Microbially induced calcite precipitation (MICP) harnesses the natural metabolic action of bacteria to induce the precipitation of calcium carbonate and alter soil engineering properties. This paper presents the results of using MICP to improve the cyclic resistance of Fraser River sand specimens. The formation of calcite cementation among sand particles is confirmed using scanning electron microscopic images and X-ray compositional analysis of cemented sand clusters. The results show that the velocity of a shear wave (VS) traveling through the specimen starts to increase just as the calcium solution is introduced into each specimen. Liquefaction resistance of sand samples is subsequently measured in a series of cyclic direct simple shear tests. MICP-treated samples exhibit cyclic resistances of up to 67% higher than those of the untreated sand. Post-liquefaction volumetric strain and changes in cyclic resistance in a repeated cyclic loading are also assessed and compared for the original and the treated sand specimens.

Introduction

Despite being the least abundant element in the Earth’s crust [1], carbon is commonly found on the planet’s surface as large reservoirs of organic matter or as inorganic carbon in carbonate rocks such as limestone [2]. Many organisms mediate in what is known as the “carbon cycle”, by fixing inorganic carbon to form organic carbon and re-mineralizing organic carbon back to inorganic carbon. Particularly, bacteria facilitate the deposition of carbonate minerals on the Earth’s surface by precipitating calcium carbonate (CaCO3) extracellularly as a result of their metabolic process [3–۵]. In an environment with a sufficient concentration of calcium ions (Ca2þ), CaCO3 precipitation can be stimulated by a microbial metabolism that increases the pH and the concentration of carbonate ions (CO3 2 ) on the cells’ surface. These cells in turn serve as nucleation sites for the precipitated mineral. This process is generally referred to as a “microbially induced calcite precipitation (MICP)” and can occur via different metabolic processes. MICP via urea hydrolysis involves the use of the microbial enzyme urease (urea amidohydrolase; EC 3.5.1.5) to hydrolyze or break down the organic compound urea. Urease positive bacteria, such as Sporosarcina pasteurii, use urea as a source of nitrogen and energy [6] and produce the enzyme urease at different levels depending on the bacterial strain [7]. Precipitation of CaCO3 typically begins with the formation of an amorphous form of CaCO3 with low stability and high solubility, followed by a transformation into a metastable and transitional phase known as vaterite, and ending in the subsequent transformation into a more thermodynamically stable state as calcite [8]. In a soil, the precipitated calcium carbonate can cement soil particles and fill void spaces. An alkaline environment with pH ¼ ۸٫۳ to 9.5 [6,9] is critical to trigger the hydrolysis of urea. If the pH level becomes acidic (<7.0), the precipitated CaCO3 will begin to dissolve as opposed to fostering further precipitation in the above chain of reactions. A local rise in pH may also cause the microbes themselves to serve as nucleation sites for calcite formation on the surface of bacterial cells [10].