How India's Bullet Train Tunnels Under the Sea in Mumbai

0:00 · A railway under the sea

India is digging a railway under the sea. The hardest part has only just begun.

This is the Mumbai to Ahmedabad bullet train. Five hundred and eight kilometres, built with Japanese Shinkansen technology, almost all of it on bridges. Except the last twenty-one kilometres. Those run underground. From a station deep below Bandra-Kurla Complex, under the city, the Mithi River and Thane Creek, to Shilphata.

About seven of those kilometres are under the creek. The builder, the National High Speed Rail Corporation, calls it India's first undersea rail tunnel.

How do you dig a hole that big, under a city and under the sea? Let's draw it.

0:50 · Why not a bridge?

Why not a bridge? At BKC, there's no room. And the creek is a protected flamingo sanctuary, lined with mangroves. A tunnel passes underneath without touching them.

1:02 · The section: shafts and depths

Here's the section. One: BKC station, thirty-two and a half metres down. Two: a shaft at Vikhroli, fifty-six metres deep. A twenty-storey building, upside down. Three: a shaft at Sawli, near Ghansoli, thirty-nine metres. Four: the portal at Shilphata.

Most of the tunnel runs twenty-five to fifty-seven metres down. Under the Parsik hill, it's a hundred and fourteen.

And it's one tube. Both tracks side by side, in a single bore thirteen point one metres across.

1:38 · Drill and blast, or bore

There are two ways to dig it.

The eastern five kilometres, from Ghansoli under the Parsik hill, used the New Austrian Tunnelling Method. Drill, blast, spray concrete on the fresh rock, measure how it moves, adjust. It broke through in September twenty twenty-five.

The other sixteen kilometres run under buildings, a river and the creek. No blasting there. So, two tunnel boring machines.

2:06 · Inside the tunnel boring machine

Let's take one apart.

It's about ninety-six metres long and weighs over three thousand tonnes. Both were made by the German company Herrenknecht.

One: the cutterhead. Thirteen point six metres across, as tall as a four-storey building. It turns at up to four revolutions a minute. Eighty-four steel discs crack the ground. A hundred and twenty-four scrapers clear it.

Two: a chamber of pressurised slurry.

Three: the shield, a steel tube that holds the ground up.

Four: the erector, an arm that builds the tunnel wall.

Five: four gantries trailing behind. Pumps, power, grout, and a refuge chamber for the crew.

2:52 · The real enemy: water pressure

Under a creek, the enemy isn't rock. It's water.

Take forty metres down, as an example. Water pushes on every square metre of the face with about forty tonnes. The face is about a hundred and forty-five square metres. That's nearly six thousand tonnes. Roughly twice the weight of the machine.

If nothing pushes back, the face collapses and the water pours in.

3:19 · How the Mixshield holds the face

So these are Mixshield machines. The chamber behind the cutterhead holds bentonite slurry, a heavy clay-and-water mix, under pressure. It pushes back, and the face holds.

The slurry also carries the cut rock out through pipes. A crusher breaks big stones first. On the surface, a plant cleans the slurry and sends it back down.

3:41 · Rings, seals and sensors

Behind the cutting, the machine builds the tunnel. Every two metres, a ring. Nine curved concrete segments, then one key that locks them. Each half a metre thick. A full ring weighs about a hundred tonnes.

Hydraulic rams push the machine forward off the last ring. And it can keep cutting while the next ring goes in.

Sixteen kilometres means seven thousand seven hundred rings. Seventy-seven thousand segments, all cast at Mahape.

Every joint is sealed twice. Two rubber gaskets, plus a seal that swells when wet.

On the surface, sensors watch whether buildings move. Inside, they track methane, oxygen, carbon monoxide and carbon dioxide.

4:30 · The honest part: which bit is under the sea?

Now, the honest part.

One: which part is really under the sea? The five kilometres already finished run on the mainland, under the hill. The creek crossing is the second machine's ten-kilometre drive, from Sawli to Vikhroli. It only started on the eighteenth of July, twenty twenty-six. Since then, the builder hasn't said how far either machine has gone.

4:54 · What "undersea" means

Two: what "undersea" means. Thane Creek is a tidal zone. Open water in the middle, mudflats and mangroves on each side. Early planning reports said less than two kilometres runs under the open channel.

5:09 · The machines were late

Three: the machines were late. They were built at Herrenknecht's factory in China. Reports say they sat at a Chinese port for months, until India raised it through diplomatic channels.

5:21 · The pace maths

Four: the pace. Officials have told reporters each machine should do about three hundred metres a month. At that rate, the first machine reaches BKC in early twenty twenty-eight. The undersea drive finishes in early twenty twenty-nine. Officials have also quoted March twenty twenty-eight. That needs close to five hundred metres a month.

5:44 · The money

Five: the money. The project was estimated at one lakh, eight thousand crore rupees. About twelve billion dollars. In January, the Railway Board chairman put the revised figure near one lakh, ninety-eight thousand crore. About twenty-two billion. The ministry blames taxes, land, inflation, and costs the first estimate left out. Japan's aid agency, JICA, lends most of the original amount, at zero point one percent over fifty years. Reports say the extra will come from India's own budget.

6:21 · The trains

Six: the trains. The line is designed for three hundred and fifty kilometres an hour, to run at three hundred and twenty. But Japan's next Shinkansen, the E10, isn't expected until the early twenty-thirties. So the first section, Surat to Vapi, is due in August twenty twenty-seven. With an Indian-built train, designed for two hundred and eighty. Mumbai opens last.

6:47 · When will it open? Your guess

In twenty twenty-five, the railway minister said Thane would come in twenty twenty-eight, and BKC the year after. In August, reports said the full corridor could slip to twenty thirty.

What's certain is the engineering. Two machines, each pushing a four-storey cutting wheel through rock, mud and seawater. One ring at a time. Seven thousand seven hundred times.

So here's our question. The minister said twenty twenty-nine for BKC. The machines' own pace says that's tight. What's your guess? Tell us in the comments.