## 3 Results and Discussion
### Biocementation of Fine-Grained Soil Using CA-Cu Nanofluower  
#### 3.4.1 Unconfined Compressive Strength  
Figures 9 and 10 show the fine-grained soil specimens after treatment with cementation solution, free CA and CA-Cu using injection and mixing, respectively. Table 4 summarises the results of each case for the control, CA and CA-Cu. According to the results, the maximum UCS of the biocemented soil increased by 85% for CA only, whereas for CA-Cu hybrid carrier mixed in the soil it increased by 319% compared to the control. On the other hand, the injection method showed a 116% increase for CA only and only a 38% increase for the CA-Cu nanoflower. Although this study aimed to use injection as a preferred method for nanoflowers, the results indicate the difficulty in injecting the CA-Cu into the fine-grained soil due to the size of the hybrid carrier, compared to the narrow pore throats in the soil matrix. This is reflected in the lower UCS values achieved and is confirmed by the non-uniform calcite precipitation in the specimen discussed later. Another possible reason for the observed UCS values upon injection is the rapid reaction of the CA enzyme involved in bioprecipitation. As shown in Figure 5d, the bioprecipitation process occurs spontaneously due to the action of the CA enzyme and the reaction is completed quickly. The rapid reaction naturally caused bioclogging of the top surfaces of the soil column, restricting the movement of both enzymes and the cementation solution. This severe bioclogging was observed in all CA-Cu cases; hence, the lower UCS values of the specimens treated by injection in Figure 9, as the treatments appear to have biocemented only the top surfaces of the soil specimens. Namely, in Figure 9, a thin layer of crust is seen to have formed on the top of all treated soil specimens (whether the treatment was caused by chemical reaction without enzymes (Figure 9a), chemical reaction with CA enzymes (Figure 9b) or CA-Cu carriers (Figure 9c) and for all solution molarities used); this crust was identified as calcium carbonate (see Figure 8). This biocement crust reduces the soil permeability on the top of the soil specimen by clogging the soil pores; it thus prevents the ingress of further treatments. Similar results have been frequently reported in previous studies for both the EICP and MICP processes. These studies have also shown the formation of a water-impermeable and high-strength crust layer on the soil surface (usually sandy soils) rather than a more uniform bioccentation of the soil at further depth [56; 57; 58; 59]. Figure 7: Bioprecipitation at different equimolar concentrations of CaCl\({}_{2}\) and NaHCO\({}_{3}\) (**a**) chemical reaction without enzyme, (**b**) carbonic anhydrase (CA) enzyme only, (**c**) CA-Copper nanoflower. Comparatively, the mixing method (Figure 10) was superior to the injection method (Figure 9), according to the results of this study. This was evidenced (a) by the higher strengths achieved as reported earlier; (b) by the absence of a localized biocement crust on the surface (as opposed to the injection method); (c) by an observed more uniform soft-rock-like consistency throughout the soil sample. Generally, the results showed that the synthesized CA-Cu enzyme was highly effective in biocementing the soil. Specifically, the CA-Cu carriers yielded over 300% increase in UCS compared to the control, when treatments were implemented by mixing; conversely, with the injection method a 38% increase was obtained using CA-Cu carriers. Mixing was therefore still the best way of treating fine-grained soil investigated in this study and this is consistent with the literature [38; 39]. As shown in Figure 1, the investigated soil consisted of 39% clay. This is why it was difficult to treat the specimens by injecting enzyme carriers of a size of several microns.