Desiccation cracking of soils
Lau, Jacky Tak Kwai
The depth of cracking of soils is often required for boundary value and limit equilibrium analyses in geotechnical engineering. At present, the depth of cracking is customarily expressed as a function of shear strength using the Rankine theory of lateral earth pressure. The objectives of this thesis are to study the mechanism of desiccation cracking in soils and to propose a mathematical model for the prediction of crack depth. Observations obtained from the laboratory program indicated that the locations of desiccation cracks and the crack spacing were highly dependent upon the inhomogeneities in the soils. Based on the experimental results, desiccation cracks were initiated at a matric suction of less than 10 kPa for silty and clayey soils. Silty soils are expected to require a higher matric suction at cracking than do clayey soils. The volumetric shrinkage strain at cracking for Indian Head Till was about 7%. Regina Clay is expected to require a lower volumetric shrinkage strain at cracking than does Indian Head Till. Two mathematical expressions were derived using the volume change (i.e., elastic equilibrium analysis) and shear strength (i.e., plastic equilibrium analysis) behavior of unsaturated soils for the prediction of crack depth. Based on the result of a parametric study, it was found that the crack depth predicted by the plastic analysis was almost twice as deep as that predicted by the elastic analysis. Since desiccation cracks are formed as a result of soil volume reduction, the elastic analysis is expected to be more appropriate for the prediction of crack depth, although the validity of the analysis must be confirmed with future studies on the desiccation crack depth in the field.