Water and Salt Migration with Phase Change in Saline Soil during Freezing and Thawing Processes
Water and salt transfer coupled with phase change may cause serious damage to engineering structures in saline soil regions. In this study, the migration of water and salt in silty clay collected from the Qinghai‐Tibet Plateau is explored experimentally and numerically during freezing and thawing processes. The results revealed that there are significant differences in the variations of liquid water content and solution concentration for different initial salt contents, due to salt crystallization and dissolution. The temperature‐induced water migration is determined by the soil properties, which can be well explained by the thermodynamics of mass transfer. The amount of salt migrated upward during cooling is slightly larger than that transported downward in the warming period, implying that salt may be accumulated in the surface soil after a large number of circulations and finally result in soil salinization.
Experimental investigation of the geotechnical properties and microstructure of lime-stabilized saline soils under freeze-thaw cycling
Abstract The Song-Nen Plain in northeastern China holds one of the three largest expanses of soda saline-alkali soil in the world. Due to its disadvantageous geotechnical properties, the saline soil in this area is unable to meet the needs of construction projects. This study evaluates the stabilizing effect of adding lime to such soils. Specifically, it investigates how the physicochemical and mechanical properties and microstructure of soda saline-alkali soil are altered by lime and how these changes are affected by both freeze-thaw cycling and the duration of curing. The results reveal that adding lime to saline soil transforms clay particles into sand and silt particles and causes the pore size distribution (PSD) to become bimodal, with small-pore and macropore populations. An increase in lime content leads to an increase in the unconfined compressive strength (UCS) but a gradual decrease in both the cation exchange capacity (CEC) and plastic index. Freeze-thaw cycles break up coarse particles and aggregate finer particles, increasing the diameter of macropores and decreasing the USC; however, the effect on specimens with >9% lime is negligible. It is further shown that the geotechnical properties of the soil change immediately after the addition of lime, indicating that the cation exchange reaction takes place very quickly. Furthermore, the pozzolanic reaction leads to an increase in the percentage of sand-sized particles and a decrease in porosity after 28 days of curing. Overall, these results indicate that it is feasible to use lime to improve the engineering properties of soda saline-alkali soil in this area.
Field instrumentation for real‐time monitoring of periglacial solifluction
Here we describe new field instrumentation recently installed on a non‐permafrost solifluction slope in Dovrefjell, Norway, and a continuous permafrost solifluction slope in Svalbard. The equipment is designed to provide continuous data on soil thermal status, hydraulic condition, phase change, soil volume strain and soil shear strain. Supporting frames were constructed from steel scaffolding tubes and provide a stable mounting for two LVDT displacement transducers. These are arranged in an inverted triangular configuration with the triangle apex linked to a small steel footplate embedded in the ground surface, allowing continuous monitoring of frost heave, thaw settlement and downslope surface displacements. In addition, thermistors and Druck PDCR 81 miniature pore pressure transducers were installed to a depth of 0.8 m in Dovrefjell and 1.2 m (approximately 20 cm below the permafrost table) in Svalbard. Thermistors were also mounted above the ground surface to 2 m to measure air temperature. All instrumentation was logged at 1 h intervals using a Campbell CR23X logger and multiplexer. In Svalbard an automatic digital camera recording one image per day was installed adjacent to the site in order to monitor snow depth. Simple Rudberg columns will be excavated after several years to observe the profiles of soil movement. Copyright © 2007 John Wiley & Sons, Ltd.