An imposing prospect – tunnelling the Bangalore metro

On a sizzling summer season’s day in Bangalore, ambassadors, politicians, lobbyists, financers, press and the cream of the Indian tunnelling neighborhood crowd collect excitedly in a big marquee erected alongside the Majestic Station shaft, eagerly awaiting the launch of the primary slurry TBM to be utilized in India.

Police patrol the location and line the edges of the tent. With one hand resting on their batons they survey the throng as Karnataka state Chief Minister Shri Yeddyurappa enters to a hail of applause, flash pictures and adoration from the baying crowd.

The visitor record for the TBM launching ceremony on Bangalore’s metro reads as a who’s who for the Indian building trade.

Addressing this crowd, Yeddyurappa proudly stamps his identify on the undertaking by congratulating those that have introduced it this far.

“The gods of the underworld have to be more than happy,” says the grasp of ceremonies.

Following a ceremonial smashing of a coconut and lighting of fires, the Chief Minister heads for the launch shaft and pushes the button that units the cutter wheel in movement.

Yeddyurappa claims the 6.44m Hitachi Zosen TBM is the primary TBM for use in a Southern Indian state, and the primary slurry TBM for use within the nation.

Bangalore east-west metro

“The visitor record for the TBM launching ceremony on Bangalore’s metro reads as a who’s who for the Indian building trade.”

As the third largest metropolis in India, Bangalore faces congestion issues just like these present in Delhi, Mumbai and Kolkata. Its inhabitants is rising at greater than three per cent per annum, with a document annual development of 12%.

The 5.4 million residents place a huge demand on the restricted public transport system. Consequently, automobile possession is growing. Some 1050 new autos are registered in Bangalore day by day. However with excessive salaries within the 800 sq. km metropolis and multinational firms properly established, the town can afford an improve.

There are two metro strains below building in Bangalore, the north-south hall and the east-west.

A three way partnership of Taiwanese contractor Continental Engineering Company (CEC) and Indian contractor Soma is executing the east-west line, with the TBM beginning works some ten days after the launching ceremony final month. CEC’s Indian arm CICI is finishing up the tunnelling works whereas Soma is excavating the stations.

The 4.45km of dual bore tunnels can be absolutely excavated by TBMs with the east and west ramps, the stations and a brief pocket observe being excavated by lower and cowl. The tunnels may have a completed inside diameter of 5.6m, lined with precast segments 1.5m large and in a 5 plus one association. The segments are manufactured close to the work website by Ya Li, a part of the Far Jap Group.

The two TBMs can be staggered about one month, or 50-100m, aside and may have a minimal overburden of 1 diameter, although it’s extra typical alongside the alignment to have an overburden of round 10m.

Eastbound drive

Majestic station is the interchange station for the east-west and north-south strains. The contract for the station has not been awarded as the ultimate value for the undertaking has not been agreed. There are some quiet issues this might sluggish the commissioning of the undertaking however for now the main focus is on the tunnelling works. 

Challenge supervisor for CEC Russell Brown says the primary 140m of the drive that TBM Helen is endeavor is among the many most testing for the undertaking. With a shallow overburden of simply 1.5 diameters, the TBM is fastidiously edging its method below the fragile buildings of Bangalore above. It’s the solely stretch of the undertaking the place the staff weren’t capable of preserve all buildings exterior the zone of affect. Nigel Butterfield, undertaking supervisor for the final guide, says at this level the consumer, Bangalore Metro Rail Company (BMRC), will settle for slight injury to native buildings, which should not exceed cracks of 5mm in width.

TBM Helen and TBM Margarita will attain Central School Station in September and October, says Brown. Soma is setting up the station, however when the TBMs arrive the primary excavation won’t have been accomplished. A restoration shaft on the west aspect and a launching shaft on the east will permit the TBMs to leap frog the station and relaunch.

“Yeddyurappa claims the 6.44m Hitachi Zosen TBM is the primary TBM for use in a Southern Indian state.”

The remaining drive of the japanese leg for the 2 Hitachi Zosen TBMs will sort out the 479m run to Cricket Stadium Station. The drive will cross by clayey silt, extremely weathered rock and arduous rock. Brown says “on the drive from Vidhana Suda to Cricket Stadium there may be plenty of arduous rock developing into the face.

It occurs about 5 – 6 instances, and specifically one of many challenges for us is the final 200-300m. We come spherical a really tight 270m-radius curve by the GPO constructing – a really ornate constructing with a big set of steps and columns to it. We sit proper below the entrance facade of that constructing, it’s proper in our zone of affect.”

BMRC stipulates that solely negligible injury, outlined as hairline cracks, is acceptable on heritage buildings. Passing by the combined floor in shut proximity to the GPO constructing, the tunnellers are going to should fastidiously monitor subsistence as they cross by the curve.

Westbound drive

By the tip of June 2012 the TBMs ought to have damaged into the receiving shaft at Cricket Stadium. They’ll be dismantled and shipped again previous the Majestic Station website to the west finish of the pocket observe for launch on the ultimate quick 227m drive. The 170m lengthy observe can be constructed utilizing lower and canopy and can be excavated utilizing drill and break up.

The quick 227m drive from the pocket observe lower to Metropolis Railway Station is but one other delicate stretch for the TBMs. The machines should cross below eight lively railway strains operated by Indian Railways. It’s anticipated to take a month to barter the route below the railway, fastidiously monitoring any settlement utilizing an array of displays over the tracks.

Ramps and stations

“Tunnelling work for the east-west line must be accomplished by the tip of summer season subsequent yr.”

The shallow alignment permits every of the stations to be excavated by lower and canopy.

Every station is 272m lengthy except for the far west 242m lengthy Metropolis Railway Station which is restricted by rail strains on the east aspect and the ramp on the west aspect.

Soma’s Shri R Richardson Asir says the stations are being excavated by drill and blast and constructed utilizing both the highest down or backside up method.

Soldier piles and rock dowels are used to assist the excavation and the station is forged inside the excavated pit.

Scientist and head of rock blasting and excavation on the Nationwide Institute of Rock Mechanics in India, Dr H S Venkatesh, says “for the success of any blasting, particularly in an city surroundings, one must work on the weaknesses quite than the strengths.” In this case the weaknesses are floor vibrations, air overpressure, management of flyrock and injury to adjoining rock mass. Venkatesh experimented with totally different blasting patterns, explosive varieties and volumes and cussions and obstacles to develop an appropriate blasting methodology for the excavation. At some stations, buildings are in very shut proximity to the blasting works so hoarding was wanted to make sure no flyrock made it off the location. The main points of the analysis to beat these weaknesses would be the topic of a later paper.

The excavation of the east and west ramps that be part of the underground part to the elevated sections is being undertaken by CEC. Venkatesh says that “although blasting occurs to be the most secure, least expensive and quickest technique of excavation, many-a-time city conditions could not allow blasting, and alternate technique of excavation should be adopted, which can be costly and sluggish.” CEC’s Brown provides that for the east and west ramp, blasting was dominated out due to city restrictions. He explains there are robust restrictions on the variety of blasts allowed every day, the dimensions of the blast and the times you’re allowed to blast. To extend the flexibleness within the working sample Brown has opted for the slower technique of drill and break up that makes use of hydraulic splitters rather than explosives to interrupt up the rock.

Tunnelling work for the east-west line must be accomplished by the tip of summer season subsequent yr.

First revealed within the July 2011 version of tunnels & tunnelling.

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