Investigation On The Characteristics Of Subgrade Mud Pumping And The Mechanism Of Fine Particle Migration In Heavy-haul Railway

In recent years,the rapid development of China’s economy has brought with an increase demand for resource transportation,and the heavy-haul railway freight with a large axle load and long marshalling has become a main form of railway freight.The heavier axle loads of these trains enhances the interaction between the train and the track,and increases the dynamic stress in subgrade,the resulting subgrade problems such as mud pumping have become more and more serious.The subgrade mud pumping will not only pollute the overlying ballast,affect its mechanical and hydraulic properties by changing its pore structure,but also result in differential settlement of the track,which seriously affect the railway tracks stability and its operation safety.The research on the characteristics of subgrade mud pumping has gradually become a popular topic.However,most of these studies focus on the description of mud pumping phenomenon and experience summary in remediation of mud pumping.And there are controversial conclusions on the mechanism of the occurrence and development of subgrade mud pumping.In this paper,we analyzed the conditions for the occurrence of subgrade mud pumping under the action of train load.Then a self-developed mud pumping test model was used to study and analyze the characteristics of subgrade（saturated silty clay）mud pumping under cyclic loading.The criterion for soil prone to mud pumping was put forward,and the mechanism of subgrade mud pumping was revealed.In addition,the Computerized Tomography（CT）was used to analyze the characteristics of fine particle migration during the process of subgrade mud pumping and its effect on the microscopic pore structure of the overlying ballast.The main contents in this paper are as follows:（1）The experimental study was carried out on the saturated silty clay（subgrade）and gravel（ballast）sample under cyclic loading by a self-developed mud pumping test model.And the evolution characteristics of axial stress,axial strain,excess pore water pressure and dynamic stress of soil were obtained.A two-dimensional calculation model of saturated silty clay subgrade was established by using the software COMSOL Multiphysics^?to study the fluid-solid coupling characteristics under cyclic loading.This calculation model can well predict the excess pore water pressure evolution characteristics of saturated subgrade soil under cyclic loading,and the characteristics of fine particle migration.（2）The influences of the particle size distribution（Kaolin content）,plasticity index,initial dry density of silty clay,cyclic loading amplitude,and loading frequency on the axial strain of sample,excess pore water pressure in silty clay,and mud pumping height were analyzed.Based on the test results,a criterion for soil prone to mud pumping was put forward,and the mechanism of subgrade mud pumping was revealed.（3）With the help of CT scanning,the characteristics of fine particle migration in the process of subgrade mud pumping under cyclic loading were analyzed.Based on the test results,an empirical formula for the mass evolution of fine particle migration was established.The accuracy and applicability of the mass evolution formula of fine particle migration proposed in this paper were verified by experimental data in existing literatures.（4）Based on the CT scanning results,the effect of fine particle migration on the pore structure parameters（such as plane porosity,porosity,pore size distribution characteristics and pore connectivity,etc.）of the overlying ballast layer was analyzed from a mesoscopic perspective.Based on the porosity evolution law of ballast layer,an empirical model of porosity evolution of ballast layer was established and verified.（5）The inhibition effect of geotextile on the fine particle migration under cyclic loading was studied by the self-developed mud pumping test model.And the remediation effect of geotextile laid at the interface between saturated subgrade and ballast layer on the problem of subgrade mud pumping was evaluated.The results showed that the addition of geotextile can inhibit the migration of fine particle and thus mitigate the problem of railway subgrade mud pumping under cyclic loading.However,with the increase in cyclic loading amplitude,the inhibition effect of geotextile on the fine particle migration gradually weakened.