Deformation Mechanism And Stability Analysis Of Soft Rock Mining Subsidence Area Under High-Speed Railway

The Guizhou-Nanning High-Speed Railway(Guinan HSR)is a transportation link between two important provincial capitals,Guizhou and Nanning.It is also a critical connection point for the "Belt and Road" initiative.The design of the Nanning section of the project faces the challenge of crossing through coal mining subsidence areas due to space limitations in the urban area.The stability of the overlying strata in the coal mining subsidence area and whether it can withstand the high-speed train’s load are key issues that need to be addressed.In order to tackle this complex problem,this paper relies on the second route selection project of the Guinan HSR(Nanning section).It comprehensively employs various technical means such as drilling,geophysics,and geotechnical engineering survey.It integrates the disciplines of soft rock engineering mechanics,coal mining subsidence,engineering surveying,PSP-In SAR monitoring,numerical analysis,and other theoretical and methodological approaches.The study systematically investigates the effects of coal mining disturbance and high-speed train dynamic loads on the settlement and deformation of the overlying strata in the soft rock goaf.Experts have approved the research findings and have scientific significance and practical engineering value for China’s future high-speed railway network construction.The main research content and achievements of this paper are as follows:(1)Based on a comprehensive survey of engineering geology,hydrogeology,and coal mining in the mining area,a combination of seismic imaging,high-density resistivity,and drilling was used to explore the size,distribution,and filling conditions of the soft rock goaf.The study shows that the rock layers in this area are mainly composed of mudstone and siltstone,which are prone to large deformations.Geophysical results from survey lines WT-4,WT-7,WT-14,and WT-19 indicate the existence of water and mud-filled areas at different heights.Survey lines WT-3 and WT-6 show low-frequency reflection anomalies at depths of 60 m and 80 m,consistent with the drilling results,indicating the existence of coal seams and weak interlayers.(2)Uniaxial compression,splitting,shear strength,lateral constraint expansion,disintegration,and acoustic tests were conducted to obtain the physical and mechanical parameters of typical rock and soil samples.XRD and SEM experiments were used to analyze siltstone and mudstone’s mineral composition and microstructure characteristics.Finally,based on the above research,an engineering mechanics evaluation was conducted on the soft rock formations in the study area.The experimental results show that the uniaxial compressive strength of siltstone,mudstone,and coal rock in the coal mining subsidence area is less than 5 MPa,indicating that they are extremely soft rocks.Mudstone is the main affected formation in the coal mining subsidence area,and it is a poor-quality,highly expansive,and extremely soft rock.(3)Based on the team’s independently developed "Visual Evaluation System for Resource Damage in Mining Subsidence Environment," the surface deformation and boundary range of the moving basin caused by mining activities were calculated for the Da Panjing goaf.Three-dimensional numerical simulation methods were used to study the deformation pattern of the ground subsidence caused by coal mining,considering factors such as the distribution of fault effects and soft rock formations.The results show that line position scheme 1 is not affected by mining.The section affected by mining is about 1240 m in scheme 2,and the vertical settlement distribution is significantly affected by faults.The vertical settlement of scheme 3 is significantly smaller than that of scheme 3,but its inclination is larger.Affected by the characteristics of soft rock and fault effect,the surface deformation caused by coal seam mining is large and dominated by vertical settlement,with a maximum value of 1330 mm.However,it is not located in the geometric center of the goaf,but in the hanging wall area of the fault.(4)The PSP-In SAR technique was adopted to monitor the large-scale surface subsidence in the coal mining subsidence area of Dapanjing.By combining the deformation of surface buildings,the stability of the underlying goaf was comprehensively evaluated.The residual deformation and duration of deformation of the goaf were calculated.The study shows that the deformation rate of most PS points in the goaf of Dapanjing is-8～4 mm/a,and the annual average subsidence meets the regulatory requirements.As the mining area has been abandoned for 35 years,70% of the residual subsidence has already occurred.In view of this,it can be considered that the goaf of Dapanjing tends to stable after coal seam mining.(5)On the basis of exploring the spatial distribution of goaf and fault and obtaining the physical and mechanical parameters of soft rock,the numerical calculation model of soft rock mining subsidence area under the dynamic load of high-speed railway was established by using PLAXIS finite element software.The effect of high-speed railway dynamic load on the surface subsidence and stress of the soft rock mining subsidence area was studied by considering the effect of the goaf and the fault.The results show that the fault effect significantly affects the dynamic response.As the depth increases,the distribution and peak value of the dynamic stress change from being dominated by the fault slip to being jointly controlled by the fault and the goaf.The depth of the influence of dynamic stress caused by the high-speed railway crossing the soft rock goaf is 40 m,and the peak value of vertical subsidence increases first and then decreases.When crossing the fault,the peak value reaches a maximum of 2.21 mm.The subsidence values meet the regulatory requirements,and the stability of the high-speed railway crossing the soft rock mining subsidence area is good.Therefore,line position scheme 2 is feasible.