The making of a metro tunnel
That it was going to be an uncommon assignment became clear as we were asked, ‘What is your T-shirt and shoe size?’ After attending a presentation on super complex engineering techniques, we were given a safety jacket, helmet, and boots with steel toes. And there we were, ready to walk from Shivajinagar to M G Road in Bengaluru through a 900-metre underground tunnel.
It was our date with Avni, the tunnel boring machine (TBM) deployed by the Bangalore Metro Rail Corporation Ltd (BMRCL) to carve circular tunnels through soil and rock strata. Together with Lavi, another TBM that is parked 120 metres north of M G Road, Avni will clear the way for the construction of 12 underground metro stations in Bengaluru.
When we arrived last month, Avni had burrowed from Shivajinagar to reach M G Road and was getting ready for the onward journey of 1.1 km to the Rashtriya Military School station on the 22 km Gottigere-Nagawara corridor.
The on-ground construction for Namma Metro has been a sore point in Bengaluru. It is marked by incessant delays and causes traffic snarls. Is it any faster 60 feet under? How do people work underground? How hot does it get? Does it get slushy when it rains? Is there enough oxygen? What happens in case of fire?
Questions raced through our minds as the engineers led us down a makeshift stairway, steep, winding, narrow and flanked by a green safety net. We were under M G Road now, at a site hidden behind blue metal sheets.
It wasn’t dark, dusty or dank like a cave, as we had imagined. Wearing gloves, knee-length boots, jackets and colour-coded hard hats, about 20 workers were milling around machines, tools, metal beams and wires that dwarfed them greatly. The visuals reminded us of TV series like ‘Megastructures’, and ‘Machines: How They Work’. Thankfully, chief engineer (underground) Subrahmanya Gudge was by our side to decipher their workings for us.
Cuts and challenges
We went straight to the spot where Avni was parked. We were looking at its cutter head on one end. With an external diameter of 6.8 metres and an internal finished diameter of 5.8 metres, it looked like a carnival giant wheel.
“It rotates, creating a pressure by which the machine is thrust forward against the strata. The broken pieces of earth and rock are pushed into the slurry pipes, located just behind the cutter head. This movement helps the machine manoeuvre,” Subrahmanya explained. Concrete slabs are placed along the tunnelling path to prevent the exposed soil from caving in.
A train-like structure trailing the TBM caught our attention. Picture seven open compartments joined back to back. It is the nerve centre of the TBM. Subrahmanya said: “It has an operations room, a control room, an electrical system with a transformer and high voltage current, a computer dashboard to increase and decrease the cutter speed, and a grouting system to manage the slurry pipes. If the soil strata are favourable for cutting, the TBM can move 10-20 mm per minute. If the rocks are hard, it advances only 1-2 mm per minute. Avni is covering four metres every day.”
Imagine a truck with 10,000 kilos of load. Now put 35 such trucks together and there you have, the weight of a single TBM, that too, without backup parts. The beast that the TBM is, you would think it can bulldoze anything in its way. Nope. It can get stuck. “In such cases, we have to make a well as big as the cutter head to remove it. We have had to do this once (at the north ramp near Sampige Road station),” he talked of the tedious process.
Surveys for safety
We then visited the other end of the tunnel, 900 metres away, in Shivajinagar. The tunnel body was made of six interlocked cement arcs. It was circular, grey and curvy, lined by overhead lights and lateral shadows, flanked by poles and pipes. A deafening hum pervaded the air. The closest parallel we could draw was the underground pit of CERN in Switzerland, where the world’s largest particle accelerator sits.
When we marched inside, we realised it was no place to type on the phone and walk idly. We had to watch our step, bend and crawl under machines, take gulps of water as it got stuffy inside. There were water cans with tumblers tied to them, resting areas for workers, intercoms, and a WiFi strong enough to stream songs!
Unlike the M G Road site, we could spot no worker as far as our eyes could see. ‘I walk a lonely road’, we hummed Green Day’s hit until, half an hour later, we saw a man walking towards us, carrying a red toolbox. A crippling thought: What would workers do if the power tripped, or a fire broke out? Tunnel manager Dhananjay Kumar Singh assured us safety measures were in place and added that workers are never allowed to go inside alone. “A minimum of two go in,” he said.
As we got accustomed to the site, Subrahmanya told us about the preparation prior to the boring. “We do soil investigation. If it is homogeneous, that is, either rock or soil, it is easy, and the cutting will be smooth. If it is mixed, the cutting can slow down. If we encounter a boulder, we press cement grouts to soften the rock — it is all trial and error. If there are multiple boulders, the challenges multiply. The dolerite rock, largely found in Bengaluru, is hard to crack at its core,” he continued.
The soil investigation is not fool-proof — it gives a “rough idea”. The Indian Institute of Science in Bengaluru did experiment with ground penetration radar technology but the results didn’t fully reveal the existing strata.
