Part of Chennai's arterial Anna Salai caved in on Sunday afternoon trapping a bus and a car in the giant crater.
This is the second such road cave-in on the city’s arterial road Anna Salai in the last ten days.
According to metro officials, the soil could have loosened due to tunneling work in the stretch for Metro Rail. The Metro Rail work has been temporarily suspended.
An expert said that from the look of the cave-in it seems like after the after boring the tunnel, the top was not filled and sealed properly.
"When an underground metro is built, there are usually two methods of construction. One is cut and fill. They cut the ground, make the tunnel and close the ground. Or else they bore the tunnel right through the soil (with a Tunnel Boring Machine (TBM)). If they used the cut and fill method for the Chennai metro, they may not have filled the top properly. That seems to be the only possibility. A remote possibility is that there could have been a well there which collapsed," the expert said.
MG Devasahayam, a retired IAS officer, however blamed it on the government saying that the project was approved by the DMK government arbitrarily and that the city does not even require such a metro service.
"When the metro was announced, no public consultation or stakeholder consultation took place," he said.
Stating why he strongly opposed the Chennai Metro Rail, he stated, "Firstly, Chennai does not need a metro like this. Unlike other cities- we have mass transit. We just needed to optimise existing transport. Secondly, being near the beach, the quality of soil was suspect from Day one. You have to dig and go down and this is bound to destabilise the ground. Anna Salai is just 2 km from the coast and this kind of tunnelling is not supposed to happen here," he claimed.
In a statement that countered Devasahayam's second point, the expert said that the soil is studied carefully before any such project is initiated and measures are taken to avoid accidents due to construction. So it is unlikely that the cave-in happened due to soil quality.
KV Dinesh, an independent consultant based in Mysuru with 30 years of experience in construction and designing with MNCs, is of the opinion that it is difficult to say whether the crater was caused due to metro work.
"As we all know, Chennai is a coastal city. The surface below the road (soil) is actually sub-grade due to the high presence of moisture. A road is also a civil engineering structure, it has to be first designed and then built. And if the water levels are shallow in that stretch, it could have collapsed on its own and formed a crater or it's possible that they dug a much bigger hole for the metro foundation."
He states that in this case the civic body is trying to shift the blame on the Metro and get away from its responsibility.
"I would not buy the argument that the Metro construction is the prime reason for the mishap, it is possible that the Metro foundation may have caused it. But if that was the case, then why is it only at that location," he says.
He adds that the lack of designing and engineering skills of the civil administration could have resulted in the cave-in.
"In Chennai, the soil is saturated with water. So, while constructing, the entire soil desk has to be replaced. Sometimes, it has to be even replaced up to 1.5-2 metres. It might be the case that it was not done in this area. Often, civic bodies across India put asphalt in the base. Unfortunately, many don’t have designing and engineering skills, so they try to put the blame on someone else."
Cave-ins in Chennai
In December 2015, a giant crater was formed along the Anna Salai-Vijayaraghava Road in Teynampet. On that occasion to, the road cave-in was blamed on the metro construction.
A 10-foot stretch on Poonamallee High Road, another arterial road in Chennai caved in in June 2015 owing to tunnelling work for the Chennai Metro Rail. The cave-in even witnessed a car getting stuck in the crater. However, the occupants in the vehicle escaped unhurt.
An official of the Chennai Metro Rail Limited (CMRL) told The Hindu last year that 11 tunnel boring machines (TBM) had been brought in the city for tunneling work for the metro.
The official also said that "The soil conditions have been hard to predict. In some areas, we had to bore through hard rock, while in others, it was mixed soil. Sometimes, loose soil gave way and that led to caving of road surfaces in various places."
The "complex soil structure" had also delayed work in the 300-metre stretch between Rajiv Gandhi Government General Hospital and Chennai railway station for over six months.
"Too much pressure on the soil will affect the roads and buildings, if their foundation is weak. We have no choice but to work slowly and carefully. We still have about eight percent of work to complete there," an official told The Times of India.
With inputs from Soumya Chatterjee and Anna Isaac