## 3 Results and Discussion
### Potential for Beach Sand Solidification by MICP Treatment
#### 3.3.1 Effect of Bacteria Population and Reactant Concentration
This filtering mechanism is more significant in this case as the virgin beach sand used here consists of a significant percentage of fines (10.5% fines smaller than 125 \(\upmu\)m). Additionally, the precipitated calcium carbonate crystals may act as a filter and trap bacterial cells from being washed out to the lower portions [7]. The other pertinent reason is the inhibition of the activity of the aerobic bacterium as the availability of oxygen diminishes from top to bottom [58]. The combination of these phenomena is suspected to impair the efficiency of MICP far away from the injection point. Further, the variation in strength due to the effects of input bio-chemical concentration was investigated in detail by SEM imaging. Figure 9 highlights the differences in the microfeatures of the crystals precipitated for 0.5 M and 1.0 M cementation solutions. Overall, for all the treatment variations, rhombohedral calcium carbonate crystals were predominantly observed under SEM which is consistent with the previous findings [27; 35; 41; 59]. For the 0.5 M cases, the CaCO\({}_{3}\) crystals showed a uniform precipitation pattern where the grains were covered with similar sized crystals which were very well distributed spatially on grain surfaces over the entire length of the sample. The contact areas were also covered uniformly and for the case with two bacteria injections, contact cementing and matrix supporting in the column top was characterized with larger crystals of 20-25 \(\upmu\)m in size (Figure 9a). This pattern gave rise to a highly favorable homogeneous strength improvement in the column making it the most suitable treatment condition. But, for the case with single bacterial culture treatment, although uniformity was still maintained, only thin layers of 2-3 \(\upmu\)m (CaCO\({}_{3}\) envelopes) covering individual sand particles and slightly bridging the adjacent grains were achieved which did not provide a satisfactory strength improvement. In the case of 1.0 M treatments, although grain surfaces were predominantly covered with uniformly sized crystals, the precipitation had become more random as more calcite formed. Larger crystals of approximately 30 \(\upmu\)m in size were randomly distributed, filling the pore spaces in the top portion and clogging near the injection point (Figure 9d). [PERSON] et al. [54] stated that flow would be obstructed even without any specific binding of soil particles as long as precipitated crystals are larger than the pore throat. Moreover, only thin layers of differently sized CaCO\({}_{3}\) crystals were present away from the injection top, thus showing greatly hindered precipitation (Figure 9f). It is reported in literature that a change in precipitation pattern in this manner can be explained by the theory behind the competition between crystal growth and crystal nucleation [24; 56]. [PERSON] et al. [60] in their work unraveled that the nucleation of new crystals would face a competition with the process of crystal growth if the nucleation of new crystals is permitted over the growth of existing ones. In the case of high Ca\({}^{2+}\) and urea concentrations (1.0 M in current study), the cladding on grain surfaces by crystals is a result of crystal nucleation in new sites which is the initial dominant process. As the number of injections are repeated, higher and more localized pH rise around some bacterial cells take place, resulting in supersaturation due to the presence of more urea molecules and there is a greater tendency to accumulate precipitate over the initial smaller crystals rather than forming new crystals. This process will then lead to the formation of larger crystals which fill the void spaces and subsequently retard the flow of cementation solution. This has been further proved by [PERSON] et al. [61] who stated that the average particle size of precipitate becomes larger as the carbonate concentration becomes higher.