JOIN USMetro underground stretch costs a fortune
Anyone who has been a victim of the pell-mell on the roads due to the ongoing Metro work would naturally argue for underground work. It requires neither a traffic diversion nor land acquisition. But, the prohibitive cost comes in the way. A tunnel boring machine itself costs Rs 100 crore. The estimated cost for the 42.30-km first phase of Namma Metro is Rs 11,609 crore.
The underground stretch of the Metro extends for a length of 8.82 kilometres and comprises eight stations. The project has been split into three packages. The work on the north-south corridor – from Rajiv Gandhi Circle in Seshadripuram up to KR Road entrance – has been awarded for Rs 707.90 crore. The stretch comprises two stations.
The contract for the underground stretch from Chinnaswamy Stadium to Magadi Road entrance has been awarded for Rs 996 crore. Four stations need to be constructed on the east-west stretch. The tender process for the work on two stations near Majestic is yet to be completed.
The east-west corridor traverses a portion of the Kempegowda Bus Station at a depth of eight metres while the north-south corridor traverses the same stretch at a depth of 12 metres. Stations on Hessaraghatta-Puttenahalli route and Byappanahalli-Mysore road route will come up at the intersection. Commuters can change Metro rail to travel to various parts of the City at this point. Apart from the two stations, a multi-storey office of the Metro has been proposed at the location. All these projects come under a package.
The work on the seven-km Reach-I (Byappanahalli-M G Road) has almost been completed at a cost of Rs 1,000 crore.
The project cost would have doubled if the work was carried underground, sources said.
Concrete rings
The tunnel boring machine which drills the tunnel of 21-foot diameter will also fix concrete rings in the inner surface. According to a BMRCL source, it requires 3,350 concrete rings for the 2.4-km stretch between Majestic and Minsk Square. Each ring will be fixed in six segments and each segment weighs about 3.50 tonnes. As many as 125 rings have already been manufactured. The tunnel borer will fix the rings as it edges ahead every one-and-a-half meter into the earth’s crest.
Before beginning the tunnelling, the BMRCL has conducted tests to ascertain the presence of water sources, hard rock and soil beneath the ground. Experts said ‘Dynamic probing’ tests have been conducted at a gap of every 50 metres.
Dynamic probing is a simple test which involves drilling a sharp-edged rod into earth’s surface. The rod stops its downward movement once it encounters a hard surface.
Studies have also been conducted on the strength and withstanding ability of the buildings situated over the underground stretch. The tunnelling work generates vibration in a radius of about 20 feet. In fact, 80 feet radius on either side of the track has been considered as an area which would bear the effect of tunnelling, officers said.
The Taiwan-based Continental Engineering Corporation, one of the contractors, has named the two tunnel boring machines Helen and Margarita.