New Methods for Determining the Thermophysical and Hydraulical Properties of Unconsolidated Rocks

Abstract

The heat conductivity and diffusivity as well as the hydraulic properties of unconsolidated rocks are the most important parameters to quantify the conductive and convective heat transfer in the subsurface. Soil mechanical and mineralogical constraints such as thermal conductivity of the individual grain fractions, the dry bulk density and the pressures remain constant. On the other hand, temperature and water content in the unsaturated zone are highly variable in time and space. The change in the ratios of the gas phase and water phase in the pore space has a thermal insulation due to the low thermal conductivity of air. In addition, the partially saturated hydraulic conductivity is a function of water content of the unconsolidated rocks. The air in the pore space reduces the conductive cross-section, which leads to a decrease in hydraulic conductivity. Increasing drainage induces a negative pressure in the pore space and due to hysteresis in drainage and irrigation thus leads to a different distribution of the water. Established computational models provide approximations for the thermal conductivity as a function of water content and other constraints such as temperature. However, for the additional determination of convective transport behavior the unsaturated hydraulic conductivity and water retention function, largely depending on the tortuosity of the pore space, has to be determined simultaneously. Here, for the integrated study of soil mechanics, hydraulic and geothermal properties of unconsolidated rocks, a heat and temperature conductivity meter has been developed. This aparatus includes a control panel, data acquisition, and analysis software. It allows measurements of samples either under constant pressure of up to 7.6 MPa or at constant volume. The temperatures of the samples can be variied from -10 to +80 ° C. The samples can be measured under varying water contents. Additionally to the parameters temperature, pressure, volume and water content the capillary tension is recorded during the measurement. Based on tests in tubes where water was drained and measured with respect to water content, water potential and thermal conductivity in different levels of the column, an evaporation test has been developed. For the simultaneous study of water transportation characteristics and the unsaturated conductivity of undisturbed unconsolidated rock samples it is equipped with a full-space line source to determine the thermal conductivity. This allows the simultaneous detection of thermal conductivity, water retention characteristics and hydraulic conductivity of a soil sample up to the entry point of air into the ceramic tensiometer (approx. -780 hPa). With the methods described tests were carried out on a large number of soils. The results serve to create numerical models of shallow geothermal systems and heat transfer of burried cables.