Numerous studies have been conducted to assess the effect of microbially calcium carbonate precipitation on the permeability and strength of poor soils. Changes in compressibility, permeability and strength of the treated soil depend on numerous environmental conditions which interfere with microbial response towards specified reagents by so doing affecting their ability to precipitate calcite. For the past few years, utilizing the ability of bacteria to precipitate calcium carbonate (CaCO3) has been found to be a promising eco-friendly approach of soil improvement (DeJong et al., 2006). The evolvement of an alkaline microenvironment as a result of bacterial physiological activity leads to the deposition of calcium carbonate (Douglas and Beveridge, 1998).
Ureolytic bacteria had been employed in conducting Significant investigations on carbonate precipitation by bacteria (Stocks et al., 1999). Ureolytic bacteria enhance the precipitation of CaCO3 through the production of the enzyme urease. Urease enzyme catalyses the hydrolysis of urea to CO2 and ammonia thereby causing an increase in pH concentration and carbonate (Stocks et al., 1999). Bacterial precipitation of calcium carbonate has been found to elevate the bearing capacity of soil (Lo Bianco and Madonia, 2007; Dejong et al., 2006). Bacterial calcium carbonate precipitation has been utilised in the crack repair of granite and concrete (Gollapudi et al., 1995, Ramachandran et al. 2001; Bang et al. 2001; Ramakrishnan, 2007; Jonkers et al., 2009). These precipitations were found to fill pores, reduce permeability through the enhancement of particle bonding (Ivanov and Chu, 2008; Whiffin et al., 2007).
Previous researches have pointed the bacteria Bacillus pasteurii - which exhibits high urease production capability – as a potential candidate suitable for been utilised in biocementation (Bang et al., 2001; Dejong et al., 2010; Bachmeier et al., 2002, Sarda et al., 2009). The researches indicate that biocementation can serve as an effective technique in reducing soil permeability. Because of the damage that moisture causes to building foundations, the need arises for altering the permeability of the foundation soil. Despite the fact that numerous researches have been conducted with the aim of reducing soil permeability, there is limited research on the utilization of bacteria in this field. Ferris et al., (1997) and Whiffin et al., (2007) have observed that biocementation in sandy soil reduced permeability significantly. Nemati and Voordouw (2003) found that calcite cementation in sandstone reduced permeability by 98%. Biocementation arises due to microbial activities that lead to the production particulate binding materials thereby improving the soil structure (Ferris et al., 1997; Nemati and Voordouw, 2003; Whiffin et al., 2007). In addition to enhancing the shear strength of tropical soil, bacterial calcite precipitates have been reported to reduce the permeability of tropical soil, however, high salinity has an inhibitory effect on calcite precipitation by the bacteria (Soon et al. 2013-2014). 22 – 75% soil permeability reduction has been reported by Whiffin et al. (2007). On treating soil with the enzyme urease, Yasuhara et al. (2012) reported a permeability reduction of 60 - 70%. So also, inoculating soil with the bacteria Bacillus megaterium has been reported to induce a 90% reduction in hydraulic conductivity (Soon et al., 2014; Umar et al., 2016.
Calcite precipitation reduces the pore cavities of soil and by extension effecting permeability reduction (DeJong et al., 2010). Chu et al. (2012) studied shear strength reduction and hydraulic conductivity of soil using the ureolytic bacteria Sporosarcina pasteurii isolated from tropical coastline soil. They also found that the cracking modulus of the lean calcium carbonate layer formed on the soil surface was 35.9 MPa, described Microbially induced Calcium Precipitation has been described as a breakthrough technique of soil improvement (Filet et al. (2012.
Bio-mineralized calcium carbonate has been shown to be effective in bioassay of soils and can be used in geotechnical engineering (Ivanov and Chu, 2008). The investigators stressed that these strategies can serve as substitutes to conventional techniques which tend to be expensive and in some cases impact negatively on the environment. Yasuhara et al., (2012) utilized urease enzyme obtained from sources other than microbial in catalysing urea hydrolysis within the proximity of calcium chloride and found out that dramatical increase in strength and 60% reduction of hydraulic conductivity of the treated soil samples occurred. Canakci et al., (2015) investigated the effect of bacterial calcium carbonate precipitate on the compressibility and strength of organic soil and found that bacterial treatment influenced the compressibility and shear strength of the organic soil. Although, several researches utilizing Bacillus sp in sandy soil improvement have been conducted, significant studies investigating the effect of time-dependent Bacillus sp. treatment on the strength and permeability of poorly graded soils have not been conducted.