Here's why the wet spraying machine is necessary at that stage:
1. The Role of the Pipe Shed
Primary Support: The pipe shed (a series of steel pipes drilled horizontally or at an angle over the tunnel crown) acts as a strong, flexible umbrella or canopy. It provides primary support for the loose soil or rock mass ahead of and above the tunnel face.
Pre-deformation: It helps to control ground settlement and prevent collapses before the main excavation of the tunnel portal section begins.
Grouting: Often, these pipes are later injected with cement grout to consolidate the ground further, creating a reinforced soil arch.
2. Why Wet Shotcrete is Applied Immediately After
Once the pipe shed is in place, the next immediate step is to "close the system" and create a continuous supporting shell. This is where the wet spraying machine becomes essential:
Sealing and Integrating: The individual pipes of the shed have gaps between them. Wet shotcrete is sprayed onto the excavated face and over the pipe shed structure. It embeds the pipes, seals the gaps, and creates a continuous, early-strength lining that works together with the pipes as a unified support system.
Preventing Weathering and Unraveling: The exposed ground between and around the pipes is vulnerable to weathering, water erosion, or simply unraveling. Shotcrete provides an immediate seal that prevents this deterioration.
Initial Lining Layer: It forms the first structural layer of the tunnel's permanent or primary lining, transferring loads evenly to the pipe shed and the surrounding stabilized ground.
Safety for Further Excavation: With this combined pipe-shed-and-shotcrete shell in place, it becomes much safer to proceed with the sequential excavation (e.g., using the Bench method or NATM) under the protected canopy.
3. Advantages of Wet Shotcrete for This Application
While both dry and wet mix shotcrete exist, wet shotcrete is strongly preferred for this critical stage:
Higher and More Consistent Quality: The mix is pre-batched in a plant, ensuring better control over water-cement ratio, leading to higher strength, better durability, and less rebound (wasted material).
Improved Working Environment: Produces significantly less dust, which is crucial for visibility and worker health in the confined portal area.
Higher Application Speed: Allows for faster placement of larger volumes, which is important for quickly stabilizing the portal.
Better Bonding: The consistent, homogeneous mix typically provides superior adhesion to the soil/rock and the steel pipes.
Standard Construction Sequence at Tunnel Portal:
Slope Cutting & Face Stabilization: Prepare the entrance slope.
Pipe Shed Installation: Drill and install the canopy of steel pipes. (Grouting may follow).
Wet Shotcrete Application: Immediately spray a thick layer (e.g., 20-30 cm) over the entire supported face and pipes.
Excavation: Begin careful excavation of the pilot tunnel or top heading under the protection of this composite support.
Installation of Additional Support: As excavation proceeds, install steel ribs, rock bolts, and more layers of shotcrete to form the full primary lining.
Conclusion:
Your statement is accurate. The wet spraying machine is necessary to complete the initial support system. The pipe shed provides the skeletal framework, and the wet shotcrete provides the flesh and skin, creating a safe, continuous, and load-bearing shell. This combined system is fundamental to modern soft-ground or weak-rock tunnel portal construction, ensuring stability and safety from the very first meter of excavation.
