Developing Mechanism Of Soil Cave In Karst Overburden And Its Influence On The Stability Of Heavy Haul Railway Subgrade

Karst geology is widely distributed in China.The development of hidden caves causes hidden dangers to the stability and safety of railway subgrades.The heavy haul railway are characterized by large weight,high target and long marshalling,which increases the possibility of risk accidents.The formation of hidden soil caves in karst areas is random and has a long development time.For some railway sections in karst areas with early design,a low grade,incomplete foundation treatment,or an unsatisfactory grouting effect,the hidden dangers of soil caves cannot be completely eradicated.With the increase in railway axle load,increase in railway service time,and long-term impact of natural climate and groundwater,the hidden dangers of underground karst caves still exist in weak areas.Therefore,to further the research on the stress characteristics of the subgrades of heavy-haul railways in karst areas with hidden soil holes,evaluate safety,and provide relevant guidance,targeted comprehensive research was conducted from the perspectives of the karst formation environment,disaster causing factors,foundation soil properties,soil hole development process,monitoring and early warning,soil hole subgrade stability,and stress response.Through laboratory tests and taking the buried soil tunnel subgrade of a heavy-duty railway in a karst area as the research object,the physical and chemical properties of the soil for the subgrade filler under a dry–wet cycle were examined,and the mechanical properties were investigated through a static triaxial test.Using on-site comprehensive monitoring technology,experiments were carried out on the collapse of a typical karst soil cave in the study area,and the variation characteristics of the soil water and the soil parameters at different depths around a soil cave under rainfall conditions were analyzed.The background environment,soil cave expansion mechanism,and soil parameters were analyzed fully.Based on the results,the discrete-element and discrete-element––finite-difference coupled modeling methods were used to analyze the soil and water fineness of the subgrades of hidden soil caves under fluid–structure coupling conditions and different types of load.The characteristics of the soil cavity were studied in depth,and a simulation analysis and risk assessment of the development of the soil cave,the stability of the roadbed,and the stress response were performed under different working conditions.The most important and innovative research results are as follows.(1)An undrained unconsolidated triaxial test and scanning electron microscopy test of fine breccia soil under different degrees of compactness,initial moisture contents,dry–wet cycle times,and soil burial depths were systematically performed using soil collected at different depths near a heavy-haul railway section in a karst area in Jiangxi Province,China.Based on the triaxial test results,the concept of the dry–wet cycle strength residual ratio was introduced,and a normalized stress–strain model of fine breccia soil considering the effects of the confining pressure and dry–wet cycle was constructed.(2)The corresponding relationship between the mechanical properties and microstructure of fine breccia soil in Jiangxi Province was examined.Through image information extraction technology,the micro characteristic parameters of fine breccia soil under different test conditions were correlated with mechanical property indices.Suggestions for the filling indices of subgrade fill are provided.(3)Based on a multifactor field monitoring test of soil cave development in the test area,the comprehensive change characteristics of parameters of soil cave development induced by rainfall were obtained.As a result of a multiple logical data traversal search,a two-level early warning system and the corresponding threshold of comprehensive risk of soil cave development based on multifactor comprehensive criteria are proposed.Based on water content monitoring data,a two-dimensional fluid model was constructed using the soil cave collapse monitoring points as the simulation prototype,and the soil hydraulic inversion parameters were determined.(4)Based on the discrete-element fluid–solid coupling theory and the discrete-element–finite-difference coupling theory,three-dimensional simulation models of a discrete medium and continuous discrete medium under the action of a fluid–solid field and dynamic load were constructed,respectively.The fine simulation and analysis of the meso characteristics and macro response of the concealed soil tunnel subgrade of a heavy-haul railway under the action of seepage,subgrade dead weight load,train dynamic load,different subgrade structures,various soil tunnel characteristics,and different load characteristics were examined.(5)Through the use of the double index system and orthogonal analysis method,the influences of different factors on the performance of a soil tunnel subgrade were revealed and ranked.The fuzzy comprehensive evaluation method was employed to develop a risk evaluation system for hidden soil tunnels for railway subgrades in operation periods,and the risk was evaluated under different conditions.