The safety of structures above the ground, Subrahmanya admitted, is a source of anxiety. While the subterrain in Bengaluru is not prone to earthquakes, the metro authorities still cordon off some areas to evacuate people when the underground tunneling work is on. The TBM deploys a soil support system to prevent the roads from caving in.
This is preceded by a survey of buildings on the tunnel route and the aim is to gauge their ability to withstand drilling vibrations. “If we detect a small crack during the study, we fix a monitoring device on the crack. We monitor this daily and even a slight movement of the building is noticed by us,” said Subrahmanya. “Any crack above 10 mm is a cause of worry,” added Dhananjay.
An hour was up, we had strode down from Shivajinagar to M G Road and back. We felt disoriented as we emerged outside — there was daylight when we had descended and now it was dark. Dhananjay’s words came back to us: “Rain, sunshine, day, night, we lose track of the outside world when we work underground.”
We met bosses and few engineers over tea and biscuits before winding up. How do they respond to the criticism over the delays? Our question did not receive a straight answer, but they said the pace was comparable to that of projects in other countries. Delays are caused by “external factors”, some said. We shot them another question. “How many of you use the metro for your daily commute?” None, we learnt. We laughed.
It is a high-skill job
Contrary to common belief, tunnelling is a work of skill, not just hard labour. And so, the engineers, operators, riggers, foremen and supervisors in this line of construction are limited and much sought-after. They often move in groups from one project to another, across cities and countries, and rival companies are out to poach them.
Tunnel manager Dhananjay Kumar Singh has worked on metro projects in Mumbai, Delhi, Singapore, Malaysia, Qatar and Dubai. Almost all 300 employees he is working with are old associates, hailing mainly from Uttar Pradesh, Bihar and Odisha. “You don’t have to train them from scratch, which can take three years,” the 40-year-old said, his words drowning in the cacophony of the machines whirring in the background.
His colleagues joined in the chat and told us why they chose underground careers. One followed in his uncle’s footsteps. One was fascinated with larger-than-life buildings in Saudi Arabia, where his father worked. There were many machine-fanatics, who aspired to work on Multi-service vehicles (MSVs), and we could see that thrill in their eyes.
Tunnelling is also about physical endurance. Working underground on 12-hour shifts is not easy, we were told. Sometimes, the team has to work on machines submerged in water. At other times, they have to deal with muddy discharge. And the summer heat can be unforgiving. Workers with strong lungs and hearts are chosen after a 96-hour training session, which outlines safety protocols and responsibilities. Safety slogans are hung all around the site.
Women uninterrupted
There were all of three female employees at the tunnelling sites but their enthusiasm made up for the poor representation.
A BMRCL employee, Priyankari (21) from West Bengal ensures the TBM moves in the right direction, and this is her “dream job”. Pallavi Madav, also from West Bengal, is the only woman on the L&T team to work on-site, monitoring the navigation system of the TBM. She likens tunnels to “a different world”. Gowri, from Bengaluru, oversees the underground operations of Lavi. “In 2011, I saw the cutter head for real. I had only seen it on Google before,” she recalled. Even today, the process of geological investigation and everyday troubleshooting excites her.
Did you know?
* Bengaluru is one of the toughest terrains to build tunnels, comparable only to Singapore. It has mixed strata, made up of hard rock and soil, which causes constant damage to the cutter head, and slows down digging.
* TBM is the latest boring equipment, comprising seven parts, which are assembled in Chennai and brought to Bengaluru on multi-axle vehicles.
* If a TBM gets stuck, it takes six to nine months to get it moving.
* The BMRCL found canals, and cannons from the 18th century under the City Market station while boring.
* Metro work goes on 24 hrs in two shifts.
* The boring rate depends on the strata type. The standard is 30 metres a day.
* The 22-km Gottigere-Nagawara stretch is estimated to cost Rs 9,000 crore. Of which, the underground metro line from Dairy Circle to Nagawara is 13.89 km and estimated to cost Rs 6,000 crore (only civil works). The Gottigere-Nagawara corridor is slated to complete by 2024-end.
Fascinating names
The tunnel boring machines (TBMs) in Namma Metro’s subterranean fleet bear names inspired by nature and mythology: Avni (earth), Lavi (lion), Urja (strength), Vindhya (mountain), Rudra (name of Lord Shiva), Varada (other name of Lord Ganesha, also the first machine bought), Tunga (river), Vamika (a name of Goddess Parvathi) and Bhadra (river).
Critical parts like the cutter head are installed using cranes while circuits and pipes are manually assembled. It takes 45 days to launch a new machine. TBMs are as expensive as they are heavy, coming in at about Rs 80 crore each. “If we fail to pay the sellers, they can stop the working of the machine remotely,” said Subrahmanya.
Tally to track
Before we got into the tunnel, we were given a metal coupon embossed with a number. A duplicate coupon was hung on a ‘tally board’ at a kiosk and our names against the numbers recorded in a register. In case of fire, the authorities rely on the numbers to identify people trapped inside.
Deccan Herald is on WhatsApp Channels| Join now for Breaking News & Editor's Picks