NEW OBSTACLE FOR NAMMA METRO

NEW OBSTACLE FOR NAMMA METRO
NIRANJAN KAGGERE


Engineers have just discovered digging a tunnel for the Metro won’t be so easy. They were led to believe the 8.8 km underground stretch of the rail track was just 20 per cent hard rock. It now turns out it is 70 per cent.
They are spending a lot of time trying to fix a problem they hadn’t factored in.
Metro engineers had assumed they would have to dig the tunnel through softer rock, and are saying an earlier report, prepared by government organisation Rites in association with the Delhi Metro, had left them unprepared.
Bangalore Metro Chief Engineer N P Sharma estimates that at least 70 per cent of the underground stretch is made of hard and weathered rock.
This rocky layer is just 10-15 metres below the surface. Tunneling would involve high intensity cutting and controlled explosions. That’s not such a good thing for buildings along the rail tracks.
And the project, already running eight to 10 months behind schedule, is likely to get further delayed.
“Unlike Delhi and other cities where Metro rail work is in progress, taking the train underground in Bangalore is a massive challenge,” a BMRCL official told Bangalore Mirror.
The problem assumes serious proportions in the stretch between Vidhana Soudha and Majestic. The rocky bed, expected to be encountered at a depth of 40 metres, was sighted at 10-15 metres. This is one of the reasons underground work in the central part of the city has been delayed.
Sharma’s report said: “During the DPR stage, soil investigations were carried out at 250-metre intervals. But we later carried out investigations at 50-metre intervals and were surprised by the findings.”
BMRCL officials are pointing fingers at the DPR for the mess-up. “We went by their report, and later realised tunnel work would not be that easy,” the BMRCL official said.
The official added: “This is not like Delhi, where we had largely alluvial soil, not much risk. We are busy evolving methods to control the possible damage, and that is consuming much of our time.”
BMRCL’s own investigations revealed that the subsoil profile of Bangalore is varying, with very hard rock discovered at 10-15 metres.
“This kind of geological composition entails specialised construction, controlled blasting and designing for water pressure from all sides,” a BMRCL technical officer said.
Besides, the tunnelling machines require special cutter heads and the entire construction has to be earthquake-proof as the city has witnessed quakes in the range of 2 to 5.5 on the Richter scale, he revealed.
Sharma’s report stated that hard rock was discovered at 9.8 to 28 metres between the Chinnaswamy cricket stadium and Minsk Square, as well as between Vidhana Soudha and the cricket stadium. Besides, the nature of the rock under the Vidhana Soudha was highly weathered rock and hard rock.
ROCK TALK
State government: Only 20 per cent of the 8.8 km underground stretch is hard rock. Namma Metro: No, it’s at least 70 per cent! You guys got it all wrong.
DETAILED REPORT WAS NOT SO DETAILED
The detailed project report (DPR) for Namma Metro was prepared by a firm called Rites in association with the Delhi Metro Rail Corporation Limited.
The geo-technical studies for the report were conducted by the Bangalore-based Torsteel Research Foundation in India (TRFI).
So how did they get it wrong?
An expert from Torsteel said,“Soil stratification varies in Bangalore. We get hard rock and soil at the same depth a couple of metres apart.We have to understand Bangalore has one of the oldest deposits of rock. Huge variations were observed even during the construction of the Sirsi Circle and Richmond Circle flyovers.”
Investigation for the report was done at large intervals and at representative locations to get a general idea. This can vary when a detailed invetigation is taken up at close intervals, he said.
Metro engineers base their views on addditonal investigations made at intermediate points. DPR investigations can change as they work with time and budget constraints, said M S Sudarshan, Managing Director (Technical) of Torsteel Research Foundation in India.
Mirror wrote to Delhi Metro MD E Sreedharan’s office seeking a response and it has received no reply so far.