## 4 Field Test Findings  
The Linghuang interval tunnel of Qingdao Metro Line R3 in Huangdao District mainly passes through a medium and coarse sand layer, silty clay layer, and moderately weathered breccia tuf layer, and the risk assessment is level II. In the interval section, YSK14+187-YSK14+342 on the right line; half of the tunnel face is made up of sand, silty clay, and other unfavorable geology, and the lower half comprises moderately weathered breccia tuff. The construction of the left line has reached ZSK14+267.5, and the surrounding rock conditions of the tunnel face have significantly changed. The tunnel face of the upper step is made up of sand, silt, silty clay, and other unfavorable geology, which is similar to the stratum exposed on the right line. The single hole of the tunnel has a width of 6 m and a clear height of 7.2 m. Part of the geological profile is shown in Figure 10.  
A concrete retaining wall needs to be constructed in advance to better complete the grouting work and prevent accidents such as mortar running and tunnel face collapses during the grouting process. Accordingly, the tunnel condition could be used to verify whether the theoretical formula is reasonable. According to the geological profile, each stratum's average thickness and effective unit weight were determined through laboratory tests, as shown in Table 2.  
Figure 9: Relationship between final grouting pressure and safety coefficient. The average groundwater level in this interval is \(-3.6\) m. The different effective unit weights of each stratum could be determined by soil data and laboratory tests and calculated by Equation (19):  
\[\gamma^{\prime}= \frac{(G_{\text{s}}-1)\gamma_{w}}{1+e} \tag{19}\]  
where \(G_{\text{s}}\) is the specific gravity of the soil particles, \(\gamma^{\prime}\) is the effective unit weight of the stratum, \(\gamma_{w}\) is the unit weight of the groundwater, and \(e\) is the particle void ratio. The results are shown in Table 3.  
Using the theoretical formula in this paper to make the calculation, we determined that \(q=0.16\) MPa and \(\alpha=1.64\). To verify the accuracy of the theoretical formula, field tests were carried out under the following conditions. The concrete specification used in these tests was C30, and the basic mechanical parameters were the same as those of the calculation example. The cement was P.O 32.5 cement produced by Shandong Shanshui Cement Group Co., Ltd. The fine aggregate was medium sand and the fineness modulus was 2.7. The coarse aggregate was granite gravel with a 5-20 mm continuous gradation. Table 4 shows the empirical formula (\(K_{0}=1\) or 2) and the calculated values of the theoretical formula in this paper, along with the experimental reference values. The final grouting pressures of the three working conditions were 0.6 MPa, 1 MPa, and 1.4 MPa, respectively. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Stratigraphic** & **Cohesion** & **Internal Friction** & **Average** & **Density of** \\
**Type** & **Force/kPa** & **Angle of Sand/\({}^{\circ}\)** & **Depth/m** & **Stratum/kN/m\({}^{3}\)** \\ \hline Miscellaneous fill & 5 & 5 & 0–5.1 & 17 \\ Silty clay & 10 & 10 & 5.1–10.6 & 19.5 \\ Medium and coarse sand & 4 & 25 & 10.6–14.2 & 14.7 \\ Clay sand & 30 & 30 & 14.2–17.8 & 12.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Classification table of different strata. Figure 10: Geological profile. The field test results show that:  
1. For the right tunnel, in the three working conditions, except for slight local deformation of the concrete retaining wall during a certain period of grouting, no water seepage, cracking, or other accidents occurred, which verifies the safety of the thickness of the concrete retaining wall obtained using the theoretical formula. 2. The concrete retaining wall was damaged to varying degrees under the three working conditions for the left tunnel. To avoid accidents, grouting must be stopped to further strengthen the concrete retaining wall.