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Tunnelling methods and supports

Short answer

Tunnel methods include drill-and-blast, TBM, NATM, and mechanical excavation. Supports prevent rock fall, control deformation, and stop water inflow, protecting workers and machines. They are realized by scaling, immediate shotcrete with mesh/fibres, rock bolts, steel ribs with lagging, pre-support in weak zones, and final concrete lining, all selected based on rock classification.

Discuss various tunnelling methods used in hydropower projects. Why do you provide tunnel supports? How are they realized?

Tunnelling methods in hydropower projects

  1. Drill and blast (conventional method): the most common method in Nepal. The cycle is drilling holes, charging with explosives, blasting, ventilation, mucking (removal of rock), scaling and installation of support. Large sections are excavated in stages: top heading then benching.
  2. Tunnel boring machine (TBM): a rotary cutter-head that excavates the full circular section and installs the precast segments or supports; fast and gives a smooth surface, but costly and not suitable in squeezing or highly variable ground. Used for long, uniform tunnels.
  3. NATM / sprayed-concrete method (New Austrian Tunnelling Method): the excavation is supported at once by shotcrete, rock bolts and mesh, and the rock itself carries the load; the deformation is monitored.
  4. Mechanical excavation (road-header) in soft rock, and cut-and-cover for shallow sections.
  5. Adits and shafts are used to open extra faces; raise boring is used for shafts.

Why tunnel supports are provided

  • To prevent the fall of rock and collapse of the opening, and so protect workers and machines.
  • To control the deformation and the stress redistribution, particularly in weak, jointed or squeezing rock and fault zones.
  • To stop the loosening of the rock and the inflow of water.
  • To provide a base for the final lining.

How the supports are realized

  1. Scaling of loose rock after each blast.
  2. Shotcrete (5-15 cm) with wire mesh or fibre is sprayed on the crown and walls straight after excavation.
  3. Rock bolts (grouted or friction), 2-4 m long at 1-2 m spacing, are installed through the shotcrete to bind the rock.
  4. Steel ribs/arches (I-sections or lattice girders) with lagging in poor rock, installed close to the face and encased in shotcrete.
  5. Pre-support (spiles, forepoling, pipe umbrella) in very weak ground and fault zones, ahead of the face.
  6. Final concrete lining (and steel lining for pressure sections) after the excavation is finished, followed by contact and consolidation grouting.

The type and amount of support follow the rock mass classification (Q-system, RMR) and are adapted to the rock seen at the face.

✓ Answer checked by Claude Opus 5.5 (high effort)

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