MEASUREMENT AND MODELING OF THERMAL PROPERTIES OF COARSE-GRAINED SOILS

Abstract

The purpose of this study is to experimentally determine and mathematically represent the thermal resistivity or thermal conductivity dry-out curves (TRDCs or TCDCs) quantifying the relation between thermal resistivity or thermal conductivity and pore water saturation for coarse-grained soils (sands). TRDCs or TCDCs were measured for thirteen sandy soils representing a range of grain characteristics and grain-size distribution. Experiments were conducted using three laboratory methods for comparison, including: (i) a modified hanging column apparatus instrumented for concurrent measurement of the TRDC and soil-water characteristic curve (SWCC), (ii) a staged-drying method involving use of remolded cylindrical specimens subjected to discrete drying increments, and (iii) a multiple-specimen method involving preparation of several identical specimens at different water contents for independent thermal resistivity measurements. Quantitative comparison of resulting TRDCs and error analyses were performed. The modified hanging column method produces the most robust and well-defined TRDCs, but potential overestimation of thermal resistivity may occur during initial drainage, possibly due to forced convection caused by water movement and free convection caused by a larger heat pulse created by the SH-1 (dual needle) thermal probe. The staged-drying and multiple-specimen methods have an advantage of relatively rapid testing time. Producing TRDCs using these methods required 10 days and 1 day, respectively, whereas the hanging column method required an average of 23 days. Analysis of variability shows that observed measurement scatter for the staged-drying and multiple-specimen methods are likely attributed to sample heterogeneity. Based on observations of the measured TCDCs, two new mathematical models were developed for modeling the thermal conductivity of unsaturated coarse-grained soils at room temperature. The models are based on observations of four different regimes in measured TCDCs. Correlations between the model parameters and soil physical parameters are examined. Modeled TCDCs are shown to provide good match with measured TCDCs. Averaged coefficients of determination are 0.957 and 0.974 for the two different models, demonstrating that they may provide better predictions than many previous models for coarse-grained soils