# Case Study of an Underpinning Pile Foundation for an Interval Tunnel Crossing an Existing Bridge
## 3 Pile Foundation Underpinning Program  
### Construction Scheme of Pile Foundation Underpinning  
As Guiyang Rail Transit Line 3 crosses the bridge piles of Guihuang elevated bridge, some of the original bridge piles are truncated by the tunnel. In order to ensure the structural and operational safety of the Guihuang Viaduct, the pile foundations of piers 15 and 16 of the Guihuang Viaduct need to be underpinned. The construction solution is to use passive underpinning. The plan for the underpinned pile foundation is shown in Figure 4. A total of six bridge piles within the upstream and downstream lanes need to be underpinned; the piles to be underpinned are 15-5, 15-6, 16-1, 16-2, 16-4, and 16-5 of the Guihuang Viaduct, and the diameter of the pile to be replaced is 1.8 m, which is cast with C30 steel concrete. The diameter of the underpinning pile is 1.5 m, with a total of 16 piles. There are four underpinned bearing platforms, and both the underpinned bearing platform and underpinning pile are cast with C35 reinforced concrete. A total of three foundation pits are excavated, to an excavation depth of 5.4 m, from Huaguoyuan West Station to Huaguoyuan East Station in the direction of construction. The relationship between stratigraphy and pile foundation location and the profiles of the underpinning pile foundations 1-1, 2-2, and 3-3 are shown in Figure 5 (unit: cm).  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Stratigraphic Name** & **Depth (m)** & \(\gamma\) **(kN/m3)** & \(E_{\text{m}}\) **(GPa)** & \(\mu\) & **c (kPa)** & \(\varphi\) (\({}^{\circ}\)) \\ \hline Miscellaneous fill & 5.6 & 19 & 0.006 & 0.3 & 8 & 17 \\ Red clay & 1.4 & 17.76 & 0.015 & 0.35 & 40 & 10 \\ Medium- & & & & & & \\ weathered & - & 27.1 & 2.0 & 0.26 & 450 & 40 \\ limestone & & & & & & \\ Rock surrounding & & & & & & \\ reinforcement area & 2.0 & 27.1 & 4.0 & 0.26 & 650 & 60 \\ \hline \hline \end{tabular} Note: \(\gamma\) is the stratum unit weight; \(E_{\text{m}}\) is the deformation modulus; \(\mu\) is [PERSON]’s ratio; c is cohesion; and \(\varphi\) is the angle of internal friction.  
\end{table}
Table 1: Main physical and mechanical parameters of the stratum.  
Figure 4: Pile buttress replacement plan location.  
The construction process of the underpinned pile foundation is divided into a total of six construction stages, with the following construction steps: (1) underpinned pile and retaining pile for foundation pit construction; (2) the excavation of the foundation pit, pouring a 10 cm C20 concrete bedding layer at the bottom of the foundation pit, and welding connection steel on the side of the existing pile cap; (3) the underpinning pile cap is cast with C40 concrete; (4) in the underpinned bearing platform bottom, the local excavation of a 0.3 m\(\sim\)0.5 m deep trench, the wire saw cutting of the existing pile into a working surface, using the mechanical vibrationless linear cutting process to cut the existing pile, and backfilling the trench at the bottom of the platform with mortar; (5) backfilling the foundation pit to the original ground level, restoring the surrounding ground and road surface; and (6) tunnel construction, breaking up the existing pile foundation when encountered. These steps are shown in Figure 6.  
Figure 5: Underpinned pile foundation cross-section diagram: (**a**) Section 1-1; (**b**) Section 2-2; and (**c**) Section 3-3.