Abstract: A recently described phenomenon, the shear induced decrease of moisture suction in saturated clay-water systems and its subsequent recovery, has been investigated with the aid of a parallel plate shearing device. The apparatus allowed a more quantitative description of the phenomenon than was previously possible. In 4–6% Na-montmorillonite pastes it demonstrated a well defined suction response to shear oven for shear angles as small as one degree. The observed decrease in suction was rapid at first. It terminated within a few minutes and was followed by an approximately exponential, occasionally incomplete recovery. All the tests carried out exhibited the same general features of suction change.
It is postulated that shear induces a displacement or change in configuration of particles and that subsequently they return to their original states due to thermal motion. The shapes of the recovery curves can be interpreted in terms of the relaxation spectrum functions encountered in linear viscoelasticity theory. By utilizing a characteristic relaxation time for these spectra, rate process theory has been employed to interpret the bonding mechanism in terms of the experimental activation free energy. The results suggest that the bonds which are re-established during the recovery are primarily of the Coulombic type.
Shear induced suction changes should be considered when dealing with deformation theories and structural models of wet soils and clays. Indeed they provide a means of testing certain aspects of soil structure.