## 3 Materials and Methods  
### MICP HSRB Formation Method  
The MICP technology based on urea hydrolysis is the most commonly used method for producing calcium carbonate crystals. _S. pasteurii_ is one of the most commonly used bacteria in the application of MICP technology in geotechnical engineering, which is a natural underground non-pathogenic bacterium [42,43]. _S. pasteurii_ has high urease activity because it can produce a lot of urease, can survive in a high alkaline environment, and has high calcium ion concentration [44,45]. In this study, _S. pasteurii_ was selected as the strain of HSRB to be formed by MICP technology. The solutions designed to be injected into the confined aquifer by MICP grouting technology included a bacterial solution (_S. pasteurii_) and cementation solution (urea and CaCl\({}_{2}\) solution). When the bacterial solution and cementing solution were injected into the soil, the urea in the cementing solution was hydrolyzed rapidly to NH\({}_{4}\)\({}^{+}\) and CO\({}_{3}\)\({}^{2}\)- under the catalysis of urease produced by _S. pasteurii_, as shown in chemical Equations (1)-(4). As the extracellular polymer of the metabolites of _S. pasteurii_ was a negative ion group, the negative charge on its surface would continuously absorb the Ca\({}^{2+}\) provided by the surrounding CaCl\({}_{2}\) solution, aggregate on the external surface of bacterial cells, and combine with the CO\({}_{3}\)\({}^{2}\)- continuously decomposed under the action of urease, thus forming calcium carbonate crystals with bacterial cells as crystal nuclei (see chemical Equations (5) and (6)). \[\mathrm{CO(NH_{2})}_{2}+\mathrm{H_{2}O}\rightarrow\mathrm{H_{2}COOH}+2 \mathrm{NH_{3}}, \tag{1}\]  
\[\mathrm{H_{2}COOH}+\mathrm{H_{2}O}\rightarrow\mathrm{NH_{3}}+\mathrm{H_{2}CO_ {3}}, \tag{2}\]  
\[2\mathrm{NH_{3}}+\mathrm{H_{2}O}\leftrightarrow 2\mathrm{NH_{4}^{+}}+2 \mathrm{OH^{-}}, \tag{3}\]  
\[2\mathrm{NH_{3}}+\mathrm{H_{2}CO_{3}}\leftrightarrow\mathrm{CO_{3}^{2-}}+2 \mathrm{H_{2}O}, \tag{4}\]  
\[\mathrm{Ca^{2+}}+\mathrm{Cell}\rightarrow\mathrm{Cell}-\mathrm{Ca^{2+}}, \tag{5}\]  
\[\mathrm{Cell}-\mathrm{Ca^{2+}}+\mathrm{CO_{3}^{2-}}\rightarrow\mathrm{Cell}- \mathrm{CaCO_{3}}, \tag{6}\]  
MICP HSRB was introduced where the slurries included _S. pasteurii_ solution, CaCl\({}_{2}\) solution, and urea solution. The particle size of the solute was small and could be seeped into the fine sand layer without damaging the aquifer structure. The recharging well was used to form a stable seepage field to drive the slurry to the designated position. \begin{table}
\begin{tabular}{c} layers \(\varnothing\) and \(\oplus\) \\ were summarized as a \\ confined aquifer \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Checking calculation of anti-gushing stability of each confined aquifer of the No. 4 Foundation Pit (safety factor is 1.05). Sporosarcina pasteurii (ATCC 11859) purchased from the Shanghai Bioresource Collection Center (SHBCC) was used in this study. Laboratory experiments on the formation of MICP HSRB using the ATCC 11859 in a confined aquifer had been performed by the authors for deep excavation. In accordance with the solute migration parameters preiously determined by the authors [41], the solute had small adsorption and a weak retardation effect in the silty fine sand layer. Using the seepage field formed by the recharge wells, the solute was driven to migrate to the silty fine sand layer and cover the bottom of the foundation pit. When water was injected into the aquifer through the recharge well, the groundwater level around the recharge well continued to rise. The water level formed a water head difference with the surrounding groundwater level. The seepage field formed by the recharging well was used to drive the solution used in MICP to form HSRB. The site injection steps were as follows:  
(1) The foundation pit was divided into several concentric ring areas and the recharging wells were installed along the separating lines, as shown in Figure 3a.