- 2082 Baisakh · 8 marks
Tunnelling in hard rock and lining types
What are the methods of tunneling in hard rock? Explain different types of lining used in tunnel.
Methods of tunnelling in hard rock
- Drill and blast (conventional) method - the cycle is drilling holes (jumbo), charging with explosives, blasting, ventilation, mucking out (loaders and trucks), scaling and rock support. It can be done by:
- Full-face method - the whole section is excavated at once; suitable for small, good-rock tunnels.
- Heading and bench method - top heading is advanced first and the bench below is excavated later; common for medium-large sections.
- Pilot (drift) method - a small centre or side drift is driven first, then widened to full section; used when rock is fractured.
- Top heading and multiple-drift methods for very large sections or poor rock.
- Tunnel Boring Machine (TBM) method - a rotating cutter head with disc cutters cuts the rock mechanically; open-gripper TBMs are used in hard rock. Fast and gives smooth walls but needs high capital cost and long straight alignments.
- NATM / sprayed concrete lining (SEM) - excavate by drill-blast or roadheader and support at once by shotcrete, rock bolts and steel ribs, using the rock itself as the main support.
Types of lining used in tunnels
| Type | Description and use |
|---|---|
| Plain cement concrete lining | Cast-in-situ with forms, 200-450 mm thick; for good rock to give a smooth surface and water-tightness |
| Reinforced concrete lining | With steel bars, in weak ground or high pressure, e.g. hydropower pressure tunnels |
| Shotcrete lining | Sprayed concrete 50-150 mm with mesh/fibre, used as initial support in NATM and as final lining in good rock |
| Rock bolts and steel ribs | Bolts anchor loose blocks; steel ribs or arches carry load in fractured rock; with lagging or shotcrete |
| Precast concrete segments | Used with TBM shield tunnelling; segments erected into rings |
| Masonry / brick lining | Older railway tunnels; used for small tunnels |
| Steel lining | In penstocks and tunnels under high internal pressure |
| Cast-iron / composite | Rare, for special conditions |
Lining supports the rock, controls seepage, protects against weathering and gives a smooth surface; in sound hard rock it may be limited to local support.
✓ Answer checked by Claude Opus 5.5 (high effort)
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