Soil Dynamics and Earthquake Engineering 131 (2020) 106034 Available online 16 January 2020 0267-7261/© 2020 Elsevier Ltd. All rights reserved. Liquefaction resistance of Fraser River sand improved by a microbially-induced cementation Guillermo Alexander Riveros , Abouzar Sadrekarimi * Department of Civil and Environmental Engineering, Western University, London, Ontario, Canada A R T I C L E INFO Keywords: Cyclic simple shear test Liquefaction resistance Microbial-induced calcite precipitation Shear wave velocity ABSTRACT 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 confrmed using scanning electron microscopic images and X-ray compositional analysis of cemented sand clusters. The results show that the velocity of a shear wave (V S ) 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. 1. 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 lime- stone [2]. Many organisms mediate in what is known as the “carbon cycle”, by fxing 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 (CaCO 3 ) extracellularly as a result of their metabolic process [3–5]. In an environment with a suff- cient concentration of calcium ions (Ca 2þ ), CaCO 3 precipitation can be stimulated by a microbial metabolism that increases the pH and the concentration of carbonate ions (CO 3 2 ) on the cells’ surface. These cells in turn serve as nucleation sites for the precipitated mineral. This pro- cess is generally referred to as a “microbially induced calcite precipi- tation (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 Spor- osarcina 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 CaCO 3 typically begins with the formation of an amorphous form of CaCO 3 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 fll void spaces. An alkaline environment with pH ¼ 8.3 to 9.5 [6,9] is critical to trigger the hydrolysis of urea. If the pH level becomes acidic (<7.0), the precipitated CaCO 3 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]. The biological nature of MICP has made it an appealing and environmentally-friendly process to improve the cyclic resistance and liquefaction behavior of sandy soils [11–15]. In dynamic centrifuge model tests on a thick deposit of loose Ottawa 50/70 sand, Montoya et al. [12] demonstrated that MICP treatment increased liquefaction resistance and reduced excess pore pressure following the application of a cyclic load. Han et al. [13] found that microbial cementation improved the cyclic liquefaction resistance of a poorly-graded sand by as much as that achieved by chemical silica grouting, but within a much shorter time period. In cyclic triaxial tests on three different types of silica sands, Zamani et al. [15] observed that the improvement in cyclic resistance due to MICP treatement was dependent on the size and the shape of sand * Corresponding author. E-mail addresses: griveros@uwo.ca (G.A. Riveros), asadrek@uwo.ca (A. Sadrekarimi). Contents lists available at ScienceDirect Soil Dynamics and Earthquake Engineering journal homepage: http://www.elsevier.com/locate/soildyn https://doi.org/10.1016/j.soildyn.2020.106034 Received 31 July 2019; Received in revised form 29 December 2019; Accepted 2 January 2020