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Water Adsorption and Swelling of Clay Minerals in Soil Systems¹

ABSTRACT

Volume changes of undisturbed (U) samples of Poplimento and Iredell B2 horizons, and of samples compacted at optimum (CO), below optimum (CL), and above optimum (CH) moisture were measured during wetting, drying, and re-wetting. Poplimento contained 9% montmorillonite and 11% vermiculite while Iredell had 48% montmorillonite and 15% vermiculite on a whole-soil basis. During the initial wetting of Poplimento samples, variations in volume change between treatments were small. This suggests that changes in soil structure with increasing moisture during compaction counteracted normal moisture-dependent differences by reducing space for expansion into larger voids or by increasing expansion from entrapped air pressures, strain relaxation, and differential strain. Compacted Iredell samples displayed an inverse relationship between moisture at compaction and swelling during the first wetting. Undisturbed Iredell samples exhibited considerably less expansion than compacted samples.

Expansion during the second wetting followed the order U CO CH > CL for Poplimento and CO > U CH > CL for Iredell. Swelling attributable to entrapped air pressures accounted for 59, 50, 39, and 8% of the total expansion for U, CL, CO, and CH Poplimento samples and for 23, 13, 27, and 22% of similarly treated Iredell samples. These values do not appear to reflect the inverse relationship between entrapped air pressures and probable pore size which is suggested by the Laplace equation. Rate of water uptake and resistance to expansion may explain this discrepancy. Estimated osmotic adsorption accounted for 30, 61, 32, and 31% of the volume change of U, CL, CO, and CH Poplimento samples and 46, 51, 43, and 46%, respectively, for Iredell samples. Intracrystalline swelling accounted for only 3 to 6% of the total expansion in Poplimento and 8 to 9% in Iredell.

The discrepancy between these three components of swelling and measured expansion was taken as an indicator of the magnitude of swelling caused by crystal strain relaxation and differential strains between adjacent crystals or aggregates. These components appeared to increase with increasing compaction moisture for Poplimento, while in Iredell they were unaffected by treatment. For the CL Poplimento sample the sum of these components exceeded the measured swelling suggesting accommodation of osmotic swelling within relatively large pores.

¹ Contribution from the Agronomy Department, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061.

² Assistant Professor and Associate Professors, respectively.

Received for publication November 17, 1980. Accepted for publication February 8, 1982.