Study On Frost Heave Behavior And Mechanical Properties Of Fine-grain Filler In Heavy Haul Railway

The heavy haul railways,put into operation in the 20th century in China,use fine-grained soil as the subgrade filler according to local conditions.The freeze-thaw effect has caused various engineering freezing damages to the fine-grained subgrades in cold regions,affecting the transportation efficiency and safety of busy freight lines.The research on the frost heave and mechanical properties of fine-grained fillers for heavy haul railways is of great significance for guiding the operation and maintenance of existing lines.This paper discusses the types and causes of frost damage in the severe frost damage section of the Shenshuo Railway through field investigations,studies the ground temperature and frost heave of the subgrade in the cold season through on-site monitoring.The paper also studies the frost heave characteristics,mechanical properties and mitigation of frost heave for fine-grained filler through laboratory tests.The main work and conclusions of this paper are as follows:(1)On-site investigation shows that the cold temperature and shallow phreatic water are the main causes of the severe freezing damage in the fine-grained subgrade with frost heave sensitivity.The freezing effect mainly causes large frost heave.After the melting in spring,the mechanical properties of the filler attenuates,and the subgrade is squeezed and sinks under the load of the train.The on-site ground temperature monitoring shows that the frost depth of the subgrade affected by shallow phreatic water decreases,and the freezing start time and the thawing end time of the formation are also delayed.The leveling measurement shows that the frost heave of the shoulders is not less than 8cm and exhibits differentiated deformation characteristics.The Stefan equation can also predict the maximum frost depth of the subgrade with shallow phreatic water according to the surface freezing index.(2)The one-dimension frost heave orthogonal test with different step cooling paths,saturated water content and load type was carried out.the frost depth,frost heave and frost heave ratio during the test,considering the load effect of heavy-haul trains and water supplement conditions,were analyzed.the influence degree of the test factors on the results of the specimens at the end of the experiment were also discussed.The step cooling path has the greatest effect on the frost depth of the specimens,and the factors that have the greatest effect on the frost heave ratio vary depending on the load type and the water supplementation conditions.The mean and frequency of cyclic load have little effect on the frost heave ratio,indicating that the increase in vehicle axle weight and the increase in operating speed have little effect on the frost heave characteristics of the filler.(3)Setting the thermostatic-curing(T-C)group as the control group,the mechanical properties of the specimens in freeze-thaw(F-T)group at different confining pressures,freeze-thaw times,cooling temperature and water content were comparatively analyzed,the shear strength,internal friction angle and cohesion affected by the test conditions were discussed,and the applicability of the Duncan-Chang model to deal with stress-strain curves is verified.The cohesion of the specimens in the T-C group is higher than that in the F-T group.When the water content is 14.5%,the internal friction angle of the T-C group is smaller than that in the F-T group.When the water content is 17.6%,the opposite is true.When the initial water content is different,the cooling temperature that is most unfavorable to the mechanical properties of the specimen is different.For specimens with water content of 17.6%,the parameters K and n of the Duncan-Chang model in the F-T group are smaller than those in the T-C group,and the damage ratio R_f in F-T group is greater than that in the T-C group.(4)The method of filling the pressure-relief hole with rubber-asphalt-fiber mixture can significantly reduce the frost heave of the specimens.When the strain is less than 5%,the elastic modulus of the unfrozen soil is significantly larger than that of the mixture.The moisture content of the soil filled with the mixture is lower than that of the soil filled without the mixture along the height direction.The main mechanism for mitigating frost heave is that the mixture has a smaller elastic modulus and a certain water-proofing effect.