12 Kolkata: The Orange Line's underground tunnel construction by the box-jacking method below the airport approach was hit by subsidence on Thursday, prompting the authorities to summon experts for urgent soil reinforcement and strategic planning. Given the site's proximity to the airport, work for India's largest and longest box-pushing operation will resume only after thorough implementation of the experts' recommendations.
TOI on Jan 29 reported the commencement of the engineering marvel beneath the airport flyover. The New Garia-Airport corridor or Orange Line's Metro tunnels will stretch 135m beneath the airport access. When completed, it will stand as an unprecedented engineering feat.
Implementing agency Rail Vikas Nigam Ltd (RVNL) advanced only around 2m from the site next to the airport flyover before the cave-in occurred, a railways official said. They said a water tank and a lavatory of the airport were dismantled for the metro work. "The razing activity may have destabilised the surrounding ground," an official said. The subsidence measures around 3.5m from ground level.
"Thankfully, the setback has come in the beginning. The subsidence location is quite far from the airport utilities. Henceforth, comprehensive precautions will be taken to prevent recurrence," the official said.
The "box-pushing technique" typically serves micro-tunnels or underpasses not exceeding 30m in length, enabling construction without traffic disruption. The method involves advancing reinforced concrete (RCC) box segments through soil to create subterranean structures. "The 135m of box-pushing shall proceed cautiously to maintain live traffic flow so that there is unhindered access for flyers," an engineer explained. The operation involves pushing 14 precast boxes beneath the airport flyover span using hydraulic jacks.
Following expert advice, soil nailing has started to secure the crater. This process involves embedding 3m thin iron rods into the soil with concrete casting for ground stabilisation and prevent collapses. The boxes shall advance gradually along this "nail" bed. Subsequently, concrete grouting shall reinforce the sides, ensuring the safe movement of the boxes. This precedes electrical resistivity tomography (ERT) along the alignment for weak zone assessment.
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