Spatio-temporal distribution of temperature in an expansive soil under a low-cost house

Abstract

Expansive soils are rich in smectite with shrink and swell behaviour in response to changing water contents. These soils are a worldwide problem causing damage to engineering structures, as is the case in Land Type Dc17, east of Bloemfontein. In the built environment, soil temperature is the main driving force behind soil water migration, responsible for volume changes in expansive soils. Little is known on soil temperature underneath houses built on expansive soils and its contribution to structural deterioration. This study was conducted with the goal of characterizing soil temperature underneath a basic house built on an expansive soil and to better understand thermal behaviour of these soils. This was attained through a number of specific objectives: (1) To classify and categorize expansive soils in Land Type Dc17; (2) To assess the effect of soil water content and (3) temperature on thermal properties (thermal conductivity, volumetric heat capacity and thermal diffusivity) of expansive soils; and finally (4) To characterize temperature variation underneath a low-cost house in comparison to bare soil in Botshabelo (Land Type Dc17). Five soils were selected in the study area and classified through laboratory analyses as Sepane (1210), Bonheim (1210), Swartland (1121), Valsrivier (1120) and Arcadia (1100). The soils had higher contents of Mg2+ than Na+, thus low hydraulic. Arcadia soil form had high swelling potential, with Sepane, Bonheim, Swartland and Valsrivier having a medium swelling potential. To study the effect of increasing soil water content on thermal properties, different water content ranges (from moderate to near saturation), were created in situ by saturating profile pits and sampling during the desorption period. Generally, with increasing water content, thermal conductivity and diffusivity increased, but decreased as near saturation was reached. Volumetric heat capacity increased with increasing water content to near saturation. The significance of these trends depended on individual soil forms. The “moist” water range samples were subjected to increasing temperatures (0 to 60°C) to study the effect of temperature on thermal properties. In all the soil forms, these properties decreased from 0 to 10°C and increased with further increase in temperature from 10 to 60°C. The significance of these trends was however depended upon individual soil form. To characterize temperature variation underneath the house in comparison to bare soil, capacitance probes were installed to a depth of 1 m under the foundation and in bare soil adjacent to the house as a control, and monitored for two years. Over a 24 hour period, there was practically no variation in soil temperature under the house (profile average). In summer and spring, temperature under the house was cooler during day time, while at night it was warmer than bare soil. In winter and autumn the profile temperature under the house was warmer throughout the 24 hour period. Seasonally, temperature of the soil profile under the house fluctuated less and was cooler than bare soil in summer and spring. In winter and autumn, temperature under the house was warmer than in bare soil. For both surface treatments, soil temperature decreased with depth in summer and spring, with cooler temperatures under the house at all depths. In winter and autumn, soil temperatures for both surface treatments increased with depth, with warmer temperatures under the house at all depths.