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Abstract

The concept of using biological process in soil improvement which is known as bio-mediated soil improvement technique has shown greater potential in geotechnical engineering applications in terms of performance and environmental sustainability. This paper presents a review on the soil microorganisms responsible for this process, and factors that affect their metabolic activities and geometric compatibility with the soil particle sizes. Two mechanisms of biomineralization, i.e. biologically controlled and biologically induced mineralization, were also discussed. Environmental and other factors that may be encountered in situ during microbially induced calcite precipitation (MICP) and their influences on the process were identified and presented. Improvements in the engineering properties of soil such as strength/stiffness and permeability as evaluated in some studies were explored. Potential applications of the process in geotechnical engineering and the challenges of field application of the process were identified.

۱٫ Introduction

Recent studies on applications of bio-mediated soil improvement method have proved the viability of the approach for effective performance and environmental sustainability. The promising outcomes of these studies have shown greater potential of exploring a wider application of the technique in geotechnical engineering. Bio-mediated method of soil improvement has been considered as an inventive and new approach in geotechnical engineering that can be utilized to prevent liquefaction and landslide in loose sand which usually results in foundation deformation and/ or failure (Alvarado, 2009). The great promise of the use of biological treatments has been demonstrated in many applications, such as improving the shear strength and decreasing the permeability of soils (Whiffin et al., 2007; Ivanov and Chu, 2008; Harkes et al., 2010; van Paassen, 2011), improvement in strength and durability of concrete and mortar, remediation of cracks in buildings (Qian et al., 2010; Achal et al., 2013), improvement in engineering properties of soil, and cementation of sand column (Achal et al., 2009a; Dhami et al., 2013). Bio-mediated method of soil improvement generally refers to the biochemical reaction that takes place within a soil mass to produce calcite precipitate to modify some engineering properties of the soil (DeJong et al., 2010). Meanwhile, utilizing the interdisciplinary knowledge of civil engineering, chemistry and microbiology to alter the soil engineering properties in the subsurface has emerged recently (Whiffin et al., 2007; Ivanov and Chu, 2008; Mitchell and Santamarina, 2005; DeJong et al., 2010). The technique utilizes soil microbial processes, which is technically referred to as microbially induced calcite precipitation (MICP), to precipitate calcium carbonate into the soil matrix. The calcium carbonate produced binds the soil particles together (thereby cementing and clogging the soils), and hence improves the strength and reduces the hydraulic conductivity of the soils. MICP can be a practicable alternative for improving soil-supporting both new and existing structures and has been used in many civil engineering applications such as liquefiable sand deposits, slope stabilization, and subgrade reinforcement (DeJong et al., 2006; Cheng et al., 2013). It was revealed that microorganisms influence the formation of fine-grained soils and change the behavior of coarse-grained soils such as strength and hydraulic conductivity. They also facilitate chemical reactions within a soil mass, promote weathering and change the chemical and mechanical properties of specimens after sampling. Hence, the effects of these microorganisms on mechanical properties of soils are still not fully discovered in geotechnical engineering field (Mitchell and Santamarina, 2005). Though it was understood that there are more microorganisms in the subsurface than on the ground, and studies of many years have proved the relevance of biological activities in influencing soil behavior, less work has been done in exploring the importance, relevance, usefulness and application of biology in geotechnical engineering. Meanwhile, it is expected that a clear understanding of the impact of microorganisms and biological activity on soil behavior can lead to proper soil characterization and/or classification and even alternative geotechnical engineering solutions. This paper reviews the concept of biomineralization and its applications in improving the engineering properties of soils.

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