A fundamental study of unsaturated flow in compacted clayey soil

Proper design of covers and liners for waste impoundments requires accurate unsaturated permeability functions for compacted clayey soils. Methods of measuring these functions are time-consuming and prone to error. Current methods for determining the unsaturated hydraulic conductivity from measured soil-water characteristic curves (SWCC) result in under-prediction when applied to clayey soils. Therefore, this research embodies a fundamental study of unsaturated compacted soils, with the goal of increasing understanding of unsaturated flow and developing an improved method to predict unsaturated hydraulic conductivity.; Review of the literature reveals that current hydraulic conductivity functions are based on assumptions inapplicable to clayey soils, especially as they do not consider the alteration of pore size during desiccation. The pore-size distribution (PSD) as measured by mercury intrusion porosimetry (MIP) is chosen as a tool for studying the evolution of microstructure during SWCC tests. The PSD is found to be strongly correlated with changes in both water content and volume for soils compacted wet of optimum. Measured PSD curves change significantly in size and shape during SWCC tests.; Analytical methods are developed to predict the SWCC from PSD curves measured before and after the SWCC test. The methods model the phenomena of pore shrinkage, accessibility, and isolation. Model predictions show good agreement with measured SWCC data.; To predict unsaturated hydraulic conductivity, a random network model is developed, which is found to make good predictions of both the SWCC and measured PSD curves. An algorithm to model pore shrinkage as a function of suction is proposed, in which the stiffness of individual pores is assumed to be an inverse function of their current radius. The new algorithm is very powerful, as it is able to predict the measured PSD at any suction given the PSD at the beginning of the SWCC test. It also is able to model compression curves from consolidation tests.; The unsaturated hydraulic conductivity of a compacted soil is measured indirectly using a modification of an existing method, which can be performed simultaneously with an axis-translation SWCC test. The accuracy of the method is evaluated by numerical unsaturated flow modelling. Predictions of unsaturated hydraulic conductivity from the network model agree closely with values measured by the method.; New equations for the SWCC and unsaturated hydraulic conductivity of compacted clayey soils are proposed. The equations are valid for water contents above the shrinkage limit. The predictions of the new permeability equation agree much more closely with measured values than predictions employing standard methods.