Here's a breakdown of its crucial role in both applications:
1. In Foundation Pit Support
Foundation pits (or excavations) are dug for building basements, subways, or other underground structures. The walls of these pits need immediate support to prevent collapse and protect adjacent buildings.
Application: Wet shotcrete machines are used to build Soldier Pile and Lagging walls or, more commonly, Shotcrete Diaphragm Walls (also known as Soil Nailing with Shotcrete).
Process:
Soil nails (long steel tendons) are drilled and grouted into the soil around the excavation.
A reinforcing mesh is installed against the soil face.
The wet shotcrete machine sprays a thick layer of concrete onto the mesh, encapsulating it and creating a continuous, structural "skin" or wall.
Purpose:
Immediate Stabilization: Seals the exposed soil face instantly, preventing erosion and minor spalling.
Structural Wall: Bonds with the soil nails to create a coherent, reinforced earth structure that actively holds back the soil.
Water Sealing: Helps to control minor groundwater seepage.
2. In Tunneling
This is arguably the most famous application of wet shotcrete machines. They are indispensable in the NATM (New Austrian Tunneling Method) or SEM (Sequential Excavation Method).
Application: Used right after the tunnel boring machine (TBM) or after each round of drill-and-blast excavation.
Process:
A section of the tunnel is excavated.
Initial support elements like rock bolts and steel ribs (or lattice girders) are installed.
The wet shotcrete machine sprays a thick layer of concrete onto the freshly exposed rock/soil, embedding the support elements.
This creates a closed, supportive ring that works in concert with the surrounding ground.
Purpose:
Ground Support: Prevents immediate loosening and deformation of the tunnel walls and crown (roof).
Load-Bearing Shell: The shotcrete layer acts as the primary lining, bearing the ground loads and transferring them to the support system.
Safety: Creates a safe environment for workers to proceed with the next excavation cycle and install the final, permanent concrete lining.
