Study On The Heaving Mechanism Of Coarse-grained Sulfate-laden Soil As Subgrade Fillings Of High-Speed Railway

Coarse-grained sulfate soils are mainly distributed in Central Asia,East Asia and the northwestern China.The sulfates soils there are problematic under the severe climate conditions,and infrastructures built on the sulfate soils usually suffers many problems.Sulfate-induced heave in the High-Speed Railway(HSR)subgrade has been reported recently,which has strongly deteriorated the service performance of HSR.This work mainly studies the heaving mechanism of coarse-grained soils by in situ investigation,monitoring,laboratory test,and theoretical calculation.The main work and conclusions are summarized as follows.(1)The theoretical relationship between the partial and total salinity of gravel was established.The partial salt contents of manufactured saline gravels were analyzed by ion chromatography.The results show that,the sieve passing rate ratio of the fine particles and salts mainly ranges in 1-2 and it is rarely influenced by the amount of total salt content;the sieve passing rate varies with different types of salt;results from passing a 5 mm sieve are more stable and representative;if the total salt content is low enough,the interaction among the salt in the analysis can be ignored,and then the theoretical model is simplified for the ease of use.Finally,a novel mothed is proposed to analyze the total salt content of saline gravels.(2)A new phase composition curve model was developed by simplifying the correlation between the soil matrix and pore water.the model is capable for the phase composition characteristics analysis with some examples in the parametric study.The model was also validated by previous laboratory data,and it will assist in the interpretation of the behavior of the second phase transition of saline soil.(3)The volumetric and thermal behaviors of coarse-grained soil was studied by salt expansion tests of soil samples with various initial salt and water content.An empirical model of critical precipitation point was established.An empirical salt heave prediction model was also established based on the theoretical mirabilite production.During the cooling progress,the temperature ranges of sample severe heave are mainly from 30? ?10 ? and from 0 ? ?-10 ?.The amount of salt expansion has a positive correlation with the salt-water ratio,while the situation would be more complex if frost heave occurs due to a peak of heave at a relative low level of salt content.(4)The mechanical properties of the frozen coarse-grained soil were obtained by triaxial test.The results show that,the mechanical properties of frozen saline soil are significantly affected by the solubilization of salt before frozen.The elastic modulus is mainly governed by the salt content,while the water content affects the threshold value of it;the impact of confining pressure on the elastic modulus varies with the water content and temperature;there are some threshold values of cohesion and internal friction angle with the change of salt content.(5)Suffered subgrades were selected and investigated,and the sulfated-induced problem in HSR subgrades were summarized.Two sections were selected and then the salt-temperature-moisture of them were monitored and analyzed;sulfate-attack induced heave of cement stabilized soil were studied by laboratory test,and the results show that the heave ratio increases with increasing sulfate content and decreasing cement addition.