Up and Down Soils—A Reexamination of Swelling Phenomena in Earth Materials

In his first published paper in 1960 Professor Charles C. Ladd stated that several soil composition, state, and environmental factors govern the swelling behavior of a soil. He summarized the theoretical basis for analysis of interparticle forces and swelling in terms of diffuse double layer theory and osmotic pressure. He contributed data on the influence of compaction conditions and pore water chemistry on the swelling behavior of compacted fine-grained soils. When examined from the perspective of today's understanding of soil behavior, the principles he stated 40 years ago still apply. Theoretical and experimental studies carried out in the intervening years, and summarized in this paper, have provided further clarification of the most important factors controlling swelling and the qualitative and quantitative applicability of double layer and water adsorption theories of swelling. The most important single factor influencing the swell potential of a soil is the effective specific surface. The smectite clays (montmorillonite) have the greatest potential for expansion owing to weak interlayer bonding and, therefore, the large specific surface available for water adsorption. In the presence of high concentration salt solutions and/or potassium or divalent adsorbed cations interlayer swelling is suppressed, and the amount of swell is much less than when sodium is the dominant adsorbed cation. Current research on the fundamentals of water adsorption and clay swelling employs Monte Carlo and Molecular Dynamics modeling to obtain knowledge of the distribution and energy states of water and cations adjacent to clay particle surfaces. There may be applications of the results of these current studies to practical problems in geotechnical engineering in the future.