Water-salt migration laws of aeolian sand subgrade in desert area

Based on aeolian sand subgrade soil in desert area, the chemical compositions of samples at different depths at the typical pavement-distressed section were tested, and the migration characteristics of salinity and moisture in subgrade were analyzed based on soil water potential principle.The soil was layered filling with the compaction degree of 95%, the moisture content and electric conductivity of soil pillar model were tested by using self-made experimental device, and the influence of temperature gradient on the water-salt migration rules in subgrade under the condition of optimum water content was analyzed.Research result shows that the subgrade soil of distressed section is fine sand, and sodium sulfate and sylvite are main salt components in both base and subgrade soil.Na₂SO₄ ·10H₂O generates at low temperature -6 ℃-0 ℃to lead to the volume expansion of soil.The salt expansion of subgrade soil and the upheaval destruction of pavement are aggravated because of the entrance of external moisture.After a week of subgrade compaction, the water content reduces by 1%-2% and the content of sulfate radical reduces by 0.05%-0.06% at depth of 5 cm, and the water content improves by 0.5%-0.8% and the content of sulfate radical reduces by 0.12%-0.14% at depth of 35 cm.Under the dual-action of gravity potential and compaction, quick and obvious water-salt stratification occurs in the uniform soil.Under the external temperature, the temperature difference at the depth of 25 cm is 20℃-30℃, but the difference is about 1℃ when the depth is more than 25 cm.So the temperature gradient variation gradually decreases with the increase of depth, and becomes zero eventually.The distributions of water and salt first decrease and then increase with the depth in aeolian sand subgrade, and appear hooklike rule.The water-salt migration in aeolian sand subgrade results from the mixing effect of gas-liquid states.The hydrosphere migration is primary within the depth of 10 cm from subgrade top surface under high temperature.However, because the effective access channels of capillary water can not form below 10 cm in aeolian sand composed of fine sand, the moisture mainly migrates in the form of pellicular water.The liquid water with salinity rises under the cooling effect to lead to salt accumulation effect on the top surface of subgrade.