Analysis of the geometrical characteristics of vertical and horizontal shrinkage cracks

In swelling soils it is, sometimes, observed that in the depth range of macro shrinkage (where crack width > 1 mm) a volume and width of vertical and horizontal cracks are of the same order of magnitude. In the depth range of micro shrinkage, however, it is assumed that the volume and width of horizontal cracks are appreciably smaller than those of vertical cracks. Such behaviour of shrinkage cracking can strongly influence the hydraulic conductivity. The objective of this presentation is to propose a model for explaining this behaviour and for predicting such situations in actual conditions of shrinkage cracking in swelling soils. The model for dry soils is based on the model of multiple cracking and fragmentation originally developed for rocks. In the case of cracking soils, the characteristics of the model become specific functions of depth. The generalized model has been applied for estimating width and volume of vertical cracks in clay soils. It is assumed that appearance of horizontal cracks is caused by faster drying, and correspondingly stronger shrinkage, of thin layers at the walls of the vertical cracks than of the inner moist soil matrix, at the same depth. Accordingly, horizontal cracks start as ruptures in the stretched layers of the drying walls of vertical cracks. The specific volume and mean width of horizontal cracks are determined by a characteristic of the average inhomogeneity of soil subsidence. Satisfactory agreement (quantitative for characteristics of vertical cracks and qualitative for those of horizontal ones) has been found between the model predictions and available experimental data for soil depths up to the maximum crack depth. (C) 1999 Silsoe Research Institute.