The 41 men stuck in the tunnel

The 41 men stuck in the tunnel


For now, though, they are being supplied food, water, medicines—and air—through small pipes. In parallel, efforts are being made to drill through to where they are and install a pipe through which they can be evacuated. None of this is easy, especially in an area already known to be geologically sensitive to such operations. Which, in turn, may be why the collapse happened in the first place.

I’d like to examine some of the numbers involved in this rescue effort. Caveat: These are all taken from news reports. I have no first-hand knowledge of the situation.

The government of India approved the construction of the Silkyara Bend-Barkot tunnel in February 2018, estimating that it would take four years to complete. If and when finished, the tunnel will be 4.5 km long and about 8.5m high. It is meant for road traffic in both directions, reducing the travel distance between Yamunotri and Dharasu by about 20 km.

From the Barkot end, some 1.7 km of the tunnel has been completed. From the Silkyara end, about 2.3 km. This means that what was left to be drilled through, as of 12 November, was about half a km in the middle. The collapse happened about 250m from the Silkyara end. It has left debris blocking a 60m stretch of the tunnel. That’s what keeps the men trapped.

Right there is the immediate problem with getting the 41 men out, that 60m wide pile of debris. The plan is to drill through it and insert a pipe. Actually there already are two pipes through the debris. Only, one is just a few centimetres wide. It was being used to send dry fruits and water to the men. I assume these are propelled through the pipe by powerful bursts of air. This was the men’s lifeline for several days, keeping them alive. On Monday, a second, six-inch pipe was forced through to where the men are trapped. This allowed for a camera to be inserted, so there were video clips of conversations with them. This larger pipe also allows them to be supplied with hot food. Plastic bottles filled with khichdi made their way to the men.

But of course, rescuing the men means pushing a much larger pipe through the debris—one that’s at least 1 m in diameter, so that the men can make their way out through it. This means drilling a hole through the debris that’s at least that wide. How is this to be done?

In essence, this is no different from drilling holes at your home—say, for inserting a nail into the wall so you can hang up a favourite family photograph. You use a drill and a bit of the appropriate size. The drill rotates the bit at high speed—typically, 2,000-3,000 rotations per minute (rpm). As you press the drill against the wall, the sharp edges of the rotating bit bite into the brick (or wood, or cement, whatever). The grooves on the bit fling the rubble that’s generated backwards and out.

But there in Uttarakhand, it’s no nail in the wall. The rescuers brought in a tunnel boring machine (TBM) to drill through the debris. A typical such machine has a cutting head rotated by a motor—in this case, the head is about 1m in diameter. It has certain fitted elements that can cut through rock. The machine also has jacks that allow it to exert a forward force, pushing the head against the rock just like you would push the drill against the wall. Typically, the jacks are supported by the already-drilled section of the tunnel the TBM is drilling. In effect, the TBM braces itself against the tunnel walls and that allows the cutter head to move forward. Behind the head is a long solid helix that sends the rubble generated by the drilling back and out.

In many ways, about the same as a hand-held drill. Here’s a significant difference, though. The cutter head does not rotate at a few thousand rpm. Remember the size of the machine and imagine the power it will need to rotate at a few thousand, or even a few hundred rpm. Or consider this: If a 1m diameter cutter head rotates at 2,000rpm, that would mean its edge whirls along at over 360kmph, faster than Japan’s bullet trains. Except that it would not whirl very long. Whatever components are near the edge of a typical cutter head will experience an outward centrifugal force of something like 3 million Newtons. Never mind what that means; suffice it to say that if subjected to that kind of force, the cutter will explode and cause untold carnage all around the machine.

So instead, the typical cutter head rotates sedately at between 1 and 10 rpm. If you’ve ever used a drill, that may seem incredibly slow. But it is enough to bore through the rock. The question that matters to those 41 men, then, is how fast does this boring progress?

Naturally, that depends on the kind of rock the TBM is up against. Typically, these machines can bore through about 15-20m per day, and that’s what we can expect in the Uttarakhand tunnel. From news reports again, it looks like the first TBM was installed at the Silkyara end on 16 November and had drilled about 24m by the next day. Given the 60m to be bored through, this was encouraging progress. But then the cutting head hit some kind of obstacle, and a loud “cracking” sound was heard. Fearing additional collapses, the rescuers halted the drilling.

Meanwhile, the rescue teams are also exploring other drilling options. One is from the Barkot end of the tunnel. That will mean boring through 500m of rock. The other is from above—that is, from the slope of the hill above the tunnel. Two sites for this vertical drilling have been identified. Wherever they are located, the contours of the hill suggest it will also mean boring through a few hundred metres of rock. In all three cases, this is virgin rock, not the collapsed debris at the Silkyara end. Also, the drilling from the Silkyara end did resume, and made good progress till Thursday morning, when only about 12-15 m of the debris was left. But another obstacle stalled progress.

I don’t know which of these four efforts—and there may be others—will reach the 41 men quickest. All I can say as I write this is, again: I hope they are out as you read this.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun.

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Updated: 23 Nov 2023, 11:12 PM IST


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