Theoretical Study On Stability Evaluation Of Goaf Overburden Under Surface Load

In China,the contradiction between the widespread distribution of coal mining subsidence areas and the tightness of land used for surface engineering construction is increasingly prominent.The use of land in coal mining subsidence areas is an effective way to alleviate this contradiction.At present,many engineering constructions have to be built above the goaf,and engineering construction above the goaf is inevitable.The engineering construction of the surface above the goaf area is a complex system problem.The stability evaluation of the overburden in the goaf area is a prerequisite for the construction of the surface engineering above the goaf area.Although there are many methods for evaluating the stability of the overburden in the goaf area,other key scientific issues are still lack of in-depth research,such as,the method for detecting the structure of the overburden rock in the goaf area,the mechanical criteria for the failure of the goaf area under the load,and the combination of the failure of the goaf area and the prediction of surface subsidence,et al.In view of the above problems,with the help of physical detection methods and similar/numerical simulation research methods,the mine rock strata control theory?voussoir beam,key strata,etc.?is introduced to detect the overburden rock mass structure in the goaf area and the failure of the goaf area under load.Stability criteria and parameters for predicting residual settlement on the ground have been studied.The key research results were obtained as follows:?1?Taking Pingshuo Anjialing coal mine and Huaibei Xiangcheng coal mine as examples,two geophysical methods were used to detect the overburden rock mass structure in the goaf,such as,EH-4 conductivity imaging system and cross-well seismic wave tomography technology.The application exploration shows that:?1?Using EH-4 or cross-well seismic combined with drilling,the complete strata,relatively complete strata,fragmented strata,and scattered structures of the overburden rock mass in the goaf are basically identified.The structure types of overburden rocks in goafs are well determined and classified.?2?The case study shows that the method of geophysical prospecting combined with drilling can be used to classify the overburden structure of the mined-out area.?2?Similar material model experiments was used to study the deformation and failure characteristics of overburden rock when the surface load acts on the center of the goaf and directly above the boundary,the results show that:?1?The secondary compression or instability of residual voids in goaf under the action of surface load is the root cause of new large surface deformation in goaf,especially shallow goaf.?2?There are mainly three types of residual voids in overburden strata in thegoaf area:Separation,fissures and voids.The rock structure in the separation space has been balanced and will exist for a long time without external disturbance.The fissures are mainly distributed in the collapse zone and the fracture zone.Although they have experienced long-term compaction,there are still inevitably fissures;the voids are mainly distributed near the mining boundary.?3?When the goaf is deep?the surface load does not affect the collapsed overburden rock?,the continuous loading is just above the center of the goaf,the overburden layer is compacted and closed,and there is no obvious change in the fracture zone rock layer.Under the model conditions,the overburden is stable when loaded to 1.17 MPa,and the maximum surface subsidence remains at 640 mm and does not increase.When the depth is relatively small,the load directly acts on the damaged cover rock in the old goaf,which will accelerate the deformation and failure of the cover rock,and then generate new large deformations on the surface.When the load is 0.3 MPa under the model conditions,the maximum surface subsidence Up to 2430 mm.?4?The ground surface continues to be loaded above the shallow goaf boundary,and longitudinal cracks develop above the remaining voids at the goaf boundary.As the load increases,the overlying rock fractures along the longitudinal cracks integrally until it cuts to the surface,and the surface deformation shows obvious Unevenness and discontinuity.?3?The numerical simulation method was used to explore the relationship between the goaf and load loading at different positions,the goaf being subjected to different loads,different roof management methods,different overburden properties and different mining depths.The results show that:?1?when the surface load is located at the center of the goaf,the residual deformation around the load is slower than when the load is located at the boundary of the goaf.?2?At the same load location on the ground surface,the deformation and failure of the overburden in the shallow goaf is proportional to the load applied on the surface.The larger the load,the greater the amount of residual deformation on the surface,and the more serious the secondary damage to the overburden in the goaf.?3?Affected by the load applied on the ground surface,the total collapse method is adopted to manage the residual deformation of the surface of the goaf formed by the roof,which is much larger than the other two methods.?4?Affected by the load on the surface,the deformation and damage in the overburden in shallow goafs range from weak to medium to hard and hard overburden conditions.?5?Affected by the surface load,under the same conditions of overburden properties,the degree of deformation and failure in the overburden from goaf is the total collapse method,strip method and room-pillar mining method;under the same roof management method The overall order of deformation and damage in goafs from overburden to weak to medium hard and hard overburden conditions.Considering the maximum residual subsidence on the ground surface and the degree of deformation and damage of the overburden in the goaf,different roof management methods have greater influence on the overburden deformation in the shallow goaf than in different overburden properties.?6?The fixed applied load and mining thickness remain unchanged.As the mining depth increases,that is,the ratio U₁ of the depth of the goaf to the sum of the damage height of the overburden and the depth affected by the building load increases,the surface subsidence of the goaf increases There are the following characteristics:when U₁>1.2,with the increase of U₁,the surface subsidence value does not change much;when1.2>U₁>0.8,with the increase of U₁,the surface subsidence basically increases linearly;When U₁<0.8,there was a sudden increase in ground subsidence.After the sudden increase in subsidence,the amount of subsidence did not change significantly with the decrease of U₁.?4?The instability mechanism of overburden in goaf under the load from surface engineering construction objects was analyzed,and then the structure pattern of overburden in goaf formed by longwall mining was given.Drawing on the Boussinesq analytical solution for the calculation of additional stress in the soil mechanics foundation,the energy conservation method is used to calculate the additional stress in the foundation under the geological conditions of the goaf area based on the double-layer medium.Calculation formula of vertical additional stress under the effect of concentrated load and vertical uniform load?rectangular foundation and circular foundation?were given.Based on the control theory of rock strata such as voussoir beam/key layers,the mechanical expressions of the overburden instability in the goaf area formed by the longwall collapse method under the action of surface loads are analyzed,and the instability criteria are given.The instability mechanism of overburden in the goaf under the long-wall collapse method under the surface load is mainly the compaction of rock blocks in the collapse zone,the instability of the broken rock block structure in the fracture zone;mainly the instability of coal pillar,and the collapse of cavities in layers or sudden instability.The overburden structure of the longwall mining goaf under the action of surface loads is still a"voussoir beam"structure.The analysis diagrams of the key rock masses for the stability of the overburden rock under the effect of the surface load are given,and the calculation formulas of the stresses for the key rock masses of the overburden structure under the surface load are obtained.Based on the"S-R"stability theory of the key rock block of the"voussoir beam"structure,the"S-R"stability criterion for the overburden structure of the goaf under the influence of surface loads is given.?5?It is proposed that the residual subsidence of the goaf formed by longwall mining under the effect of surface loads.When the probability integral method is used for prediction,the selection of the residual residual subsidence factor should take full account of the surface load size and action location,the compaction of the collapsed zone,and the instability of the fractured rock block structure in the fracture zone.The two areas are respectly calculated,according to the collapsed rock block compaction zone,and the fractured block structure of the fractured rock block.The calculation method of the corresponding residual subsidence coefficient of the ground is presented.?6?Using the research results of the paper,the stability of the foundation affected by coal mining in a residential area in Beipiao City,Liaoning Province was evaluated.The builder built the building according to the evaluation conclusion and handed it to the owner.The current condition is good and no problems have occurred.