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Drainage in Tunnel

Load the mind with new topic of tunnel engineering which is Drainage of Tunnel 

                      

BASICS ABOUT DRAINAGE IN TUNNEL

In a tunnel water comes from shafts and portal which is being dug in order to achieve the alignment of tunnel, water which is being used during washing  holes of drill and progressing the tunnel during the time of construction,The water obtained from the first source cannot be easily determined and water obtained from the second cause requires analysis and investigation and can be prevented by RCC bulkhead.
Exploding charge may open up a ground water source unexpectedly omitting large quantity of water into the tunnel. Such sudden shot of water give rise to conditions 
  1. An underground pocket or lake has been trapped.
  2. An underground flow with an inexhaustible source such as river or  lake has been trapped
The progress of tunneling work and the maintenance of tunnel drainage after its construction primarily depends on good drainage of tunnel.If the drainage of tunnel is improper it leads to wear and tear and so cause serious problems in the structure 


Tunnel Drainage Arrangement

  1. Pre-drainage or preventing the entry of excess water from entering the tunnel before starting the construction work 
  2. Dewatering and pumping of the tunnel is removing the water that has entered the tunnel during the construction of tunnel 
  3. Permanent drainage in tunnel is keeping water of the tunnel after its completion

PRE-DRAINAGE

When seepage is small and comes down from the tunnel roof made to flow over  temporary pitched roof on to longitudinal drains .If exploration indicate existence of ground water it may possible to grout off heavy discharge and stabilize the formation before tunnel approaches the difficult zone 
The pre-drainage of a tunnel basically consists of preventing the surface water and subsoil water adjoining the tunnel interfering with the construction work 
Remedial measures and precautions IS taken to divert the entry of surface water or rain water through the shaft or to provide a heavy blanket of clay over the tunnel heading to prevent its blowing because of rush of water and also control the floodage.

DEWATERING (TEMPORARY DRAINAGE)

  • Taking out water from ditches and so used in hard rock bases and water resistant soils
  • It is not possible to drain off water at all grades because self cleansing velocity is not possible
  • Construction of open drains weakens the section of tunnel because it break consistency of shell 
  • Open drains are not favoured in modern tunnels and method of pumping is adopted to remove water from inside of tunnel 
  • The simplest method for the removal of water is by drawing through the open drains or ditches constructed along the tunnel on both sides at the invert

Disadvantages of Dewatering in Tunnel Drainage

  1. If the muck or debris blocks the flow of water, the objectionable ditches and pools will be created inside the tunnel
  2. The valuable working space is utilized in the construction of open drains.

PUMPING (TEMPORARY DRAINAGE) 

  1. Quantity of water is collected in sump wells and then pumped out of tunnel.For long tunnel gathering pumps are used. When the tunnel is long, it may be necessary to have more than one sump well in Tunnel Drainage
  2. Sump well also used in settlement of settlement of solid particle and are located at the interval of 300m to 500m at wet spots.
  3. Steel pipes of diameter varying from 80mm to 150mm are used.
  4. The motive power for centrifugal pumps may be electrical or pneumatic.
  5. Diesel motive power is not preferable due to extra load on ventilation system 

PERMANENT DRAINAGE IN TUNNEL

  1. Construct ditches longitudinally sloping towards shafts and portal
  2. Central Drain-  covered with facility for the inspection and cleaning
  3. If the water trickles from roof,false roof of corrugated galvanised iron sheets may be provided to divert roof water to side drains
  4. Sheet provided on side walls and thus splashing of water on pavement of track will be prevented
  5. Easy to construct cut and cover tunnel waterproof but difficult in cased of bored tunnel Seal the crack in the lining that leaks
  6. The manhole for inspection should be provided at a distance of 30 m to 50m
  7. The fissures, crevices, etc. to be grout should be cleaned out to remove any clay or unconsolidated material before starting the process of grouting.
  8. For permanent drainage longitudinal drains ,open drains ,concrete lining generally adopted
Central drainage duct in Drainage of Tunnel
                  Central Drainage Duct



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