Effects of wetting-drying cycles and desiccation cracks on mechanical behavior of an unsaturated soil

The effects of wetting-drying (W-D) cycles on the desiccation-induced cracking of a fat clay (CH) as well as the role of soil suction in the evolution of soil strength, were investigated. Cylindrical block soil samples (50 mm in diameter and 100 mm in height) compacted to a dry density of 1.3 Mg/m(3), were subjected to 3 stages of drying at a temperature of 22 +/- 2 degrees C, the first of which had an initial water content of 25%. Three parallel samples were retrieved and tested for unconfined compressive strength (UCS) and surface fracture characteristics at moisture content levels ranging from 13% to 25%. Analyses of images of crack patterns obtained using stereological software, indicate that the pattern of increases in UCS with decreasing moisture content induced by drying, is strongly affected by the intensity of cracks on the samples. Desiccation cracking only occurred in the 2nd and 3rd stages of soil drying. During the first stage of drying, the stress-strain curve of the soil exhibited increase in slope with decreasing water content. This implies the domination of suction at the initial stages. The number of crack segments observed on samples, increases with W-D cycles, thus decreasing soil brittleness. However, increase in W-D cycles does not significantly affect the soil water retention characteristics. These findings illustrate the significant role played by soil suction in strengthening soils against the degradation effects of cracks during the early stages of drying. As drying proceeds, the proliferation of cracks eventually reduces soil strength as soil suction is overcome. Therefore, in developing structures on clayey soils that will be subjected to seasonal changes in moisture content, assessments of the balance of the effects of suction and soil desiccation cracking should be made.