Monday, September 07, 2009

TUNNEL EFFECT TO TURN POWER LANE DRY

TUNNEL EFFECT TO TURN POWER LANE DRY
For the next one year, Vidhan Soudha, the high court complex and hundreds of government offices situated along Ambedkar Veedhi will have restricted water supply as a result of underground drilling for Metro Rail
NIRANJAN KAGGERE


Ambedkar Veedhi houses the states legislature, judiciary and executive, and collectively our netas, bureaucrats and government servants, who normally so lord it over the public, could, for once, be at the receiving end. Due to ongoing Metro Rail work in the area, all the government offices in the area will be seriously short of water supply for close to a year.
The Bangalore Metropolitan Rail Corporation Limited (BMRCL), as part of its underground tunnelling work, has issued a circular to seal all borewells and open wells located in Raj Bhavan, GPO, Vidhana Soudha, Cubbon Park and near K R Circle - all along a 100-metre radius. NEW TECHNOLOGY
In the circular, the Deputy Commissioner, Bangalore Urban has written to the Chief Post Master and the people in-charge of Raj Bhavan lawn, Vidhana Soudha lawn and Cubbon Park to ensure all borewells in the area are sealed and water is no longer drawn from them. Sources in Vidhana Soudha told Bangalore Mirror that the closing of bore wells is because the technology adopted by BMRCL to dig tunnels is relatively new and untried in Bangalore’s topography.
The BMRCL will be using a technique called Earth Pressure Balance Tunnel Boring System to drill the tunnel on Ambedkar Veedhi as part of the proposed metro station. The technique requires proper maintenance of soil pressure during and after construction of the tunnel. A senior official supervising the Cubbon Park section told Bangalore Mirror, “We were told that while drilling the tunnel through this technique, tremendous pressure is exerted and even a small gap in the soil structure under the earth would create problems. Hence we have been asked to close all bore wells.”
DANGER OF POLLUTION
The BMRCL circular added: “The pressure while drilling the tunnel might damage the casings of neighbouring bore wells and with the damage chemicals might seep into the underground water table leaving it polluted. Hence it has been advised to close the bore wells till the completion of tunnelling work.” Clarifying the matter, a senior official from the Horticulture Department explained, “It is more a precautionary measure and once the work is over, the water from the wells will be tested - if there is no impact of chemicals or other impurities it will be re-opened. At Vidhana Soudha there are two bore wells which cannot be used even after the construction. They will be closed for ever and instead two other bore wells will be provided on the same premises.”
PARKS WILL BE AFFECTED
The closing of bore wells along the stretch will affect the maintenance of the verdant gardens at Raj Bhavan, Vidhana Soudha and Cubbon Park. “Though there are two bore wells in Vidhana Soudha, we hardly used them, instead relying on rain water. But the rain water collection tank in front of Vikasa Soudha will be affected when the makeshift road is built as the main work beneath Ambedkar Veedhi begins. With three water sources closed due to metro work, we will have to manage with alternative sources like tank water or rely on bore wells elsewhere,” an official supervising the Vidhana Soudha lawn said.
BMRCL officials have marked a total of 23 open wells and 20 bore wells along the North-South and East-West corridors which are likely to be affected due to the underground work.
“All the identified points have been divided into four types. Those points which are within 12.5 metres from the tunnel will be backfilled by removing the casings, and those that fall between 12.5 metres and 17.5 metres will be backfilled along with the casing and bore wells that are between 17.5 metres and 30 metres will be sealed and not allowed for consumption. However, those which are beyond 30 metres away will not be directly impacted,” a BMRCL survey pointed out.
ADVERSE EFFECTS
The tunnels will be in the saprolite and fissured zone of the earth besides hard rock layer at certain places. Fissured zone has the highest permeability, and blockage of this zone (due to tunnelling) may result in substantial reduction in the effective transmission of water.
The BMRCL survey proposes that the tunnel will be drilled through the saprolite and fissured zone of the earth. But experts opine that the blockage of fissured zone which has highest permeability would affect transmission of water at later stages.
Soon after the tunnelling, if the borewell users do not take up recharging of borewells, then the water table will go down by 10 ft
The tunnelling will be taken up at a depth of 16 mtrs from the earth level in all places except Majestic where it will be 18 ft WHAT IS TUNNEL BORING MACHINE (TBM)?
Excavation of tunnels housing various projects is taken up using tunnel boring machines (TBM). Unlike the previous method of excavating tunnels using drilling and blasting (D and B) method, TBM method is said to have been more scientific and advanced.
TBMs can bore through hard rock, sand and almost any variety of soil and rock structure beneath the earth’s crust. The diameters of tunnels excavated through TBMs range from a mere One meter to 16 meters. Boring tunnel using TBM would bringdown the level of possible disturbance to the surrounding ground and ensures the smooth tunnel wall. As a result, TBMs are widely used in high density urban areas.
The front end of TBM consist of a rotating cutting wheel. Behind the cutting wheel is a chamber where the excavated soil is either mixed with slurry or discharged as it is. Akin to the movement of an earthworm, the TBM will brace against the tunnel walls and push itself forward. Tunneling in Bangalore or in any other urban areas comes with a special challenge of ensuring the minimal disturbance of ground surface. However, techniques like Earth pressure balance and Bentonite slurry are preferred to the conventional open face method due to the superior ground control.
Experts opine that the tunnel boring machine would do multiple tasks like drilling the tunnel, thrusting out the muck and sealing the inner walls of the tunnel with pre-cast concrete elements. The mud will be slightly slushy since the tunnel boring machine uses water while drilling. One machine can drill an average 20 metres a day on normal soil and 12 metres on rocky soil.x METRO IMP ACT
The work on building tunnels at both Vidhana Soudha and Cricket Stadium is expected to begin in a month. The tunnel at Vidhana Soudha metro station and other underground tunnels will be dug at an average depth of 12 meters. However, the tunnel for underground stations at Majestic-a centre point for both East-West corridor and North-South corridor, will be at a depth of 18 meters. The tunneling process across
the city would affect the nearby borewells and openwells

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