The completion of India's longest rail tunnel between Devprayag and Janasu in Uttarakhand faced significant challenges, including moments when it seemed the tunnel might collapse and jeopardise the whole project, Larsen & Toubro Limited (L&T) said on Saturday.
"The tunnel boring machine (TBM) named 'Shakti' was about five km inside the tunnel when it encountered a gush of water from all directions at the rate of around 1,500 litres per minute," Rakesh Arora, Project Director of the tunnel, told PTI.
"Besides the TBM operator, there were 200 people inside the tunnel at that time. It was one of the toughest moments when the tunnel was at risk of flooding or collapsing. However, we took corrective measures immediately," he said.
Arora said the situation did not improve for around a month. His team struggled to overcome it by using a combination of chemical as well as cement grouting to stabilise the rings and surrounding rock, consequent to which, the water flow started reducing and the engineers could stabilise the interiors successfully, he added.
"Besides, there were occasions when huge pressure from the surrounding soft rock posed a serious risk to the tunnel's progress, which was successfully handled by speeding up the excavation works with a combination of lubricating the TBM shield using bentonite," Arora added.
The 14.57-km tunnel on the 125-km Rishikesh-Karnaprayag Rail Link Project achieved a breakthrough on April 16 in the presence of Railway Minister Ashwini Vaishnaw, Uttarakhand Chief Minister Pushkar Singh Dhami and other railway and state officials.
Devprayag-Janasu tunnel, besides being the longest, was completed with the fastest speed in the world. It was for the first time that a Single Shield Hard Rock TBM of 9.11-metre diameter was used in the Himalayas to excavate the mountains.
According to the Rail Vikas Nigam Limited (RVNL), out of 14.57 km, 10.49 km was completed through TBM and the rest with the traditional drill and blast method.
"The 9.69-metre diameter Cabrera tunnel in Spain was built by a Double Shield Tunnel Boring Machine (TBM) at an average rate of 423 metres per month and the 10.49-km Devprayag-Janasu tunnel has been completed by Single Shield TBM at an average rate of 413 metres per month, becoming the second fastest in the world,” RVNL Chairman and Managing Director Pradeep Gaur had told PTI on the day breakthrough was achieved.
"As the Himalayas are the most challenging geologically, it makes the whole project extremely demanding and interesting. Out of a total of 125 km, 105 km pass through the tunnels.
"Around three km are covered through bridges and four km by adits (openings) and cross passages; the left portion is just 12 to 13 km which consists of 12 station yards," Gaur added.
Arora said the TBM's biggest enemies are water ingress and soft squeezing rock which try to obstruct them from digging a tunnel in the Himalayas. Once a TBM is stuck inside a tunnel, the project is abandoned, he added.
According to L&T officials, besides the state-of-the-art TBM machine, multiple scientific and physical investigations helped them ascertain what kind of rocks they would get ahead and they made preparations accordingly.
"There is a manual probe through drilling and studying the speed of drilling to find out the rock conditions up to 60 metres ahead. Hard rock gives us slow penetration while soft rocks make penetration faster," Arora said.
"The scientific method is Tunnel Seismic Prediction (TSP) in which seismic waves are emitted and a report is prepared on the condition of rocks ahead by studying the speed of the waves. TSP tells us what kind of rock is 100 metres ahead of TBM and 15 metres on both sides of TBM," he said.
The construction firm said they used another physical method to know the rock conditions by investigating the excavated muck.
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