## 3 Results and Discussions
### Soil Reinforcement Tests
#### 3.2.4 CCC and Efficiency of Strength Enhancement  
Previous studies have shown that strength improvement is closely related to higher precipitation of calcium carbonate [6; 48]. The efficiency of strength enhancement could be defined as the UCS per mass of precipitated calcium carbonate. Figure 8 compares the CCC and strength reinforcement efficiency  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Treatment Method** & **Elastic Modulus (MPa)** & **Failure Strain \(\epsilon_{\text{max}}\) (\%)** & **Peak Strength (kPa)** & **Peak Strain (\%)** \\ \hline Untreated & 19.1 & 2.7 & 324.76 & 1.7 \\ Skim milk powder & 27.02 & 2.5 & 442.18 & 1.5 \\ Glutinous rice powder & 23.67 & 2.45 & 345.07 & 1.6 \\ Brown sugar & 31.98 & 2.35 & 494.22 & 1.5 \\ EICP & 80.5 & 2.15 & 1167.23 & 1.45 \\ EICP + skim milk powder & 123.84 & 1.65 & 1547.95 & 1.25 \\ EICP + glutinous rice powder & 112.66 & 1.7 & 1464.52 & 1.3 \\ EICP + brown sugar & 103.96 & 2.1 & 1455.41 & 1.4 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Parameters obtained from the UCS tests.  
Figure 7: Axial stress-strain curves of representative samples treated by (**a**) EICP and improved EICP with organic material added; and (**b**) one kind of organic material.  
among different treatment methods. Compared with the original soil, the CCC of soil reinforced only with one kind of organic material was basically unchanged due to the absence of new calcium ions, and the CCC for soil treated with EICP and the modified EICP containing organic material increased nearly by 1.2-fold to about 4.3%. Compared with EICP-treated soil, the CCC of soil treated with the modified EICP changed very little regardless of which organic material was added, but the strength enhancement efficiency substantially increased by 32%, showing that the large enhancement in strength may be attributed to the improvement in the calcium carbonate crystal structure. The improvement effect of CCC is consistent with that obtained by [PERSON] et al. through the sand bio-improvement test with the addition of skim milk powder [30]. This was probably because both [PERSON] et al. [30] and our study used a lower concentration of calcium ions, which in turn led to a higher mineralization rate, and organic substrates with a higher concentration of calcium ions are more likely to improve the precipitation yield.  
The reinforcement effect was more pronounced for the improved EICP with skim milk powder added. Nevertheless, the strength enhancement efficiency for the improved EICP with skim milk powder added in this study was far from that in reference [30]. This was mainly due to the fine particles of the treated soil, the smaller total mixing amount of reaction solution in the mixing method used in this paper, and thereby a scarcity of nucleation sites provided by organic materials. The drip irrigation technology of EICP combined with organic materials can be further studied to promote the popularization and engineering application of the improved EICP technology in the reinforcement of fine-grained soils.  
Figure 9 shows the compiled data of the UCS versus CCC from other researchers. Compared with the results obtained by [PERSON] et al. [49] through MICP drip irrigation, the UCS under the same CCC in this study is higher. The strength of the soil with a 2% CCC was equivalent to that with a 6\(\sim\)17% CCC in the study by [PERSON] et al. [49]. This shows that the distribution of calcium carbonate during mixing was more even, and the soil integrity was better than that in the grouting method. The improvement effect of the modified EICP with organic materials added was better than that of MICP.  
Figure 8: CCC and strength enhancement efficiency of soil treated by different methods.