## 1 Introduction
\[\text{CO(NH${}_{2}$)}_{2}+\text{H${}_{2}$O}\xrightarrow{Utense}\text{NH${}_{3 }$}+\text{NH${}_{2}$COOH} \tag{1}\]  
\[\text{NH${}_{2}$COOH}+\text{H${}_{2}$O}\xrightarrow{}\text{H${}_{2}$CO${}_{3 }$}+\text{NH${}_{3}$} \tag{2}\]  
\[\text{H${}_{2}$CO${}_{3}$}\xrightarrow{}\text{H${}^{+}$}+\text{HCO${}_{3}^{-}$} \tag{3}\]  
\[\text{NH${}_{3}$}+\text{H${}_{2}$O}\xrightarrow{}\text{NH${}_{4}^{+}$}+\text{ OH${}^{-}$} \tag{4}\]  
\[\text{HCO${}_{3}^{-}$}\xrightarrow{}\text{H${}^{+}$}+\text{CO${}_{3}^{-}$} \tag{5}\]  
\[\text{CO${}_{3}^{2-}$}+\text{Ca${}^{2+}$}\xrightarrow{ Bacterial cells}\text{CaCO${}_{3}$}\downarrow \tag{6}\]  
The enhancement in the mechanical behavior of MICP treated sands is accomplished by a few physical mechanisms based on how calcium carbonate is precipitated with respect to the particles. Calcium carbonate crystals can grow on grain surfaces individually coating them (grain coating), or may precipitate at or near contact points of particles (contact cementing), or grow into the void spaces from particle surfaces, subsequently creating calcium carbonate bridges among sand grains (matrix-supported) [27]. Further, calcium carbonate cements can grow, filling the pore spaces without bridging the sand grains thus lowering the hydraulic conductivity of the treated samples. However,the overall improvement in the mechanical properties of loose sand following MICP treatment is due to a combination of all four aforementioned cementation mechanisms (Figure 1). Moreover, MICP can be harnessed for in-situ sand/soil applications in two different approaches: biostimulation and bioaugmentation. Biostimulation is the process of modifying prevailing environmental conditions (substrates, nutrients) to encourage calcium carbonate precipitation through enrichment of natural indigenous urea hydrolyzing bacteria [5; 28]. Although stimulated ureolytic species are more resilient for in-situ applications, their urea hydrolysis rates seem to be lower than the specialized laboratory cultivated strains [5], and level of MICP will be limited to the initial bacterial cell concentration in the soil [29]. In contrast, bioaugmentation is implemented by supplementing the soil of concern with bacterial strains having specific metabolic capabilities along with nutrient and precipitation media [28; 30]. Generally, in-situ bioaugmentation with exogeneous strains is challenging as injected foreign cultures may face competition for survival and proliferation among native communities along with poor compatibility in new environments [25; 30; 31]. Therefore, a more successful approach would be to augment the soil to be treated with native ureolytic bacteria that has been isolated and enriched in laboratory culture media in advance. The main objective of the present study is to evaluate the effectiveness of MICP treatment via bioaugmentation to stabilize loose beach sand in Sri Lanka, an island highly vulnerable to coastal erosion. As an initial step towards achieving this goal, native ureolytic strains were isolated from Sri Lankan sand/soil and assessed for their potential for bioaugmentation. Upon receiving more successful results for MICP treatment using an inland ureolytic bacterium than that from local marine bacteria for the desired engineering application, further studies were carried out to (i) optimize the growth and enzymatic activity of the isolated strain, (ii) address mechanical behavior and microstructure of the MICP-treated sand and (iii) explore the versatility of the isolated inland strain to perform biomedical calcium carbonate precipitation in nearshore environments as an alternative technique for sand stabilization. Figure 1: Cementation mechanism of microbial induced carbonate precipitation (**a**) grain coating; (**b**) contact cementing; (**c**) matrix-supported; (**d**) pore-filling.