Last mile: How underground metro took shape with minimal rumble

Unlike in Delhi, work on building tunnels for Chennai’s mass rapid transit system is set to finish without major impact on heritage buildings. Though there were several incidents of structures developing cracks, there was no collapse like in the national capital, because of the extensive study of soil structure and building stability and precautions that were taken during tunnelling. The extra caution was also one of the reasons that caused a delay of nearly two years.

CMRL had to manoeuvre its tunnel boring machines below the 2,000-odd buildings, including century-old structures, which dotted the path of the underground metro corridors. But before the machines were installed, Afcons Infrastructure, which built most of the metro rail tunnels, identified critical structures, checked the soil around the buildings, periodically conducted grouting, reduced the boring machine speed and inspected the structures after the tunnels were built.

Instruments like settlement markers, tilt and crack meters were installed on heritage structures like Victoria Hall and Ramaswami Mudaliar Choultry, to monitor impact of tunneling on the buildings. “Existing cracks and other distresses in buildings were recorded before tunnelling and were inspected after completion of tunnelling. If distresses were found to have increased, repairs were carried out,” said an Afcons official.

The ‘earth pressure balance’ methodology, based on several factors including geology, water table and the structures above was the methodology followed for all stretches. Apart from tough soil conditions, boring close or under heritage structures delayed metro rail tunnelling over the years, as the speed of the tunnelling machines were reduced to less than 8m a day from 11m in areas where there were no heritage structures.

In the first year after tunnelling began in 2012, more than 210 cracks were reported in buildings along the route in Washermenpet, Mannadi, Madras high court and Chennai Central. These also included heritage structures like Ripon Buildings and CSI Tamil Wesley Church where CMRL conducted repair work. An Afcons official referring to the initial cracks on the Ripon Buildings said the soil near the structure was weak as there were water filled pits that were dug up for construction of annexure buildings for the corporation. “As Chennai Central metro station work was in progress we were held responsible f for the cracks,” the official said.

N L Rajah, advocate and member of the Madras high court heritage committee said reports on status of heritage structures should be periodically submitted to the government. “There should be a plan for protection of buildings before tenders are floated for construction,” he said.