In recent soil mechanics literature attempts have been made to explain the engineering properties of clays in terms of surface chemical theories, particularly the Gouy—Chapman theory. Experimental confirmation of these theoretical predictions have been restricted almost exclusively to studies performed on fine grain size fractions of montmorillonite. It seemed desirable to determine experimentally the effect of physico-chemical variables on the engineering properties of a more typical engineering material. The < 2 μ fraction of Fithian illite was selected.

A study was made of the effect of variations in the pore water electrolyte concentration on the engineering properties of homoionic Ca- and Mg-illite. Pore water electrolyte concentrations ranged from 1 N down to about 10-4 N. Engineering tests included Atterberg limits, one-dimensional consolidation tests, and effective stress triaxial tests.

The electrolyte concentration was found to have only a small effect on the Atterberg limits, the maximum limits occurring at electrolyte concentrations between 0.01 N and 0.1 N. For virgin consolidation the soil seemed strongest at about the same range in electrolyte concentrations but the effect was not large. Electrolyte concentration had almost no effect on the position of the rebound (swelling) curves. The most important effect of electrolyte concentration seemed to be its effect on the geometric arrangement of the particles for samples that were sedimented from dilute suspension. The geometric arrangement of particles seems to be a more significant variable than the osmotic repulsion between particles.

There still appears to be merit in the old mechanical approach where the compression characteristics are explained by elastic bending (reversible), slippage (partially reversible), and rupture (irreversible) of particles. Double layer phenomena seem to exert a much smaller influence on the engineering properites of nonexpanding lattice clay minerals than is commonly inferred in the literature.