Tunnels are underground passages used for transportation purposes. Tunnels are the underground routes driven without disturbing the overlying soil to bypass the obstacles safely. Tunnels can be used to carry passengers & freights, water, sewers, gases, etc. Tunnels are constructed in various shapes & sizes. The shape of the tunnel cross-section is governed by the nature & type of ground to be penetrated, existing overburden stress on the rock, etc while the size of the tunnel depends on the usage to which it is subjected. The economy of tunnel construction depends on the relative cost of open cuts vs. tunnelling. The tunnel becomes more economical than an open cut beyond a certain depth.
Advantages of Tunnelling
- It reduces the route distance & travel time
- It provides easy gradients in hilly terrain
- Surface activities are not disturbed
- It remains free from the weather actions like rainfall, snow, etc.
- The tunnel becomes more economical than an open cut beyond a certain depth.
Disadvantages of Tunnelling
- The initial cost of construction may become higher
- Construction of tunnel requires skilled manpower & sophisticated equipment
- Strick supervision is necessary during construction
- Higher safety precautions are necessary during construction
- Construction of tunnel requires more time than open cuts
- A tunnel may collapse during an earthquake
Terminologies related to Tunnel Engineering
- Tunnel Portal: It is the entrance or exit of tunnel where tunnel intersects with the open area. It may be an inlet portal or outlet portal.
- Crown: It is the topmost point of the tunnel cross-section.
- Invert: It is the lowest point of the tunnel cross-section.
- Faces of Operation or Attack: It is the surface from which a boring operation is carried out.
- Adit Tunnel: It is a horizontal or near-horizontal passage that provides access for extra faces of operation/attack in addition to the inlet face and outlet face. It may also be used for the purpose of the auxiliary entrance, ventilation, drainage, etc.
- Inclined/Vertical Shaft: It is an inclined or vertical passageway that connects the surface to the underground tunnel or network of tunnels.
- Pilot Tunnel: It is a small tunnel driven, parallel & close to the proposed main tunnel, to explore geological conditions & assist in final excavation. During construction of vertical shaft, a pilot tunnel is excavated at first.
- Tunnel Linings: These are the supports erected during & after tunnel construction to ensure a safe working environment inside the tunnels. Rock bolts, steel ribs, wire mesh, shotcrete, etc are used as tunnel lining materials.
- Mucking: Mucking means the removal of blasted debris from the tunnel interior to a good distance outside the tunnel entrance.
- Overbreak: It is the over excavation beyond the intended boundaries, resulting in a larger opening or void than originally intended.
- Niche: It is the relatively small recesses or compartments excavated inside a tunnel for specific purpose such as equipment housing, utility installation, for vehicles to make turns or change direction, etc.
- Cavern: Caverns are intentionally excavated larger underground chambers for construction of specific underground structure such as an underground powerhouse. In tunnelling, niches and caverns are both types of excavated spaces within the tunnel structure, but they differ in terms of size, purpose, and construction methodology.
- Grouting: Grouting is a process where a fluid material, often cement-based, is injected into the ground to improve soil or rock properties.
- Overburden: Overburden refers to the soil, rock, or other material that lies above the tunnel roof or crown.
A. Based on Purpose
- Traffic Tunnel
- Highway Tunnel
- Railway Tunnel
- Pedestrian Tunnel
- Conveyance Tunnel
- Power Tunnel
- Water Supply Tunnel
- Sewer Tunnel
B. Based on Shape/Cross-Section
- Circular Tunnel
- D Shaped Tunnel
- Horse Shoe Tunnel
- Square or Rectangular Tunnel
- Elliptical Tunnel
Methods of Tunnelling
During tunnel construction, tunnels are lined with suitable materials parallelly with the boring operations. Tunnels are usually lined with timber, steel, cast iron, masonry, or concrete with suitable outlets to let out the enclosed subsoil water behind the linings. Other items of work include the provision of ventilation, drainage, lighting, etc. Tunnelling may have to be done in the hard rock or soft soil based on which the method of tunnelling differs. Hard rock is considered as a fully self-supporting soil that does not require much support except where a loose rock is occasionally met. On the other hand, soft soils like running grounds (eg: water-bearing sands) require instant supports all around. So, different methods of tunnelling based on the nature of the soil to be penetrated are listed below:
A. Tunnelling in Soft Ground
- Fore Poling Method
- Needle Beam Method
- Shield Method
- Compressed Air Method
- Liner Plate Method
- Army Method
- American Method
B. Tunnelling in Hard Rock
- Full Face Method
- Top Heading Benching
- Bottom Heading & Stopping
- Drift Method
- Pilot Tunnel Method
For the detailed description of each method of tunnelling listed above, the readers are kindly requested to go through ref 1.
References
- Srinivasan, R.(1958). Harbour, dock and tunnel engineering. India: Charotall Book Stall