What Is Benching in Tunnels? Benching refers to a stepwise excavation technique used in tunnel construction, where the tunnel cross-section is divided into horizontal "benches" or layers. These benches are excavated sequentially from top to bottom, enhancing stability during the process.
Purpose and Applications
1. Stability Control
① Benching minimizes ground deformation by reducing the unsupported excavation area at any stage. This is critical in weak or fractured rock formations or soft soils.
② It allows temporary support systems (e.g., shotcrete, rock bolts) to be installed between bench layers.
2. Adaptability
① Commonly used in road tunnels, mining tunnels, and large-span underground spaces where full-face excavation is risky.
② Suitable for both drill-and-blast methods and mechanical excavation.
Advantages and Limitations
Pros:
① Enhanced safety in unstable ground conditions.
② Flexibility to adjust support systems between benches.
Cons:
① Slower progress compared to full-face excavation.
② Requires precise coordination between excavation and support installation.
Key Steps in the Benching Process
1. Top Heading Excavation
① The upper portion of the tunnel is excavated first, forming the initial bench.
② Supports are installed immediately to prevent collapse.
2. Bench Excavation
① The lower bench(es) are removed in stages, following the profile of the top heading.
② Sidewalls and invert (floor) are reinforced progressively.
3. Final Lining
Once the full cross-section is excavated and stabilized, a permanent lining (e.g., concrete) is applied.
This method is widely documented in tunnel engineering practices and aligns with principles mentioned in prior discussions about structural stability in tunneling. For project-specific applications, geological surveys and engineering guidelines should always be consulted.