## 3 Results and Discussion
### Potential for Beach Sand Solidification by MICP Treatment  
#### 3.3.1 Effect of Bacteria Population and Reactant Concentration  
The reactant and the bacteria cell concentration are two primary factors that governs the degree and the homogeneity of the strength increment over the treatment depth. The local strength of sand specimens underwent different treatment conditions with respect to the aforementioned two parameters, are illustrated in Figure 7. However, the cementation reagent contained equimolar concentrations of urea and CaCl\({}_{2}\) as in [PERSON] et al. [54]. All the samples were strongly cemented and the estimated average (as well as individual) UCS values from two independent tests for all the cases were above 3 MPa. When the two samples injected with 0.5 M cementation solution were considered, it was clear that the specimen with bacteria injected twice showed a significant strength increment than that of the specimen injected with bacteria only in the beginning. According to previous records, it is deduced that higher concentrations of bacteria populations enhance the amount of CaCO\({}_{3}\) precipitate and, thus, the results of MICP treatment [41]. Generally, bacteria play two key roles in the bioccmentation process; (i) catalysis of urea hydrolysis and (ii) provision of nucleation sites [55]. The initial investigations on urease activity of the isolated strain showed that enzymatic activity diminishes over time. Besides, there are several other factors that may gradually inhibit the bacteria's affinity to perform their intended role as a catalyst for urea hydrolysis over the entire treatment time period. Possible reasons are hydraulic constraints such as cell encapsulation from the resulting precipitate or being confined inside pores, restricted movement of nutrients and precipitation components as pore spaces get narrowed after initial precipitation, space constraints in the case of saturated conditions, accumulation of metabolic wastes and also flushing out of bacteria [50; 56]. Further, bacterial cell walls act as nucleation sites for CaCO\({}_{3}\) precipitation in biochemical processes [13; 57] and some of the above mentioned factors may lower the availability of cells that can serve this purpose. Therefore, it is concluded here that the rate of urea hydrolysis and subsequent bioccmentation was retarded over time in the case where bacteria was injected only in the beginning, whereas the urease activity was regained and number of nucleation sites were improved with the reinjection in the case where culture was injected twice, leading to a better performance. This phenomenon was further confirmed by the measurement of Ca\({}^{2+}\) concentration and the pH of the samples collected from the solution draining out from the outlet (Figure 8). Better performance of microbial induced carbonate precipitation was characterized by lower Ca\({}^{2+}\) concentration and higher pH values as soon as the bacterial culture was injected. Figure 8: (a) pH; (b) Ca\({}^{2+}\) measurements of the solution collected from the bottom of the sand. Figure 7: Variation of local strength of microbial induced carbonate precipitation (MICP) treated sand under different bacteria and reactant concentrations. In contrary to that, samples treated with 1.0 M cementation solution showed a noteworthy heterogeneity in strength distribution over the length of the column as well as among the two duplicated samples. The top portion of the specimens gained strengths as similar to or remarkably higher than their 0.5 M equivalent. However, middle and bottom portions did not harden as much as the top, and all specimens supplied with 1.0 M solution had a greater tendency to clog. The strength variation vertically along the column is suspected to happen due to several reasons. One possibility is filtering and a better accumulation of bacterial cells on top and depletion of reactant concentration as they are been readily consumed for bioccmentation while flowing from top to bottom [50].