Study On Deformation Characteristics Of Coarse-grained Saline Soil Subgrade Of Tehran-Isfahan High-speed Railway

The Tehran-Isfahan high-speed railway in Iran is one of projects implemented by "One Belt,One Road" Initiative.Due to the constraints of the engineering geological condition along the line,the local coarse-grained saline soil as the subgrade filler is adopted in the high-speed railway.In view of the extremely strict deformation control for high-speed railway,how to evaluate its deformation characteristics and clarify its deformation range is one of the key technical problems faced by high-speed railway construction in coarse-grained saline soil areas.Based on the Tehran-Isfahan high-speed railway construction project,this paper comprehensively adopts research methods such as laboratory test,field test,particle discrete element numerical simulation,and theoretical analysis.The influencing factors,deformation mechanism and mesoscopic characteristics of salt expansion and collapsibility for coarse grained saline soil subgrade for high-speed railway,salt frost heave mechanism,and deformation characteristics of subgrade under freeze-thaw cycles are systematically studied.The main works and conclusions are as follows:(1)The meteorological conditions of Iran and the topography,geomorphology and hydrogeological conditions along the high-speed railway were comprehensively analyzed,and the local environmental temperature changes,rainfall,the distribution,type and salt content of saline soil along the railway were clarified.Based on results from a series of tests on basic soil properties,the particle gradation composition,salt type,salt content,optimum moisture content and maximum dry density of the subgrade fillers in coarse-grained saline soil were obtained.(2)Through laboratory test of the salt expansion,the salt expansion property of coarse-grained saline soil subgrade filler was evaluated,and the effects of cooling rate,temperature,salt content,moisture content,compaction degree and overburden load on the salt expansion characteristics of coarse-grained saline soil were analyzed.The T-type correlation degree was used to analyze the influence of each factor on the salt expansion rate.Based on the salt expansion test under the interaction of multiple factors in orthogonal design,the relationship models between the salt expansion rate of coarse-grained saline soil and various factors were established.The allowable salt expansion rate of saline soil subgrade for high-speed railway was proposed.(3)Considering the influence of soil pore size on the formation and development of salt crystals and the uniform nucleation of salt crystals,the nucleation critical radius of salt crystals and ice crystals in soil pores were determined under the assumption that salt crystals and ice crystals in soil were spherical.The relationship between crystalline salt,non phase change water and temperature was corrected.Using a method similar to the mechanical model of elastic porous media,the calculation model of salt frost heave of low concentration and high concentration coarse-grained saline soil were established respectively.(4)Based on the field collapse test of subgrade,the collapsibility characteristics of the coarse-grained saline soil subgrade filler were evaluated,and the collapse influence depth of coarse-grained saline soil subgrade was given.On this basis,the laboratory collapse tests of coarse-grained saline soil considering influences of moisture content,salt content and compaction degree were carried out by using the self-developed test device.The allowable collapse rate of saline soil subgrade for high-speed railway was proposed,and the limit salt content of collapsibility for coarse-grained saline soil subgrade was given.(5)The discrete element software PFC3 D was adopted,and the salt expansion and collapsibility numerical models of coarse-grained saline soil were established by considering the brine phase transition process.The change of soil pore structure during the process of salt expansion was analyzed.The water immersion collapse process of coarse-grained saline soil was simulated,and the variation laws of soil porosity and inter-particle contact force chain during different loading levels and water immersion collapse processes were summarized.The mechanisms of salt expansion and collapse of coarse-grained saline soil were revealed.(6)Through on-site subgrade monitoring,the variation laws of moisture and temperature with time at different depths of coarse-grained saline soil subgrade were obtained,and the sensitive depth of the subgrade affected by temperature and the significant change depth of the moisture content for the subgrade were determined.Using the self-designed freeze-thaw cycle test device,the subgrade deformation model tests under the three working conditions of closed system,precipitation and groundwater salt recharge under the action of freeze-thaw cycles were carried out respectively.The variation laws of temperature and moisture and deformation characteristics of coarse-grained saline soil subgrade with different salt content were analyzed.The deformation rates of coarse-grained saline soil subgrade under different conditions were given.The research results of this paper can not only give effective technical support for the construction of the Tehran-Isfahan high-speed railway,but also provide reference for the design of subgrades and the revision of relevant specifications for similar regional projects in the future.