Improving the Strength of Sandy Soils via Ureolytic CaCO<sub>3</sub> Solidification by <i>Sporosarcina ureae</i>

Abstract

15'Microbial induced carbonate precipitation' (MICP) is a biogeochemical process that can be applied to strengthen materials. The hydrolysis of urea by microbial catalysis to form carbonate is a commonly studied example of MICP.In this study, Sporosarcina ureae, a ureolytic organism, was compared to other ureolytic and non-ureolytic organisms of Bacillus and Sporosarcina in the assessment of its ability to produce carbonates by ureolytic MICP for ground reinforcement. It was found that S. ureae grew optimally in alkaline (pH ~9.0) conditions which favoured 20 MICP and could degrade urea (30.28 U/mL) at levels similar to S. pasteurii (32.76 U/mL), the model ureolytic MICP organism. When cells of S. ureae were concentrated (OD 600 ~15-20) and mixed with cementation medium containing 0.5 M calcium chloride (CaCl 2 ) and urea into a model sand, repeated treatments (3 x 24 h) were able to improve the confined direct shear strength of samples from 15.77 kPa to as much as 135.8 kPa. This was more than any other organism observed in the study. Imaging of the reinforced samples with scanning electron microscopy and 25 energy dispersive spectroscopy confirmed the successful precipitation of calcium carbonate (CaCO 3 ), organized as calcite, across sand particles by S. ureae. Treated samples were also tested experimentally according to model North American climatic conditions to understand the environmental durability of MICP. No significant (p < 0.05) change in strength was observed for samples that underwent freeze-thaw cycling or flood-like simulations.However, samples fell to 29.2 % of untreated controls following acid-rain simulations. Overall, the species S. ureae 30 was found to be an excellent organism for MICP by ureolysis to achieve ground strengthening. However, the feasibility of MICP as a durable reinforcement technique is limited by specific climate conditions (i.e. acid rain).

Stabilization Characteristics of Different Loose Sandy Soils using Microbial-Induced Calcite Precipitation

“…This method is a bio-geochemical process that induces carbonate precipitation within the soil matrix. This method is mainly composed of 3 constituents, namely, alkalophilic microbe, substrate solution (urea), and calcium ion solution [10][11][12][13][14], where bacteria feed on the nutrients found in media to grow the cells, urea is used as a substrate hydrolyzed material, and calcium acts as the energy source to form the calcite [15][16][17]. The cohesion of sand particles by biocementation is very useful in geotechnical engineering to minimize the effect of erosion and increase slope stability [18].…”