Study On The Influence Law Of Water Mined-out Area In Deep Salt Mine On Railway Subgrade Deformation

China is rich in mineral resources.The large-scale exploitation and utilization of mineral resources have laid a material foundation for the social development of China,and caused a large number of goaf in the deep strata,which could easily lead to ground collapse,cracks and other geological security problems.In recent years,due to the actual engineering constraints,considering factors such as economy,railway construction in our country inevitably need through mined-out region,and the mining technical conditions is complex,the formation of mined-out area has uncertainty,internal collapse of mined-out area increased and external load easily lead to formation of again lose the original stress balance,resulting in railway structure cracks,collapse and other major hazards,significant influence on railway construction safety.Therefore,this paper,relying on a new Guangzhou railway project,systematically studied the surface subsidence law of the railway underlying deep mining area and established the prediction method of the surface subsidence of the railway underlying deep mining area.The main research contents and results are as follows:(1)The study on the microstructure,chemical composition and soil dynamic characteristics of railway subgrade soil is performed.It has laid a foundation for the study of subgrade soil performance stability prediction and stability reinforcement design.The results show that the subgrade soil with the same depth has good thermal stability and can maintain good stability under the action of temperature field.The variation trend of particle size of subgrade soil with different depth is basically the same.The pore distribution of subgrade soil belongs to the disordered distribution of large pore and middle pore,which does not reflect the adsorption characteristics of connected pore and porous structure.The overall microstructure of subgrade soil is mainly composed of coarse particles and fine particles in contact with each other.The dynamic load has little effect on the subsidence of the subgrade soil.(2)By setting up long-term surface deformation observation points around the whole production well of Longgui nitrate mine in Guangzhou city,the surface deformation monitoring leveling network was established to monitor the ground surface deformation above the mine for 15 years.Based on the analysis of the monitoring data of surface deformation,the ground surface subsidence,horizontal movement,the earth's tilt,movement curvature of ground surface,cracking and ground deformation near new railway were studied,which reveals the ground surface subsidence rule above the mine and provides a direct basis for the design and construction of similar engineering project.The results show that the maximum settlement along the railway is 560 mm.After the total stoping of mining in the mining area,the settlement rate slowed down.The settlement rate of the maximum settlement point along the railway decreased from 170mm/ year to 49mm/year.The possibility of salt cavity roof collapse is small.(3)The general deformation law of overburden rock is analyzed,and the law of ground subsidence development of water-soluble salt mine is summarized,then the main factors causing ground surface subsidence that rock salt creep deformation characteristics is studies.Based on the characteristics of rock salt creep deformation of three stages respectively,each stage creep deformation theory model was established.Through theoretical analysis and comparing with the results of finite element calculation and the related test specimens,the creep model can better predict the creep behavior of rock salt.It can lay a foundation for further research on the influence of rock salt creep on ground surface subsidence.(4)Based on the finite element software ABAQUS,according to the Guangzhou Longgui salt mining characteristics,material characteristics,the finite element model is established.Based on Fortran language,the creep model subroutine is embedded in ABAQUS finite element numerical analysis,and the long-term effects of single cavity mining,multiple cavity and multiple small mining cavity that connected as a large rock-salt cavity are explorated,including stratum displacement,ground settlement and creep deformation of rock-salt cavity;On this basis,the effects of maintaining brine pressure during mining and recharging brine after mining on the above long-term deformation effects were studied.The deformation after cavity mining has a small relation with the number and causes a small deformation change.However,the size of creep is related to the number of cavity mining.When the number increases to a certain degree,creep deformation increases rapidly.The settlement of the roof increases sharply with the creep deformation after the large cavity is connected by different number of cavities,and the uplift degree of the bottom plate is almost negligible under the influence of creep deformation.The large deformation of the roof caused by the creep of surrounding rock may lead to the instability of the roof.After mining,keeping the brine pressure free from brine extraction can effectively prevent the overall deformation of the solution chamber and effectively limit the surface subsidence deformation.(5)Based on the new railway project in Guangzhou,the railway subgrade-formation-dissolved cavity three-dimensional finite element model is built.The influence of railway subgrade construction and brine injection on ground surface subsidence is studied.The results showed that after 100 years` creep,creep deformation on the ground surface is still in the second phase,solution cavity structure remains relatively stable state and there is no collapse phenomenon.After 100 years of railway construction,the subgrade will have additional settlement of 0.7087 m,far exceeding the post-construction settlement of 20 cm stipulated by the standard limit.100 years after the construction of railway subgrade by recharging brine,the maximum additional settlement of the subgrade is only 12.65 cm,which meets the requirements of the standard limit that the post-construction settlement shall not exceed 20 cm.The recharging brine is effective for controlling the creep of the surrounding rock,reducing the long-term subsidence of the ground surface and ensuring the safety of railway operation.(6)Prediction method for ground surface subsidence above deep mine is established considering formation creep deformation.It investigates relationship between the surface subsidence characteristics and the factors such as the brine,salt cavity injection Angle,salt cavity height and creep time.By comparing the predicted results with the FEM results,it proved that this model can effectively predict ground surface subsidence that above mining area.In addition,the creep of surrounding rock in salt chamber can be effectively controlled by recharging brine,and the effect of surface subsidence reduction increases with the increase of brine concentration.With the increase of tilt Angle,the maximum land subsidence gradually increases.The height of salt cavity has little effect on the surface subsidence.The total surface subsidence increases with the increase of creep time,but the increased rate is decreases.