A river is not the problem when you tunnel under one.
The problem is the ground beneath it, which is saturated sand and clay holding the whole weight of the water above.
Cut a hole in that and it does not stay a hole. It flows.
So the machine that does this cannot simply be a drill with a conveyor behind it.
It has to hold the ground still while it removes it.
So how do you excavate something that wants to move?
A pressure vessel that travels forward
The face of the excavation is sealed off behind a steel bulkhead.
The chamber in front of that wall is filled with a clay slurry and pumped up to match the water pressure outside, so the ground never gets an unbalanced push to collapse into.
The cutterhead turns inside that fluid, and the spoil leaves as a slurry through pipes rather than on a belt.
Everything depends on one balance. Too little pressure and the face falls in. Too much and slurry blows up through the riverbed above.
Behind the cutterhead the tunnel is being built at the same time, with precast concrete rings bolted up inside the tail shield and grout injected behind them as the machine shoves off the ring it just placed.
That is why it cannot reverse and cannot pause for long.
And it is why swapping a worn cutting tool means working under pressure, in an airlock at the head, which is the single most dangerous job on the site.
Fifty feet of wheel and a building behind it
The cutting wheel on this one measures about 50 feet across.
The machine behind it runs roughly 486 feet from the cutterhead to the back of the trailing gear, which is longer than the ship that laid most of the world’s early undersea cable.
All of it together weighs on the order of 4,400 tons.
It is not one device but a train: the shield and its rams, the ring erector, the slurry circuit, the grout plant, the power and the control cabins, all dragged along behind the head.
Crews work inside it for the whole drive.
The thing advances by pushing against the concrete it has already built, a few feet at a time, then stopping while the next ring goes up.
Progress is measured in rings, not hours.
What the drive actually recorded
The machine passed the 6.2 mile mark in December of 2025, and at one point covered 2,356 feet in a single month.
It broke into the receiving shaft on July 30 of this year, having cut 7.04 miles in one continuous drive.
That is the longest under river crossing yet made by a machine in this size class without coming out.
The deepest point of the bore sat 292 feet below the riverbed, against water pressure of about 131 pounds per square inch.
The full crossing runs 8.85 miles, and about 1.1 miles remain to the final shaft, with completion expected by the end of the year.
The tunnel carries a high speed line rated at 217 miles an hour, which is the entire reason for the diameter.
What breaking through does not finish
A breakthrough is the machine’s milestone, not the tunnel’s.
What exists now is a lined bore. Track, power, signalling, ventilation, drainage and the fire and evacuation systems all follow, and on a crossing this long the ventilation and escape design is a major piece of engineering in its own right.
The record is also narrower than it sounds. It is a record for continuous distance in a size class, which says nothing about cost, nothing about schedule against the original plan and nothing about how many interventions were needed at the head.
None of those figures are public.
What is verifiable is the geometry, and geometry is what separates this from the record lifts and the largest crawlers that get compared with it.
Why the diameter is the whole argument
A narrower tunnel would have been cheaper and faster to bore.
It would also have forced trains to slow down.
At speed, a train entering a tube pushes a pressure wave ahead of it, and if the tunnel is tight the wave punishes passengers’ ears and slams the structure at the far end.
The fix is cross sectional area, which is why a high speed crossing needs a hole this wide and why a road tunnel of the same length would not.
Trains will cross the river without lifting off the throttle, cutting the run from the island to the city to about seventeen minutes, as the breakthrough report set out.
A shield machine that size exists to buy that, as state coverage also recorded.
The record is a byproduct of not slowing down.
