Deformation Characteristics Of Coarse-grained Sulphate Saline Soil Subgrade Of A Highway By In-situ Test

The engineering property of saline soil is special.Under the condition of temperature change,salt expansion deformation and collapsible deformation will occur,this causes soil damage and caused great hindrance to the construction of high-grade highways.Coarse-grained sulphate soil is widely distributed in western China.In recent years,a large number of high-grade roads have been built in this area.Using coarse-grained sulphate soil as road building materials can not only save cost,but also reduce the construction period delayed by soil transportation.In previous studies,the deformation characteristics of fine grained sulphate soil were mostly studied,but the research on coarse-grained sulphate soil was less,and the research on road-based coarse-grain sulphate soil was rare.In order to understand the engineering properties of coarse-grained sulphate soil as subgrade filler and make it better used in the construction of high-grade highway subgrade,it is necessary to study the deformation characteristics of this kind of soil.Salt expansion test was carried out in laboratory by freeze-thaw test machine.The results show that with the increase of salt content,the salt expansion deformation of coarse-grained sulphate soil increases.Since there is not enough water for water absorption and crystallization,when the salt content is 3.0%,the deformation of the sample of coarse sand and gravelly sand is reduced.For the samples of coarse sand with 1% salt content under different conditions of dry density,with the increase of dry density,the salt expansion rate of the sample increases.For the samples of coarse sand with 1% salt content,when the water content is greater than the optimal water content,the salt expansion rate decreases with the increase of water content.The larger the soil particle size,the smaller the salt expansion rate.For the salt-free sample,the salt expansion temperature of the coarse sand was-1.0 ?,and the salt expansion temperature of the gravelly sand was-3 ?.When the salt content is greater than 1.0%,the salt expansion temperature of coarse sand is 25 ?,the sensitive range of temperature of salt expansion is 25~10 ?,the salt expansion temperature of gravelly sand is 20?,and 20 ~5 ? is the temperature of sensitive range of salt expansion,when the temperature is lower than 5 ?,the salt expansion curve of the gravelly sand gradually becomes gentle.The subgrade testing segment was built in the G7 highway project.Through long-term monitoring,the deformation characteristics of gravelly sand sulphate soil subgrade with different salt content and the temperature changes of different depths of the subgrade were studied.The results show that the maximum value of heave deformation of the subgrade increases with the increase of salt content.The heave deformation of unsalted gravelly sand subgrade is smaller than the other two subgrade,but the settlement deformation is larger.However,the heave deformation of the gravelly sand subgrade with salt content of 2.0% is larger than the other two subgrade,and the settlement deformation value is smaller.It is found that the closer the monitoring location is to the subgrade surface,the more drastic and frequent the temperature change is.The failure modes of gravelly sand subgrade with and without salt are different.,the deformation of gravelly sand subgrade with salt content of 1.0% is sensitive to temperature,its value of heave deformation is larger than the unsalted gravelly sand subgrade,and the value of settlement deformation is smaller,and heave deformation is the main reason of the failure of the subgrade.On the contrary,for unsalted gravelly sand subgrade,the settlement deformation is larger,the heave deformation is smaller,and the settlement deformation is the main cause of the failure of subgrade.