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CLASSIFICATION OF TUNNEL

"Classification of Tunnel" play a main role to clarify shapes, differences & accountability in design of Tunnel Structure         

classification of tunnel
  

 Tunnel classification &Design of shape and size

  1. A tunnel classification may be for  foot or vehicular road traffic or for a canal.
  2. The central portion of rapid transit network are usually in the tunnel.Some tunnel are aqueduct to supply water for consumption or for hydroelectric station or sewers. 
  3. The shape of tunnel c/s is governed by nature and type of ground penetrated while size is largely controlled by the use to which tunnel is put
  4. Theoretically circular section is best for resisting either internal or external pressure and at the same time provides the greatest c/s area for least diameter

Shapes for tunnel classification
Shapes for tunnel cross section


Disadvantage

  1. But this circular section has disadvantage too .The top curved invert make it difficult in carrying railway tracks without considerable filling of the invert.
  2. The curved bottom also make it difficult for placing concrete of the invert.
  3. Now recent practice is to design tunneling rock with semicircular arch and vertical side walls .Tunnels in soft soils or ground re usually circular or horseshoe shaped as they have to withstand horizontal pressure.
  4. The horse shoe shape is a compromise section to minimize the inconvenience in lining the invert of circular section  

Procedure of work in Tunnel classification

In the wake of driving the tunnel are lined with timber masonry or concrete with suitable outlet to let out enclosed subsoil water behind the lining.Others items of work including the provision of ventilation during and after construction ,drainage and lighting if necessary are carried out to complete the construction of tunnel. 

Tunnel classification :

A/c to tunnel alignment

1)Offspur tunnel: To negotiate minor local obstacles ,very short in length
2)Saddle or base tunnel: Constructed in valley along natural,till slope not exceed ruling gradient
3)Slope tunnel: Constructed in steep hills.For economic and safe operation  
4)Spiral Tunnel: Provided in narrow valleys in forms of loops.It increase length of tunnel and avoid steep slope

SHAPES OF TUNNEL based on Tunnel Classification


SHAPES OF TUNNEL based on Tunnel Classification

1)Circular Section:

  • Offer great resistance to external pressure
  • Best theoretical section
  • Greatest c/s for least perimeter
  • For non cohesive soil
  • Suitable for sewer section
  • Difficult for concrete lining  

2)D section or Segmental Roof section

  • Section with arched roof  and straight sides
  • Most suited in rock tunnel
  • Suitable for subway or navigation tunnel
  • Slope of 1 in 24 is provided

3)Egg shaped section

  • Adopted for carrying sewage
  • Maintain self cleansing velocity of flow
  • Provide least c/s area@bottom

4)Elliptical section

  • Not used as traffic tunnel
  • Best for carrying water & sewage
  • Provided in soft rocks.An elliptical tunnel with height of section 1.6 times its the width has advantage over circular one as tensile stress in crown are reduced

5)Horseshoe Section

  • Most suitable for soft rock
  • Use for highway & Railway section
  • Best section for Traffic Purpose

6)Rectangular Section

  • Most suitable for Hard rock
  • For pedestrain traffic
  • This section are adopted if rcc caisson used for high tunnel 

    Influence of construction on shape in tunnel classification

  1. Conventional method suitable for driving horseshoe and flat arched section and less economical for circular 
  2. Shield method restrict almost to circular section
  3. Caisson method used for rectangular and circular section 
  4. Free face method for any shape
This how the tunnel  classification is being done to differentiate based on the categories used to adopt according to their usage.



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