock degradation,and to study the mechanical characteristics of the surrounding rock,influences of in-situ stress and mining disturbance and relationship between the post-peak surrounding rock and support.The main conclusions are as follows:（1）Based on the effects of degradation,dilatancy and rheology of the surrounding rock of deep mining roadways,it was pointed out that the constitutive model should be able to describe the following 6 mechanical properties of the surrounding rock:strain softening,plastic dilatancy,three creep stages,stable and unstable creep,rheological damage and rheological dilatancy.A BVS constitutive model was established to describe the above 6 mechanical properties,and its three-dimensional incremental constitutive equations were derived based on incremental plastic flow theory and rheological theory.The main calculation steps of the BVS model are as follows:viscoelastic stress guess calculation→viscoplastic stress correction→instantaneous plastic stress correction.The BVS model was developed into a new constitutive model which can be invoked by FLAC^3D using C++language,and the calculation flow chart was provided.The model parameters were identified based on the experimental data of sandy mudstone of Kouzidong coal mine.The stress-strain response and mechanical properties of each element and multi-element under various loading conditions were checked strictly by comparing the laboratory test data,theoretical solutions and numerical calculation results.The results show that the BVS model can describe the above 6 mechanical properties of the surrounding rock of deep mining roadways.（2）Based on the BVS model,incremental plastic flow theory and rheological theory,the influences of strain softening,dilatancy and rheological damage on deformation and failure of the surrounding rock of deep roadways were analyzed,as well as the mutual effects and synergistic effects between the three mechanical properties.The results show that the surface displacement and plastic zone of the surrounding rock increase significantly due to strain softening;the surface displacement of the surrounding rock increases slightly by dilatancy;rheological damage causes the surface displacement and plastic zone of the surrounding rock to increase with time.Both strain softening and rheological damage increase the softening parameter,which accelerates each other’s process and reduce each other’s starting stress;dilatancy causes the softening parameter to increase,which accelerates the processes of strain softening and rheological damage.Considering any two mechanical properties above at the same time has a greater impact on the surrounding rock deformation and failure than the sum of the impacts of the two mechanical properties considered separately,that is,any two mechanical properties above have a synergistic effect,and then the three mechanical properties above also have a synergistic effect.（3）Considering both strain softening,dilatancy and rheological damage,the control effect of rock bolting on large deformation of the surrounding rock of deep roadway was analyzed from several different aspects.The control mechanism of a rock bolt system is as follows:improves the residual strength of the shallow part of the surrounding rock,strengthens its confinement to the deep part,inhibits strain softening,dilatancy and rheological damage,and then the surface displacement and plastic zone of the surrounding rock are significantly reduced.Therefore,a support should be timely after an excavation to reduce the degradation level of the surrounding rock and make full use of the self-support capacity of the surrounding rock.（4）Cubic fine sandstone samples with a side length of 100 mm were prepared for true triaxial tests considering the displacement constraint to simulate the degradation process of a surrounding rock element under the axial constraint,and one of the fractured samples was scanned by CT.It was found that the axial strength is higher than the in-plane strength at the post-peak stage,and all the cracks caused by in-plane failure are almost parallel to the axial direction.Combined with the numerical simulation results of inhomogeneous rock samples and on-site borehole TV images,the directional difference of the surrounding rock degradation was proposed and verified,and its mechanism is as follows:due to the axial constraint,shear cracks tend to slide and propagate in the plane cluster parallel to the axial direction,and these cracks have little effect on the axial strength,but can significantly reduce the in-plane strength,leading to the directional difference of the surrounding rock degradation.By setting the in-plane and axial cohesions unequal and the accumulation rates of the in-plane and axial softening parameters different,the different in-plane and axial yield functions and plastic potential functions were derived,and then the directional difference of the surrounding rock degradation was implemented.（5）The directional difference of the surrounding rock degradation was introduced into the BVS model by C++language as an optional module for the numerical simulation of a deep roadway.It was found that the degradation level of the surrounding rock is reduced as a result of considering the directional difference of the surrounding rock degradation,especially the decrease in the axial cohesion is significantly reduced,which delays the axial yield,and leads to the tangential stress lower than the axial stress.Ignoring the directional difference of the surrounding rock degradation results in an overestimation of the surface displacement,especially in the instantaneous elastoplastic stage.（6）Based on the BVS model and incremental plastic flow theory,the effects of in-situ stress were analyzed.It was found that with an increase in depth,the surrounding rock presents the following laws successively:no degradation occurs→degradation occurs after excavation for a period of time and tends to be stable→degradation occurs instantly after excavation and tends to be stable→degradation occurs continuously→degradation accelerates and its level rises.Under the condition that the axial in-situ stress is close to or greater than the lateral in-situ stress,the tangential stress,as maximum principal stress,decreases rapidly with the degradation of the post-peak surrounding rock,and then its curve intersects with the axial stress curve,inducing the three-directional plastic flow along an edge of the yield surface,which increases the radial strain and plastic shear strain of the surrounding rock,that is,the deformation increases and degradation accelerates.An increase in the angle between the axial and maximum horizontal principal stress directions of a roadway leads to increases in the roof subsidence and plastic zone depth,while the convergence and plastic zone depth of the two walls show a decreasing trend（non-monotonic）,therefore,the angle of 15～30°with consideration of the two walls is optimal.（7）Based on the BVS model,taking the original support section of the headgate of No.121302 working face of Kouzidong coal mine as a research object,a numerical simulation considering the mining process and time was carried out to analyze the effects of mining disturbance and time.The results show that after the roadway section is 20 m away from the heading,the tunneling disturbance is no longer evident,rheology gradually becomes a dominant factor,and the surrounding rock continues to degrade in a large range,which is characterized by small roof subsidence,dilatancy of the shoulder corner coal,inward migration of the whole wall and severe floor heave;the continuous heave of the floor mudstone drives the dilating bottom coal to squeeze into the roadway space,resulting in the severe floor heave;mining results in large scale degradation of the surrounding rock,and the nearly closure of the roadway section（on-site multiple roadway repairs are not considered）.The softening parameter of the floor induced by rheology accounting for≤16%in the whole mining process,but rheology is one of the main factors causing the stress adjustment,which indirectly leads to an increase in the softening parameter.The results obtained by the BVS model are consistent with the on-site measurement results,which reflects a good applicability of the model to deep mining roadway.（8）The viscoplastic large deformation mechanism of the surrounding rock of deep mining roadways:In high in-situ stress environment,mining disturbance induces an increase in deviational stress in the surrounding rock,which leads its mechanical properties such as strain softening,dilatancy,rheological damage and directional difference of degradation to emerge.Synergistic effect of these mechanical properties results in the plastic flow related to stress adjustment and time-dependent viscous flow in a large range of the surrounding rock,presenting a large deformation feature.