The Highways Department, which has recently begun work on the 3.5 km-long elevated corridor on Anna Salai, to facilitate a CMRL line to be established, will use a technique called Deep Soil Mixing to stabilise soil for constructing the ₹621 crore corridor. The contractor, who will build it on an Engineer, Procure and Construct (EPC) basis, has already begun the soil tests.
Sources in the Highways Department said since the soil over the two tunnels of Metro Rail was not strong enough, it would be strengthened. Bores would dig up the soil and cement concrete would be injected to form blocks. The foundation of the flyover would come up over these blocks, which would be over the two tunnels of Metro Rail.
This technique has been necessitated since during soil tests it had been found that the top soil was stiff clay and could be used for this purpose. “Construction of the foundation would be the most challenging part of the flyover and would take about a year just to complete it,” the source pointed out.
In place of concrete columns, steel columns would be used. “These would be pre-fabricated and would help reduce the weight on the soil and the cement blocks,” said another source. The State already has flyovers using steel in place of concrete. The weight of steel would be 2/3 of concrete. Though the cost would be 20% more than concrete, it would help save time and reduce the whole weight of the structure.
The flyover would help motorists zip over seven traffic signals.
In the 54 km-long phase I network of Chennai Metro Rail system, a substantial part of the Blue Line from Washermanpet to Chennai airport runs underground via Anna Salai. In two locations, the elevator corridor is built over the underground stations, Teynampet and Nandanam and majority of the corridor will be built over the tunnel along Anna Salai.
During the construction in 2010, provision has been made for the station diaphragms to hold the columns of the flyover that was still in a proposal stage at that time.
An official of Chennai Metro Rail Ltd (CMRL) said, “The discussion has been on for many weeks now and we have given the No-Objection Certificate. We have to clear their designs in the coming weeks. While it is a challenging task, it can be done without compromising safety and affecting the tunnel. The construction will be carried out under the supervision of CMRL and its general consultant.”
Officials said they would ensure during the design and construction stage that the load of the pillars doesn’t exceed a specific limit and cause any damage to the tunnel. “Similarly, the tunnel parameters and track parameters will be monitored throughout the construction phase using various equipment to check for vibration levels,” he added.
