Desiccation of a sensitive clay: field experimental observations

An excavation with three different levels was exposed to continuous evaporation during 35 days in order to observe and document the formation of shrinkage cracks in an intact and weathered sensitive marine clay. Desiccation of the intact Saint-Alban clay under restrained conditions, at an average evaporation rate of 0.018 cm/h and an initial moisture content of 103%, produced visible primary cracks after 17 h with an average spacing of 20 to 24 cm. Secondary cracks within each original polygon appeared after about 70 h of evaporation. Subhorizontal cracks at a depth of 6 to 8 cm were also created owing to differential strains induced by differential shrinkage of the soil polygons. Further evaporation resulted in the formation of a new set of vertical cracks after about 150 h of evaporation in the soil below the subhorizontal shear plane. The shrinkage induced volume change associated with these different sets of cracks produced polygons with a protuberance at their bottom part indicating that the depth of crack propagation below the subhorizontal shear plane was about 3 to 4 cm. In weathered clay at an initial moisture content of about 50%, the average spacing was 10 to 12 cm, indicating that spacing between primary cracks is related to soil type. In the cohesionless topsoil, no cracks were observed.