The Mumbai-Nagpur Samruddhi Mahamarg has given Maharashtra its longest tunnel! Afcons recently completed the civil work of the 7.78-km twin tunnel. The company is constructing what is slated to be India’s widest and Maharashtra’s longest twin tunnels in the 701-km Maharashtra Samruddhi Mahamarg. It is part of Package 14, which will connect Pimpri Saroddin to Vashala Budruk. The total length of the project is 13.1 km, which includes two viaducts, twin tunnels of length 7.78 km and an interchange. The first viaduct is a simply supported I-girder bridge of 910m, and the second viaduct is a balanced cantilever box girder bridge of 1,295m,with a maximum height of 60m. “The twin tunnelshave been executed with the New Austrian Tunnelling Method (NATM), which consists of a 70-m vertical shaft of 7m diameterand an inclined shaft of 504m,” shares Sekhar Das, Project Manager, PKG-14 - Maharashtra Samruddhi Mahamarg.“The entire carriageway will be completed using pavement quality concrete.” Construction technology The Western Ghats are an especially difficult terrain. And cutting through this mountainous terrain, the expressway project is designed for speeds up to 150kmph.“The south portal of the tunnel, viaduct and inclined shaft are surrounded by a reserve forest without proper access,” informs Das. “The project team developed access roads with proper gradient on the sloping Ghats section, including widening of hairpin bends with necessary permissions from respective departments.” For execution of the 7.78-km twin tunnel, NATM was considered the most effective method in the Western Ghats. “This can be defined as a support method to stabilise the tunnel perimeter with the help of sprayed concrete and rock anchors, and uses regular monitoring to control stability of the tunnel,” says Das. He adds that the viaducts were constructed with standardised methodology, right from foundation to the finishing works. Cut and cover The twin tunnels are an integral part of the project and the cut-and-cover technique has been adopted for the same. “As the overburden is less in the starting chainage of the tunnel, the cut-and-cover portion is required,” explains Das. He adds that this unique portion has its own challenges, which involves controlled blasting. As a slope must be maintained to reach the bottom of the tunnel face, the drill depth must be adjusted and blasted accordingly. For any of the tunnels, it is necessary to havean overburden of 1.5 times the diameter for stability. The design team thoroughly investigated and checked the feasibility of the point in achieving the same. After a length of 240m, it arrived and the main tunnel start chainage was finalised. In the construction of the cut and cover, one major challenge is to cast concrete in the shape of the tunnel, which required experience and skilled people. Apart from this, a gantry was designed in the shape of the tunnel for 12m length. The cut-and-cover section was completed in nearly 14 months with meticulous execution. Ventilation and viaduct There are two shafts provided for natural ventilation. The vertical shaft is 70m deep,with 7-m diameter. The inclined shaft is 504-m long, and 9-m wide, which connects the right side of the tunnel. The first 900-m viaduct consists of 51 piers and 255 PSC I-girders. The second 1,295-m viaduct consists of 26 main piers with 420 segments of length 5m. This viaduct also comprises nine extended span piers with 54 RCC I-girders. While the first viaduct has been completed, superstructure work is in progress for the second. The journey of this project has witnessed several achievements, including: Maximum tunnel excavation for a month: 258.4m Completion of PSC I-girder viaduct:18 months Casting of box girders in a month: 60 Box girder casting time cycle: 8 days Manpower and safety This project required skilled manpower in all aspects, andabout 2,000people were involved in this project during peak times, according to Das. As few infrastructure projects got the approval to work during the pandemic, “our team ensured all COVID prevention and safety protocols during work. We created awareness programmes for all workers and staff, with daily talks, and encouraged them to maintain personal hygiene. The ill were given regular check-ups and everyone recovered. The team arranged vaccination drivesonsite to vaccinate all staff.“ Indeed, the safety team at Afcons always encourages a safe working environment and consistently spreads awareness among workers for safe working practices. There are also two ambulances available 24/7 at the site for any emergencies. Timelines and project completion The project started in 2019, with a timeline – for the entire project comprising 7.78-km twin tunnels, two viaducts,interchange and other portions – of three years.The twin tunnels were completed in a record time of two years, without any loss of time or incident. Most tunnelling works were executed under pandemic restrictions and lockdowns between March 2020 and May 2021. Afcons achieved a series of four breakthroughs in just under five months during the pandemic, completing the end-to-end connectivity through both tunnels. All breakthroughs were achieved before time despite the COVID-19 pandemic and various associated restrictions and hindrances like shortage of manpower, disruptions in supply of material and explosives. Describing the project milestone, Anil Kumar Gaikwad, Joint Managing Director, Maharashtra State Road Development Corporation (MSRDC),said:“This is an extremely proud moment and a critical milestone that has been achieved by Afcons in PKG-14 which is the most challenging of all 16 packages in the entire project. The tunnelling has been done efficiently and proactively following the highest safety standards. We are extremely pleased that the tunnelling could be completed so swiftly without any lost time.” The total project cost is Rs 2,745 crore.