Wetting hydraulic properties of cracked soils
Date of Issue2015
School of Civil and Environmental Engineering
In the field, desiccation cracks in soil occur commonly at the ground surface. The presence of cracks results in an increase in water flow rate in this zone due to lesser resistance to flow of water through crack openings. Some models have been proposed previously to analyse flow through cracked soils. However, a model to quantify lateral flow rate through a cracked soil and a model to analyze change of water content of cracked soil incorporating an actual crack network in the soil has not been fully developed. Laboratory experiments to investigate performance the model also need to be performed. In addition, variation of crack network in the soil need to be incorporated in the model. In this study, a framework to analyze lateral flow through cracked soil was developed. The framework consists two aspects: a model to calculate flow rate through the crack network and a method to analyze change in water content and matric suction of a cracked soil. In this study, the crack network in the soil is idealized as series of linear cracks. In the proposed model to calculate flow rate through the crack network, the flow through a single crack is modelled as a flow through parallel plates and the flow rate through the idealized crack network is calculated by incorporating the conservation of mass principle and the additional head losses due to the change in crack aperture. In the proposed method to analyse change in water content of cracked soils, the idealized crack network is modelled as head boundary conditions and the boundary conditions are then incorporated in a numerical analysis. In addition to that, average water content and average matric suction was proposed to represent the variation in water content and matric suction in a horizontal plane of a cracked soil. Experiments were performed to investigate performance of the proposed model. The experiments mainly consisted of small and large scale lateral flow tests. Small scale and large scale lateral flow apparatuses were developed to perform the tests. The large scale lateral apparatus was developed to capture large number of cracks in the specimens. Cracked soil specimens were obtained by desiccating the soil in room temperature. Flow rates were measured during the lateral flow tests. Water contents were measured during and at end of tests. Instrumentations were utilized to measure change of water content during one of the large scale lateral flow tests. Numerical analyses were performed to investigate performance of the proposed model to analyze change in water content and matric suction of the cracked soil. The results obtained from numerical analyses were compared with those obtained from laboratory experiments. A parametric study was performed to investigate the effect of variation of crack network on change in water content and matric suction of cracked soils. A computer code to generate random crack networks was developed. The computer code generates random crack network from the statistical parameters of the crack network. Connectivity among cracks endpoints are incorporated in the calculation to imitate the desiccation crack network in soils which is interconnected each other. Numerical analyses using the proposed method of change in water content were performed incorporating variation of crack networks. A comparison of the predicted and measured lateral water flow rates showed that the proposed model was able to predict the lateral flow rate through the crack network quite well. Proposed method to analyze change in water content and matric suction of a cracked soil was also able to predict change in water content and matric suction of cracked soils. The results were closer to the measured values than those obtained by modelling the cracked soil as a continuum. This finding indicated that in order to model change in water content and matric suction of cracked soils, the cracks should be modelled as head boundary conditions. In addition, the parametric study showed that with the same statistical parameters of crack network, different average of water content and matric suction can be obtained. Values of maximum variations in average water content and average matric suction were found in this study.