Dynamic Response And Characteristics In Macroscopic And Mesoscopic Of Heavy-haul Subgrade

China’s heavy-haul railway is a special and top-level type of freight dedicated railway,mainly transporting large goods,and is also one of the main channels for bulk cargo transportation in China.Compared to the common high-speed passenger and freight railway,heavy haul railway transportation has the characteristics of significant axles and multiple wheel sets,which widens the scope of subgrade work area,complicates the stress path and rotation of the stress principal axis within the subgrade,and makes the evolution of soil microstructure more intense,leading to more serious deterioration of the cumulative speed of subgrade plastic deformation and service performance,and threatening the safe operation of trains.Therefore,relying on Special-funded Program on National Key Scientific Instruments and Equipment Development——In Situ Simulation System of Foundation Cumulative Deformation under Heavy Traffic Load(52027813),this article has carried out a series of studies with the heavyhaul railway as the research object.A calculation model for the dynamic response of the subgrade of the heavy-haul railway was established,and a series of dynamic triaxial sequential loading tests and dynamic triaxial long-term cyclic loading tests were conducted basing on soil samples of Datong-Qinhuangdao railway subgrade.Subsequently,a discrete element numerical model of hollow torsional shear under traffic loads was established,the three-dimensional dynamic response,macroscopic dynamic characteristics,and microscopic evolution mechanism of railway subgrade under heavy load train loads were deeply studied,and a prediction model for dynamic modulus of resilience and cumulative deformation of soil mass was established.The main research results are as follows:(1)Based on the extending Biot’s unsaturated soil theory,a coupled dynamic model of track sleeper ballast unsaturated subgrade was established,and the three-dimensional stress distribution and soil element stress path of the subgrade under heavy load train loads were obtained.Then the correctness of this model was verified by field tests.The research shows that the vertical displacement of soil element is positively correlated with saturation,train speed,and axle load,while negatively correlated with soil element depth;The stress in all directions are positively correlated with the axle load of the train,and negatively correlated with saturation and soil unit depth;Horizontal normal stress σx and σy is inversely proportional to the train speed,but the train speed is related to the vertical normal stress σz and shear stress τxz had no significant effect.It is worth noting that as the depth increases,the stress curve waveform in each direction gradually changes from "multi peak" to "single peak",and the number of rotations of the stress principal axis also decreases.(2)Based on theoretical calculations,a series of dynamic triaxial sequential loading tests and long-term cyclic loading tests under heavy haul stress levels were conducted using soil samples from the Pinggu section of the Datong-Qinhuangdao Railway subgrade.The test found that the dynamic resilient modulus of soil is negatively correlated with the amplitude of dynamic deviator stress and water content,but positively correlated with confining pressure and compactness;The plastic deformation of the sample continuously accumulates with the increase of loading times and is positively correlated with the dynamic stress amplitude and water content,while negatively correlated with the confining pressure and compactness.A prediction model for dynamic resilient modulus and cumulative plastic deformation is established,and a prediction method for dynamic resilience modulus based on genetic algorithm optimized BP neural network is proposed.(3)Based on the DEM-FDM coupling method,a dynamic three-dimensional complex stress path loading numerical model of soil mass with fully flexible boundaries was established.Through indoor tests of hollow torsional shear units,the mesoscopic parameters of the numerical model were calibrated,and hollow torsional shear tests at different stress levels were simulated.The results show that the macroscopic cumulative deformation of the numerical model for hollow cylindrical elements is positively correlated with the loading stress and dynamic shear stress,while negatively correlated with the confining pressure;The variation of porosity,average coordination number,horizontal contact number,and normal contact force is directly proportional to the axial load and dynamic shear stress,and inversely proportional to the confining pressure.