Tunnelling methods; shotcreting

Tunnelling methods in hydropower projects

1. Drill and blast: drilling, charging, blasting, ventilation, mucking, scaling and support. It is the method used most in Nepal. Large faces are excavated by top heading and benching.
2. Tunnel boring machine (TBM): full-face rotary excavation with a segmental lining; fast, and gives a smooth tunnel, but costly, and it needs uniform ground and a long drive.
3. NATM: excavation with immediate shotcrete, rock bolts and mesh, and monitoring of the deformation, which lets the rock carry part of the load.
4. Road-headers or hydraulic breakers in soft rock, and cut-and-cover at shallow portals.

Purpose of shotcreting

Shotcrete is concrete sprayed at high speed onto the excavated rock surface.

• Seals the surface, preventing weathering, slaking and loosening of the joints.
• Gives immediate support and carries the rock load together with bolts and mesh.
• Fills the cracks and bonds the rock blocks into a ring (arch action).
• Smooths the surface and reduces overbreak and hydraulic roughness (as a lining of non-pressure tunnels).
• Gives quick, flexible support at low cost.

Procedure of shotcreting

1. Scale the loose rock and wash the surface with an air-water jet to remove dust and loose particles.
2. Install mesh or add steel/synthetic fibres, and set the thickness gauge pins.
3. Prepare the mix (cement, aggregate up to 10 mm, admixtures, accelerator) by the dry-mix process (water added at the nozzle) or the wet-mix process (pumped wet concrete).
4. Spray with the nozzle held 1-1.5 m from the surface and at right angles to it, moving in small circles. Spray from the walls upwards to the crown, in layers of 5-10 cm; apply the second layer after the first has set.
5. Control the rebound (material that falls away) and remove it; do not reuse it.
6. Cure the shotcrete by spraying water or a curing compound for several days.
7. Quality tests: thickness checks, strength tests on cores or panels, and bond tests.