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 soil to be penetrated 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. tunneling. The tunnel becomes more economical than an open cut beyond a certain depth.
Advantages of Tunneling
- It reduces the route distance & travel time
- It provides easy gradients in hilly terrain
- Surace 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 Tunneling
- 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: The tunnel entrance is called a tunnel portal.
- Crown: It is the topmost point of the tunnel cross-section.
- Invert: It is the lowest point of the tunnel cross-section.
- Adit: It is a horizontal or near-horizontal passage that provides access to a tunnel. It may be used for the purpose of the auxiliary entrance, ventilation, drainage, etc.
- Shaft: It is a vertical passage from the ground surface that provides access to a tunnel. It may be used to transfer the centerline from the ground surface into the tunnel.
- Tunnel Linings: These are the supports erected during & after tunnel construction to ensure a safe working environment inside the tunnels. Stone masonry, brick masonry, timber, steel, 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.
- Faces of Operation or Attack: It is the surface from which a boring operation is carried out.
- Pilot Tunnel: It is a small tunnel driven, parallel & close to the proposed main tunnel, to explore geological conditions & assist in final excavation.
Classification of Tunnels
A. Based on Purpose
- Traffic Tunnel
- Highway Tunnel
- Railway Tunnel
- Pedestrian 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 Tunneling
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. Tunneling may have to be done in the hard rock or soft soil based on which the method of tunneling 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 tunneling based on the nature of the soil to be penetrated are listed below:
A. Tunneling in Soft Soils
- Fore Poling Method
- Needle Beam Method
- Shield Method
- Compressed Air Method
- Liner Plate Method
- Army Method
- American Method
B. Tunneling 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 tunneling listed above, the readers are kindly requested to go through ref 1.
- Srinivasan, R.(1958). Harbour, dock and tunnel engineering. India: Charotall Book Stall