Surveying Steps in Tunnel Engineering
- Surface Survey.
- Transferring the alignment under ground.
- Transferring levels under ground.
- 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 detailed survey.
- A detail survey of the geological information of strata as the cost of tunneling depends upon
- the nature of materials to be encountered.
TRANSFERRING THE ALIGNMENT UNDER GROUND
- This is the most difficult and important operation in setting out a tunnel.
- Here we transfer the centre line from surface to underground shafts
TRANSFERRING LEVELS UNDER GROUND
- 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 bench mark.
Classified based on the kind of soil penetrated as:
1.Tunneling through hard rock and
2.Tunneling through soft soil
1)Tunneling through Soft Soil
- Shield Tunnelling
- Tunnel Boring Machines
- Compressed Air Method
- This method involves the use of shield machine to drive the tunnels below the ground
- This construction method causes minimal disruption to the traffic and environment
- The method involves mechanized excavation of soils in front of a steel cylinder, called “shield” and assembling of the tunnel construction under the protection of this cylinder in its back part.
- The machine is pushed forward by a system of hydraulic jacks.
- Shield consists of circular steel ring with a transverse diaphragm
Essential parts of shield
- Cutting edges
- Propelling Jacks
- Hood – Forward extension of the ring at the top and affords protection to miners working in the forward section
- Tail – Rear section which projects back a little distance over the completed lining
- Cutting edges – Provided with proper stiffeners form the forward circular edge of the shield and cut into the earth to be excavated
- Propelling jacks – Hydraulic jacks attached along the centre-line of the central ring beam of the shield. They push the shield forward, cutting the soil
- Port holes – can be opened out to permit the muck to flow into the completed portion of the shield
Tunnel Boring Machines
- Logical improvement of shield
- Mechanized operation
2)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:
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 bench to the floor of the tunnel.
- Two plates of girder about 23cm in length are fixed at 1.8m centre to centre distance.
- A belt conveyor is fitted with a number of jack is its running on the plate girders.
- The ends of girders project beyond full face of the bench
Compressed air method
- Air compressed method, compressed air is used to counteract the hydrostatic ground pressure exerted against the cutterhead.
- These machines are specially suited for excavation in unstable soils with the presence of water.
- Air lock and accessories
- Bulk head to fix the air lock
- Blow line
- Air compressors
Air lock 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 tunnell
- Air tight diaphragm in which air lock is fixed
- Blow Line
- Consists of 100 mm diameter pressure pipe running from tunnell heading to the outside.
- To exhaust foul air
- To maintain 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 :
2.Needle beam 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.