WU Qing-bai, SUN Tao, TAO Zhao-xiang, LI Qiang
Soils with Na2SO4 are one of the special soils in salinized soil regions, from which engineering always suffers. Salt ions in the soils are partly released and crystallized, and migrated from warm section to cool section and from wet section to dry section under temperature gradient, as well as evaporation. Thus, soil expands and salt expansion occurs. A salt expansion experiment for coarse grain saline soil and coarse grain soil with salty layers was design to study the process of salt expansion. At the same time, salt expansion with and without salty layers is compared. It is found that under a constant temperature, salt expansion of coarse-grain soil is smaller when temperature in the upper layers >0℃. The salt expansion amount increases with decreasing temperature when temperature in the upper layers >0℃, and decreases with decreasing temperature after reaching a definite value, because of the overburden load. Salt expansion amount will reach the maximum when temperature at the upper layers arrives -5℃. From the view of temperature change at the upper layers and the process of salt expansion deformation, temperature at the upper layers will reach -5.0℃ after the test is conducted for 10 h. However, temperature at 3 cm depth is only about 0℃. Freezing temperature of a sample is about -2.06℃ (measured value) under this test condition. The sample has not been frozen, thus, no deformation is caused by frost heave. Deformation amount in this moment is 62.8% of the total deformation amount. When temperature at the upper layers is constant after 30 h, temperature at 6 cm depth is 0℃, freezing front is not more than 6 cm. Frost heave occurs in the upper part of the sample. Therefore, deformation is the result of salt expansion, and then enhanced by frost heave. The salt expansion deformation for coarse grain soil with salty layers is obviously greater than that for coarse grain saline soil without salty layers in any temperature range. When temperature >0℃, salt expansion deformation is quit extensive, increasing with elapsed time. The process of salt expansion is consolidation in the initial stage of the test. Then salt expansion deformation linearly changes from consolidation to expansion after 5 or 6 h. Salt expansion is obviously constricted by overburden load. This suggests that salt expansion appears at once after salty layers formed in coarse grain saline soil and encountered with water. When temperature <0℃, salt expansion is smaller than that in temperature >0℃, but the process is the same as that when temperature >0℃. From the view of salt expansion deformation when temperature <0℃, salt expansion is main deformation, and frost heave is secondary one. Temperature in the soil samples is still higher than preezing temperature when temperature at upper layers is -10℃. At the same time, rapidly freezing does not to cause an obvious frost heave. On the other hand, frost heave of coarse grain saline soil is small when water content is small. Therefore, salt expansion is still preponderant deformation when temperature <0℃, and frost heave is able to enforce the deformation process of salt expansion. Sensitive temperature region of salt expansion for coarse grain soil with salty layers is from +3 to +7℃ and +12 to +15℃. However, salt expansion intensity in other temperature range is larger than that of coarse grain saline soil without salty layers. Generally, salt expansion extensively forms in spring and autumn, when many constructions are destroyed in arid regions.
