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Surveying Steps in Tunnel Engineering










Surveying Steps in Tunnel Engineering

  • Surface Survey.
  • Transferring the alignment underground.
  • Transferring levels underground.

Surface Survey

This includes
  • A preliminary survey is done by theodolite to connect the 2 ends of the proposed tunnel.
  • A plan (map) with a suitable scale.
  • Final alignment is selected from this plan and on the basis of a detailed survey.
  • A detailed survey of the geological information of strata as the cost of tunneling depends upon
  • the nature of materials to be encountered.




TRANSFERRING THE ALIGNMENT UNDERGROUND

  • This is the most difficult and important operation in setting out a tunnel.
  • Here we transfer the center line from a surface to underground shafts

TRANSFERRING LEVELS UNDERGROUND

  • Leveling on the surface is done in the usual way and the levels are transferred underground at the ends of the tunnel from the nearest benchmark.

Tunneling methods

Classified based on the kind of soil penetrated as:
1.Tunneling through hard rock and
2.Tunneling through soft soil

Tunneling through hard rock

  • The hard rock is self-supporting, so less amount of lining or supports needed
  • Requires greater skill and accuracy while cutting or exploding the harder strata
  • Different methods are:
                          1.Drift method
                          2.Heading and bench method
  3.Full face method or shield method of tunneling
                          4.Cantilever car dump method

Cantilever car dump method

  • This method provides an improved arrangement of throwing mucks from the bench to the floor of the tunnel.
  • Two plates of girder about 23cm in length are fixed at the 1.8m center to center distance.
  • A belt conveyor is fitted with a number of the jack is its running on the plate girders.
  • The ends of the girders project beyond the full face of the bench

Compressed air method

  • Air compressed method, compressed air is used to counteract the hydrostatic ground pressure exerted against the cutter head.
  • These machines are especially suited for excavation in unstable soils with the presence of water.
Equipment
  • Airlock and accessories
  • Bulkhead to fix the airlock
  • Blow line
  • Air compressors

Airlock and accessories

  • Long air-tight cylindrical steel chamber, with two doors, one at each end both opening inwards
  • Doors are opened at the ends when the intensity of pressure in the lock reaches the higher pressure chamber in the tunnel




Bulkhead

  • The airtight diaphragm in which airlock is fixed
  • Blow Line
  • Consists of 100 mm diameter pressure pipe running from tunnel heading to the outside.
  • To exhaust foul air

Air Compressors

  • To maintain the pressure of 10 N per cm2 to 14N per cm2 during working periods

2.Tunneling through soft soil

  • Requires extra support for the soft soil through which the tunnel is cut through
  • Methods used are :
                      1.Forepoling method
                      2.Needle beam method
                      3.Case method
                      4.American method
                      5.English method
  • In compressed air closed shields, the rotating cutterhead acts as the means of excavation whereas face support is ensured by compressed air at a sufficient level to balance the hydrostatic pressure of the ground.
  • Debris is extracted from the pressurized excavation chamber using a ball valve-type rotary hopper and then conveyed to the primary mucking system.




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