In what now seems to be a familiar sight on Chennai’s roads, the arterial Anna Salai road caved in on Sunday, trapping a bus with 35 passengers on board and a car in a large crater. There were no serious injuries. Two days later, on Tuesday, a 10-metre crack was spotted at the same spot, reported The Hindu.

The cave-in on Sunday coincided with tunnel boring work conducted by the Chennai Metro Rail Limited for an underground station in Anna Salai. Metro officials said loose sand pockets along the road had probably caused the road to give way, though they added that soil tests had been conducted before the tunnelling.

Since January 2015, there have been at least seven incidents across Chennai of cave-ins on stretches where Metro construction is on, raising doubts and fear over the safety of constructing underground tunnels. Less than two weeks ago, a stretch of road on Anna Salai, near Church Park School, had collapsed during peak-hour traffic.

Recalling Sunday’s incident, the conductor of the bus, which was stationed at a bus stop, said the vehicle seemed to slowly sink into the ground. He thought at first a front tyre had been punctured, until he saw the cracks on the road. The passengers were safely evacuated and it took nearly three hours to pull both vehicles out of the sinkhole that was two metres deep, five metres wide and 10 metres long.

“This has happened without any indication,” The Times of India quoted a Metro official as saying. “We have been taking all safety precautions but will increase frequency of checks in the coming days after the stretch is repaired and traffic is restored.”

Soil theory

Many are not convinced by the Metro’s assurances, and say that laying Metro lines along the coast has loosened the soil.

But L Elango, head of the geology department at Anna University, said this was not an issue because there is technology to overcome such challenges. “All over the world, tunnels are running beneath the river and beneath the oceans also,” he said. “United Kingdom and France are linked in this manner. So there are no issues regarding the type of rock or soil found in Chennai. All over the world, people have constructed Metros in different kinds of formation.”

Chennai’s surface is made largely of charnockite, a highly compressed rock formation. However, in parts of the city, looser sediments with more clay may exist, said geologists.

“Chennai coast has many sedimentary deposits, which comprise of soft materials,” said S Ramasamy, a geologist at the University of Madras. “The interior parts of Chennai are made of hard rock and are tough and compact. But Anna Salai and Thyagaraya Nagar have soft sedimentary deposits. At one point in time, they might have been covered by sea, which may have regressed over time.”

Experts say the tunnel boring machine, or mole, is designed to not displace the soil around it. Image credit: Chennai Metro Rail/Facebook
Experts say the tunnel boring machine, or mole, is designed to not displace the soil around it. Image credit: Chennai Metro Rail/Facebook

Safe technique

Regardless of the soil type, the tunnel-boring technique is safe, according to experts. The tunnelling machine, called the mole, is designed in such a way so that it does not produce any disturbance in the soil around it. It can drill through sand or hard rock, and is commonly used in heavily-urbanised settings.

“It is unusual for a road to cave in while using the tunnel boring method,” said Elango. “There must be some kind of negligence on the part of the Chennai Metro Rail Limited while tunnelling underground.”

E Sreedharan, principal adviser to the Delhi Metro Rail Corporation and popularly known as Metro Man, agreed with Elango’s view. “With a tunnel boring machine, this should not happen at all,” he said. “We have done lots of tunnel boring but there has not been any caving of the road.”

The Chennai Metro Rail Limited, however, issued a statement saying the cave-in was the result of an “existing loose pocket along the tunnelling alignment” where the tunnel boring machine was being used. “The monitoring point has been installed throughout the tunnel alignment at 10 metres interval,” the statement read. “Continuous reading is taken at every 6 hours interval to cross check any development done to the ongoing tunnelling wok. The recent reading was taken around 2 pm just before the incident and there was no settlement recorded while taking the reading.”

Despite the assurances of periodic monitoring, recurring incidents such as the one on Sunday seem to indicate that chances of roads caving in are higher at places where Metro work is underway. And given that the mass transport system is still a few years from completion, the possibility remains that this might not be the last incident of its kind.