Effect of salt content on shear strength of sulfurous saline soil

To investigate the influence of salt content on the shear strength of salinized soil as well as the relationship between cohesion force, internal friction angle and salt content, the Da Qaidam Salt Lake and surrounding areas located in the Qaidam Basin, Qinghai Province, have been selected as the testing area.The salinized soil samples at 0~30 cm beneath the ground surface have been collected and meanwhile salt leaching tests on these collected salinized soil samples have been conducted to produce salt leached soil.On the basis of the procedures mentioned above, according to the type and salinization degrees of the collected salinized soil, anhydrous sodium sulphate(Na2SO4) with different masses has been separately added into the salt leached soil to make artificial sulphate saline soil with 6 salt gradients, and then direct shear tests have been conducted on these artificial sulphate saline soil to further investigate the influence of salt content on the shear strength, as well as the shear strength indices such as cohesion force and internal frictional angle.The results are as follows: after salt leaching tests, soluble salt ions′ content and pH value are markedly lower than the counterparts before salt leaching.Total ion content decreases from 2.165 0% to 0.74%, which is lower than the value before salt leaching and pH value decreases from 8.65 to 8.01.Moreover, after salt leaching, soil grain composition has changed by some extent, sand is reduced from 33.0% to 31.3%, silt is declined from 55.2% to 54.0%, and clay particle is increased from 11.8% to 14.7%, respectively.Meanwhile, as the soil density and moisture content are 1.41 g/cm³ and 25.68% respectively, the cohesion force and internal friction angle for salt leached soil are 14.8 kPa and 26.5°, which are higher than the counterparts(6.7 kPa and 24.6°, respectively) for salinized soil before salt leaching.With different salt contents, the cohesion force and internal frictional angle of the artificially salinized soil exhibit an trend of initial decreasing and then increasing as the salt content increases.And further study results show when salt content increases from 0.74% to 5.17%, the cohesion force and internal frictional angle for artificially salinized soil gradually decline, and as salt content increases from 5.17% to 14.17%, the cohesion force and internal frictional angle for artificially salinized soil increase gradually.And as salt content is 5.17%, the corresponding cohesion force and internal frictional angle for artificially salinized soil are 8.3 kPa and 26.1°, respectively, which are both the minimum values.So the corresponding salt content value is the threshold value of the artificial sulphate saline soil, which indicates that there exists a threshold value for sulfate saline soil.As the salt content is below this value, the cohesion force and internal friction angle decline with salt content increasing, and as the salt content is beyond this value, the cohesion force and internal friction angle increase with salt content increasing.The achievement has theoretical and practical significance in further exploring the influence of various salt contents on the cohesion force and internal friction angle as well as on the shear strength of the salinized soil.And this study can also serve as a basis for the further studies on the engineering characteristics of salinized soil.