Dual Tunnel

Key Factors Affecting Tunnels in Side-by-Side Sequential Drives

Constructing side-by-side stormwater tunnels using a slurry TBM demands careful planning, especially when the bores are driven one after the other. The first tunnel fundamentally alters the ground behavior, which in turn influences the conditions for the second drive. Here are the primary factors that can impact the success of the second tunnel:

1. Stress Redistribution and Ground Relaxation

The first drive modifies the in-situ stress field. This can cause:

• Lateral stress relief toward the excavation void.
• Asymmetric pressure loading on the second tunnel face.
• A shift in the equilibrium state that the second TBM must navigate differently, potentially affecting steering accuracy or face pressure control.

2. Settlement and Convergence Behavior

Surface and subsurface settlement induced by the first tunnel can lead to:

• Ground softening between bores, increasing the risk of overbreak or face instability.
• Longitudinal displacement or sagging of adjacent soil masses, which might affect TBM guidance or ring buildup stability for the second tunnel.
• Allowing a settlement stabilization period before launching the second drive can help mitigate this.

3. Reduced Soil Confinement and Face Stability

The soil mass between the tunnels may no longer offer the same confinement as virgin ground. This affects:

• Slurry pressure calibration—especially during start-up, curves, or shallow cover conditions.
• Risk of face blowouts or slurry loss if confinement drops below expectations.

4. TBM Trajectory and Tolerances

The altered soil response may require:

• Trajectory revalidation for the second TBM.
• Adaptive steering logic to prevent deflection caused by void migration or softened zones from the first drive.

5. Impact on Segment Performance

Differential ground movement between the two drives can lead to:

• Joint stress accumulation occurs in rings installed closest to the adjacent tunnel.
• Potential misalignment at connecting structures like cross-passages or junction boxes.

6. Instrumentation Feedback Loop

Learnings from the first drive should be systematically fed into the second. This includes:

• Real-time slurry pressure and advance rate trends.
• Settlement and deformation data.
• Face conditions and soil conditioning performance.

Proactive adjustment based on this data makes the second drive more predictable and controlled.

I'll continue updating this blog as I gather more on-site information. Share your experiences in the comments!