Effective stress in clays of various mineralogy

Effective stress changes during the drying process were determined for three clays. Suction stress decreased, remain nearly constant, and increased with increasing suction for kaolinite and illite clays but decreased monotonically for montmorillonite clay. Decreasing suction stress caused an increase in effective stress. Considerable suction stress occurred at very high suction for the illite and montmorillonite clays. The effective stress in clays possessing a significant proportion of one of the clay minerals kaolinite, illite, or montmorillonite was determined based on the suction stress characteristic curves (SSCCs) of the clays. The SSCCs were determined based on the drying soil-water characteristic curves of the clays for a suction range of 0.03 to about 219.0 MPa. One-dimensional compressibility behavior of initially saturated clays was also studied by loading clay specimens up to a maximum vertical stress of 21.0 MPa. The effective stress–void ratio responses of the clays during the drying process were compared with their saturated counterparts. The shapes of SSCCs and the magnitudes of minimum and maximum suction stress were strongly dependent on the mineralogy and the properties of the clays. For the clays with kaolinite and illite as the dominant clay minerals, the suction stress decreased, remained nearly constant, and then increased with an increase in the applied suction, whereas it decreased monotonically with increasing suction for the montmorillonite clay. A decrease in the suction stress caused an increase in the effective stress, which in turn reduced the volume of the clays. For applied suctions smaller than the air-entry value of any clay, equal magnitudes of suction stress and effective stress produced a similar volume change of the clay. The study clearly showed that suction changes beyond the air-entry value are less effective in producing volume changes in unsaturated soils, primarily because of a decrease in the effective stress due to an increase in the suction stress