Performance of unsaturated clay-based barrier under opposing thermal and hydraulic gradients

Experimental and numerical studies of coupled heat and moisture flow induced in an unsaturated soil by simultaneously imposed temperature and hydraulic gradients, in opposite directions, are presented. The experimental portion of the study was designed to provide information on the moisture and temperature distributions in relation to elapsed time and distance from the heat source. Tests were conducted on three different materials: (a) buffer (equal proportion of silica sand and sodium bentonite), (b) backfill (25 percent low swelling clay and 75 percent crushed granite rock), and (c) sand. Each material was compacted to its maximum dry density and optimum water content. The experiments were performed with the application of external hydraulic and temperature gradients in opposite directions. Swelling pressure tests were performed to quantify the local swelling pressure as well as the wall's effect on the resultant reaction pressure. The effect of swelling pressure on hydraulic pressure and local volume change has been studied using the experimental results.;In the theoretical part of this research, a numerical solution of the governing coupled heat and mass flow equations was obtained via the implicit finite difference method. The solution was then used in conjunction with Powell's optimization technique to back-calculate the transport coefficients. A sensitivity analysis of the volumetric water content and temperature with respect to the transport coefficient was performed. Using the calculated transport parameters, the moisture content profile was predicted.;The experimental results have shown that moisture flows from the hot and cold ends to the middle part of the soil column. The developed local swelling pressure appears to have significant effect on moisture movement and hydraulic pressure. The use of a sand layer between the buffer and waste container has been observed to be very useful in reducing the shrinkage of the buffer material.;The numerical results strongly support the dependence of the transport coefficients, in the governing coupled heat and moisture flow equations, on temperature and volumetric water content.