Enhancement of the thermal conductivity of sands via microbially-induced calcite precipitation

Abstract

Energy piles and ground source heat pump systems have been shown to provide sustainable alternatives for temperature regulation in buildings and other applications such as road de-icing. However, their efficiency can be undermined in partially-saturated and dry sandy soils due to the relatively low thermal conductivity (kt) of these materials. Microbially-Induced Calcite Precipitation (MICP) has been demonstrated to be an environmentally-conscious ground improvement technology capable of modifying the engineering properties of sandy soils including increases in shear stiffness and strength and decreases in hydraulic conductivity. These improvements result from the precipitation of calcium carbonate crystals at inter-particle contacts and on particle surfaces. This paper presents results from a soil column study aimed at investigating changes in soil kt during MICP treatments and subsequent desaturation using a poorly- graded sand. The results indicate that while bio-cementation can increase soil kt, the level of enhancement depends on the degree of saturation. For instance, increases of up to 330% were measured under dry conditions while only modest increases of about 15% were measured under saturated conditions. MICP treatment may therefore be most effective at enhancing the kt of partially-saturated and dry sands. In addition, the similarity between the evolution of kt and shear wave velocity (Vs) during MICP treatment suggests that kt may provide a new method to assess cementation level and contact quality.