Effect of bio-cementation on thermal properties of silty sand

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2022

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Montana State University - Bozeman, College of Engineering

Abstract

In recent years, there has been an increasing interest in the use of biological technologies in geotechnical engineering to improve thermal properties of geomaterials. Urea hydrolysis is a chemical reaction which can generate favorable conditions that result in the precipitation of calcium carbonate. Certain microbes or plant sources produce the urease enzyme which catalyzes the hydrolysis of urea to form carbonate (CaCO 3) to bond soil particles. Cementation located between the grain particles acts as a highly conductive heat transfer path by increasing the contact area between the sand particles. In this thesis, the applicability of bio-cementation via microbially induced calcite precipitation (MICP) on silty sand specimens with different fines contents of 0%, 5%, and 15% were investigated. MICP promoting fluids were injected into sand-filled columns and the resulting calcium conversion was measured. At the end of the injections, the MICP treated specimens were tested for cementation uniformity. The amount of precipitated CaCO 3 gradually decreased as the distance from the injection ports increases. The observed bio-cementation distribution could be attributed to the filtration of bacterial cells through the soil particles. The resulting effect of filtration on CaCO 3 distribution was observed to be more prominent for silty sands, presumably due to the presence of fine grains. Thermal conductivity measurements were assessed after each pulse during the MICP treatment using a TR-3 sensor. Under the saturated and untreated conditions, thermal conductivity increased with increasing fines content. In addition, MICP treatment can increase the thermal conductivity of saturated silty sands with the increasing number of treatment pulses. An increase of about 18% in thermal conductivity of the soil was achieved at an average CaCO 3 content of 10.7% presumably due to the formation of calcium carbonate bridges binding the soil grains together. The results presented herein suggests that MICP treatment can be a viable option to increase the thermal conductivity of soils in the range of fines content studied here (less than 15%). The findings of this research could be used to improve the efficiency of geothermal boreholes and other energy geo-structures using MICP by improving thermal conductivity of dry and partially saturated soil.

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