Gradation of Subgrade Soil and Its Salt-Resistance Effect in Salt Lake Area in Qinghai

A subgrade project in Qinghai salt lake area, China was in a cold and dry geotechnical environment with strong concealed salt weathering. Taking the natural saline soil samples of the above subgrade construction project and soil samples of the built subgrade structure in Chaka Salt Lake area of Qinghai, comparative tests were conducted to study the effect of the subgrade structure protection against salt weathering in the saline soil area of Qinghai province. On the basis of soluble salt content measurements, particle analysis and capillary water rising experiment, the maximum height of capillary water rising in subgrade soil layer in Chaka saline soil area was calculated theoretically. The results indicate that the rising height of capillary water in the saline soil was higher than that in the general soil; the content of fine grains (0.250–0.075 mm) was conducive to the increase of water and salt migration, while the content of coarse grains (> 0.250 mm) would inhibit water and salt migration. Under the dual action of water and salt migration and vapor salt migration, secondary salinization occurred in the subgrade soil layer of Chaka saline soil area. It was suggested to set up the gravel salt-blocking layer in the subgrade project of the Chaka saline land area; the grading range of particles should be controlled between 2.0 and 5.0 mm, and the fraction of grains in this size range should be greater than 75%, while the thickness of the gravel salt-blocking layer should be between 300.0 and 1000.0 mm. Besides, the location of the blocking layer should be on the upper part of the embankment, higher than the height specified in the general subgrade code, which can improve the effect of preventing the movement of water-salt and enhancing the effect of vapor-salt leaching.