Tunnel Engineering
A tunnel is an engineering structure, artificial gallery, passage, or roadway beneath the ground under the bed of a stream, or through a hill or mountain.
Such underground passage constructed without removing the overlying rock or soil may be used for the transportation of passengers, freight, water, sewage or gas, etc.
Advantage of tunnel
(i) For carrying public utilities like water or gas, railway lines or roads across a stream or mountain tunnels may be cheaper than a bridge or open cut.
(ii) Tunnel avoid the dangerous open cat adjacent to the structure.
(iii) Tunnels save tearing up of expensive pavements and so lesser maintenance costs and operating expenses.
(iv) Cost of hauling is decreased due to lighter grades, possibly in the tunnels. (v) Aerial warfare and bombing of cities have given intangible value to tunnels.
(vi) It is generally assumed that when the cut required will have a vertical depth exceeding 20m. it is less expensive to build the tunnel.
Classification of tunnel
1. Classification based on the purpose
2. Classification based on the quality of material
(a) Tunnel in hard rock. (b) Tunnel in loose sand and quicksand (c) Open cut tunnel
3. Classification based on position or Alignment
(a) Saddle or Base tunnel:
- To minimize the length of the tunnel, the track is led through valleys as long as the natural slope of the valley.
- In narrow valleys, the additional length for the minimum permissible radius is obtained by forming a loop into the interior of the mountain.
(c) Off spur tunnels
- Tunnels that are made to shortcut minor local obstacles.
- They are very short in length.