# Prediction of Surface Deformation Induced by Ultra-Shallow-Buried Pilot Tunnel Construction
## 3 Case Validation of Deformation Predictions in Shallow-Buried Tunnels
### Project Overview  
Beijing Rail Transit Line 13 College South Road Station is situated at the south side of the College South Road and Jiaoda East Road intersection, serving as an interchange for Line 13A and the planned Changnan Line (positional relationship shown in Figure 11). The ultra-shallow-buried pilot tunnel section within the study scope has a maximum burial depth of 8.2 m, with relevant soil layers and mechanical parameters detailed in Table 6.  
The station employs the six-pilot-tunnel pile-beam construction method, with this case focusing on the first step: deformation induced by initial pilot tunnel excavation (Figure 12). Pilot tunnels are excavated using the bench method with reserved core soil, adhering to key principles like \"advanced support and strict grouting\". The construction sequence emphasizes critical steps: advanced small duct support (DN32 steel pipes, 2.5 m length, grouting pressure 0.5 MPa), staged shotcreting (C25) and grid steel frame installation (spacing 0.5 m, main bars \(\Phi\)22 mm) (Figures 13 and 14).  
Settlement monitoring utilized the Trimble DINI03 digital level (Trimble, Navi Mumbai, Maharashtra) (accuracy 0.3 mm/km), calibrated by the National Institute of Metrology, with daily checks under stable conditions to ensure data reliability. Key calibration metrics (i-angle correction, repeatability \(\leq\)0.3 mm/km) were retained to validate measurement credibility.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Straum**} & \multirow{2}{*}{**Cohesion** (**kPa**)} & \multirow{2}{*}{\begin{tabular}{c} **Internal** \\ **Friction Angle** \\ (\(\lx@paragraphsign\)) \\ **(\(\lx@paragraphsign\))** \\ \end{tabular} } & \begin{tabular}{c} **Coefficient of** \\ **Earth** \\ **(\(\lx@paragraphsign\))** \\ **(\(\lx@paragraphsign\))** \\ \end{tabular} & \begin{tabular}{c} **Vertical** \\ **Subgrade** \\ **Coefficient Kv** \\ **(MPa/m)** \\ \end{tabular} & \begin{tabular}{c} **Horizontal** \\ **Subgrade** \\ **Coefficient Kh** \\ **(MPa/m)** \\ \end{tabular} & \
\begin{tabular}{c} **Characteristic Value of** \\ **Foundation Bearing** \\ **Capacity (kPa)** \\ \end{tabular} \\ \hline
\begin{tabular}{c} Silty Clay\(\ddagger\) \\ **Clayey Silt\(\beta\)**\(\ddagger\)**  
Figure 14: Dimensions of small pilot tunnel grid steel frame (unit: mm).  
Figure 12: Schematic diagram of main arch soil excavation by pilot tunnel and column method.  
Figure 13: Schematic diagram of small pilot tunnel lining structure.  
The three pilot tunnels were all excavated under ultra-shallow-buried conditions, with a crown overburden thickness of 4.5 m. No surface water was present within the station site. Within a depth of 40 m, there were three groundwater layers: upper perched water, phreatic water and confined water. The complexity of fault structures in the proposed site was moderate, and the current assessment of active fault hazard is low. The pilot tunnel has a horseshoe-shaped cross-section, being 4 m wide with a rectangular part height of 3.4 m and a circular part height of 1.7 m, with a center burial depth of 5.5 m. It should be noted that the single pilot tunnel in this case was the first excavated tunnel, and the stratum had not been disturbed by the excavation of adjacent pilot tunnels before. As the burial depth of the pilot tunnel was shallow and above the groundwater table, it was not affected by groundwater during construction, so the consolidation drainage effect was not considered in the model. The calculation parameters are as follows: \(H\) = 5.5 m, \(A\) = 4 m, \(B\) = 3.4 m, \(C\) = 1.7 m, \(\Delta R\) = \(-\)7.75 mm, tan\(\beta\) = 0.17, \(\delta\) = 350.89 mm. Under these parameters, the posterior variance ratio of the model is 0.31, indicating an \"excellent\" back-analysis accuracy rating. The calculation results are shown in Figure 15 at the tunnel centerline (horizontal distance = 0 m).