BENGALURU: The government finalizing the Metro link to Kempegowda International Airport (KIA) from Nagawara via Jakkur and Yelahanka may have brought cheer, but the biggest challenge for Bangalore Metro Rail Corporation (BMRC) will be to construct the underground stations on the Gottigere-MG Road-Nagawara line.The link is an extension of the Gottigere-Nagawara line, which is a part of Namma Metro Phase 2. It is a 21.25km corridor, of which only 7.5km is over the ground and 13.7km is underground. Officials said they will require more than five tunnel boring machines ( TBMs ) for the 13-km route.The line will have 12 underground stations — Swagath Cross on Bannerghatta Road, Dairy Circle, Mico Factory, Langford Town, MG Road, Shivajinagar, Cantonment Railway Station, Pottery Town, Tannery Road, Venkateshpura, Arabic College and Nagawara. These areas of central and east Bengaluru are already crowded and BMRC needs to block parts of the roads to build the underground stations.“We need nearly an acre of land on the ground for each of the 12 underground stations. The land acquisition for this has not begun. We will also need to block some parts of these roads for construction and safety reasons,” officials said.Land for six elevated stations is also yet to be acquired. BMRC has paid Rs 600 crore to the Karnataka Industrial Areas Development Board (KIADB), which has to be handed over to land losers as compensation. KIADB will now begin the acquisition process. Pradeep Singh Kharola, BMRC MD, said: “The target is to complete the civil construction of the entire 21-km line in 36 months, and that is why we will need to deploy more TBMs to complete tunnelling, which is the tough part.”BMRC will make use of the five existing TBMs that were deployed for Metro’s first phase. This means each TBM will tunnel nearly a 1-km stretch, making it a faster process.However, experts say the geotechnical conditions will be as tough as they were in Chickpet. TG Sitaram, IISc professor, told TOI, “The old areas of Bengaluru from east to north are like a plateau. The soil is a combination of soft soil and hard rocks, like in Chickpet. It will be a difficult and time-consuming process. The only advantage is the machines, which can handle this kind of soil. How much time it would take cannot be estimated so early.”The estimated cost of the Gottigere-Nagazara line is around Rs 11,014 crore; according to the Phase 2 detailed project report, BMRC had estimated it to be complete by 2023.All this only points to a probable delay of the airport link from Nagawara via Jakkur and Yelahanka even as Siddaramaiah has directed officials to prepare a detailed project report.