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.