Water and salt migration and erosion characteristics of gravel sulfate salty soil subgrade under effect of pavement covering

To clarify the water and salt migration characteristics of sulfate-containing gravel mix under the effect of airport pavement covering in the seasonal frozen area of Western China and to identify the erosion hazards at the bottom of the subgrade due to water and salt migration, based on the reconstruction and expansion project of an airport in Northwest China, independently-designed experimental equipment was used to conduct experiments on the water and salt migration and deformation of models with different combinations of salt content in upper and lower parts of the subgrade under the effect of pavement covering and the working condition of freeze-thaw cycles in the temperature range of —30 ℃ - 25 ℃ for the first time. The pavement bottom erosion characteristics under the joint action of freeze-thaw cycles and water and salt migration were analyzed by a scanning electron microscope and an X-ray diffractometer. Experimental results show that the change in water content is 0.37% between working conditions when the salt content in the upper part of the subgrade increases from 0 to 0.3%, which is an increase of 83.8% compared with the situation when the salt content in the lower part of the subgrade increases from 0.3% to 0.7%. During the nine freeze-thaw cycles, the salt content of the gravel mix in each working condition changes significantly within the range of 35 cm below the subgrade, and the migration of the salts upward in the range is greater than the replenishment of the salts migrating from the lower part. When the salt content of the upper part of the salt-containing gravel mix subgrade increases from 0 to 0.3%, the final cumulative deformation of the sample increases by 47.8%, whereas when the salt content of the lower part of the subgrade increases from 0.3% to 0.7%, the final cumulative deformation of the sample increases by only 7.4%. In the prediction model of cumulative deformation of a salt-containing gravel mix subgrade under conditions of varying salt content and cycle time, salt content in the upper part of the subgrade is the dominant factor driving deformation of the sulfate-containing gravel mix subgrade. Under the coupling effect of the water-salt phase change and sulfate erosion, when the salt content of the lower part of the subgrade increases from 0.3% to 0.7%, the salts fail to be replenished to the bottom of the subgrade to maintain the high alkaline environment required for the stability of ettringite. Instead, the amount of ettringite on the bottom surface of the subgrade decreases. The research results provide basic data for revealing the water and salt migration of sulfate-containing gravel mix subgrade under the effect of pavement covering and its erosion law on cement base in the seasonal frozen area.