Spraying with a wet shotcrete machine immediately after starting to open a tunnel is crucial for several reasons related to safety, structural stability, and construction efficiency:
1. Immediate Ground Support
Prevents Collapse: When a tunnel is excavated, the surrounding rock or soil is disturbed and may become unstable. Applying wet shotcrete right after excavation stabilizes the surface and helps prevent collapses or rockfalls.
Stabilizes Loose Material: The excavation process loosens rock and soil, which can fall and create a hazardous environment. Shotcrete quickly adheres to the tunnel walls and secures these loose materials, providing instant support.
2. Protection Against Water and Air Infiltration
Water Sealing: Tunneling often exposes areas where groundwater or moisture can seep into the tunnel. Wet shotcrete seals these areas, preventing water infiltration, which can erode the tunnel walls and reduce their stability.
Reduces Oxidation and Weathering: Exposing fresh rock to air can cause it to weather or oxidize rapidly, particularly if the rock contains iron or other reactive minerals. Shotcrete creates a protective barrier, reducing deterioration.
3. Structural Reinforcement
Early Load-Bearing Capacity: Wet shotcrete adds strength to the tunnel lining right after excavation, reinforcing the tunnel walls to support the load from the surrounding rock or soil. This is especially important in weak geological conditions.
Works with Other Supports: Shotcrete is often used in combination with rock bolts and wire mesh to create a composite support system that distributes stress and strengthens the tunnel structure.
4. Safety Enhancement
Prevents Accidents: Spraying shotcrete helps mitigate the risk of rockfalls, landslides, and other accidents that may occur when freshly excavated surfaces are left unsupported. It provides an immediate layer of protection for workers.
Improves Working Conditions: In tunnels where dust or loose debris are concerns, shotcrete helps to contain and reduce dust, creating a safer and cleaner working environment for the construction team.
5. Faster and More Efficient Construction
Rapid Application and Curing: Wet shotcrete can be applied quickly and sets faster than traditional concrete, allowing for continuous excavation and support processes without long delays.
Minimizes Downtime: By providing immediate support, shotcrete reduces the need to halt excavation for extensive temporary support measures, which can slow down the project.
6. Long-Term Stability
Controls Deformation: In tunnels, the ground surrounding the excavation tends to deform due to stress redistribution. Shotcrete helps control this deformation by providing a flexible yet strong support layer that adapts to the tunnel's shape.
Maintains Tunnel Geometry: Early shotcrete application helps maintain the tunnel's design geometry by reducing shifting or collapse during the initial stages of excavation.
