Osmotic Suction in Unsaturated Soil Mechanics

Experimental research was conducted to evaluate the effects of salt content and osmotic suction on suction measurements, moisture retention, and shrinkage behavior of a natural compacted clay soil. Results are presented for: (1) soil water characteristic curves (SWCCs) in terms of matric, total, and osmotic suction, and (2) unconfined shrinkage tests on natural and salt-amended compacted soil specimens. These experimental results show that the measurement of total suction is dominated by osmotic suction, even when only moderate amounts of salt are present. Although osmotic suction dominates the total suction measurement, its influence on soil behavior is relatively small compared to the effects of changes in matric suction. It is common practice in engineering to use total suction as a substitute for matric suction, because total suction can be readily measured over the entire range of soil moisture up to 10⁶ kPa using psychrometric techniques. Matric suction is measurable only up to approximately 1500 kPa maximum. Theory and experiments described in this paper are intended to illustrate how the use of total suction as a proxy for matric suction may be problematic when osmotic suction is large due to the presence of salts in the soil.