## 4 Case Studies
### Tunnel Beneath the Capital Airport Runway  
Figures 19 and 20 show the plan view and cross-sectional view of the Beijing Capital Airport connector line, respectively. The cross-sectional view is shown in Figure 2. The tunnel features a single-layer, dual-span, double-arch structure. It includes a 1621-m rapid transit tunnel and a 1265-m vehicular tunnel. The tunnel crosses beneath a continuously operational runway for 232 m, with a span of 23.9 m and a depth of 5.6 m. The diameter of the pipe curtain is 970 mm, the elastic resistance coefficient of the soil is 10,000 kN/m, the elastic modulus of the concrete is 230 GPa, the elastic modulus of the pipe curtain is 200 GPa, the weight is \(\gamma\) = 20 kN/m\({}^{3}\), the lateral pressure coefficient is taken as 0.5, the distance from the palm face to the initial support seal is 2 m, the wall thickness is taken as 850 mm, and the grouting pressure is 0.16 MPa.  
The settlement control standards for the airport runway and taxiway are extremely stringent, requiring total settlement to remain below 30 mm and flatness deviations to be less than 1%. Controlling settlement during tunnel construction is highly challenging, as even minor errors can lead to excessive runway settlement, collapses, or catastrophic accidents, such as aircraft destruction and loss of life.  
Figure 19: Plan view of the large cross-section tunnel beneath the Capital Airport runway.  
The geological formation is composed of silty clay with low stability. Since the groundwater level is below the tunnel invert, surface settlement caused by groundwater level drawdown does not need to be considered.  
The pipe roof was designed as a rectangular full-ring enclosed structure. The steel pipes have a diameter of 970 mm, a wall thickness of 850 mm, and are filled with concrete. The primary support and temporary support adopt a combined steel frame and mesh-reinforced shotcrete structure. The mechanical calculation model is illustrated in Figure 21. Based on Equation (10), the total surface settlement caused by tunnel excavation is calculated as \(u_{1}\) = 6.8 mm.  
The spacing between pipes in the pipe roof was designed to be 1000 mm. During single-pipe jacking, the grout was injected around the pipe for pressure maintenance and resistance reduction, and the grouting pressure was designed to be 0.3 MPa. An alternate pipe jacking method was employed to minimize the settlement superimposition effect. According to Equation (25), the total surface settlement caused by pipe roof construction is determined to be \(u_{2}\) = 3.8 mm.  
The total surface settlement resulting from the tunnel construction is calculated to be u = 10.6 mm. Figure 22 presents the measured settlements of the airport runway. It can be observed that the surface settlement caused by the pipe roof construction is 4 mm, while the total surface settlement induced by the construction reaches 11.5 mm. The accuracy of pipe curtain construction, geological homogeneity, and changes in the groundwater environment can all affect surface settlement. The deviation between the measured values  
Figure 21: Cross-section of the tunnel.  
Figure 20: Cross-section of the large cross-section tunnel beneath the Capital Airport runway.  
and predicted values at each measuring point is within 15%, and the settlement control meets the requirements for ensuring the uninterrupted operation of the Capital Airport.