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Relation between specific surface area and soil water retention

Specific surface area (SSA) is a soil property that is important for porous media characterization. Correlation between SSA and plasticity, swelling potential, sorption behavior and surface interactions have been proposed. Common methods for determination of SSA are adsorption methods where the accumulation of a material (e.g. nitrogen (N2), Ethylene Glycol Monoethyl Ether (EGME)) on surface is measured. Using water vapor as a probe molecule on surface is preferred because it can best represent the filed behavior and also penetrate to both internal and external surfaces. Current methods for SSA determination based on using water vapor (single point method or BET model) fails to predict it correctly especially for SSA>150 m2/gr. In this paper, a method for calculation of SSA based on water content in a single point (for expansive soil) or in a range of relative humidity (for non-expansive soil) is provided. Three different hydration mechanisms, around cation, on interlayer particle surface and on external particle surface, are distinguished enabling us to find the single point or a range of relative humidity corresponding to monolayer water coverage on particle surface. Monolayer water coverage can be obtained from the hysteretic behavior of a soil’s specific moisture capacity (SMC), or from the soil water retention curve (SWRC) interpreted by a modified BET sorption model. Comparison between the result of SSA based on EGME and proposed method based on water vapor reveals the reliability and accuracy of the method. This method enables us to use water vapor for SSA determination of expansive soil, while other methods based on water vapor cannot yield to the real SSA values of expansive soils.

Author(s):

Morteza Khorshidi    
Colorado School of Mines
United States

Ning Lu    
Colorado School of Mines
United States