A portion of Poonamallee High Road collapsed and formed a crater, trapping a Maruti Suzuki Swift and giving its three occupants a hair-raising experience on Friday.
The incident occurred as a result of underground tunnelling work as part of the Chennai Metro Rail project.
The car was heading to Chennai Central station opposite Madras Medical College when the road suddenly caved in. The driver stopped the vehicle on time, but the chassis was dented when it hit the hard surface. On Thursday night, the road had caved at the same place due to tunnelling work and a metal sheet had been placed temporarily over it.
The car was subsequently taken from the cavity using two cranes.
Officials of Chennai Metro Rail Limited (CMRL) said, “The road caved in because the soil underneath was loose and the road surface collapsed, unable to withstand the pressure from the tunnel boring machine (TBM). We will repair the area soon. If the car owner has to seek compensation, he has to approach the contractor – Afcons, the firm in charge of tunnelling work on the stretch”
“There was a bus behind the car. Had the bus been caught instead of the car, there would have been major trouble,” said an eye-witness, C. Geetha, who works in a nearby store. Currently, tunnel boring machines (TBM) are drilling through the earth from Madras High Court to Chennai Central station and this work is being carried out on a joint venture basis by Afcons – Transtonnelstroy.
This is not the first time that tunnelling work has caused a cave-in on a major road. Similar incidents have occurred on the same stretch and also on routes along the Metro alignment, including near Fort Station and Chintadripet among others. More recently, when houses developed cracks in Chintadripet resulting in families leaving their homes, the contractors agreed to bear the cost of repair works there.
Unlike Delhi, where the TBMs bored through rocky surfaces, in Chennai, the soil conditions are unpredictable owing to which such incidents happen, officials explained.
A professor at IIT-Madras, who did not wish to be named, however said that there are ways to predict the soil conditions. They may have done soil tests to see which type of soil the TBM will bore through. But, these would have been done at every 100 metres or 500 metres or even 5,000 metres between two places, he added. “Usually, foam is injected to strengthen the soil in front before the TBM begins the drilling process. Then, after crossing a section, the TBM automatically lays a concrete lining above. But if the machine hits a “hollow pocket” at some point, then the road could cave in,” he said.
