How did they build the Holland tunnel underwater?

Tunnel workers (affectionately known as 'sandhogs') dug through the mud by pneumatically pushing cylindrical shields through the river bottom. Those shields held back the earth, making way for construction of the thick tunnel walls, built of cast iron rings filled with concrete.



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On the British side, the digging began near Shakespeare Cliff outside of Dover; the French side began near the village of Sangatte. The digging was done by huge tunnel boring machines, known as TBMs, which cut through the chalk, collected the debris, and transported the debris behind it using conveyor belts.

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The mountain material is solid and strong enough not to collapse the hole, or the hole is lined with reinforcement that transfers load from one side to the other. This is now iron/steel rings or reinforced concrete. Tunnel boring machines build the liners as they progress, and push forward using the last ring.

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Collapse of the tunnel isn't what would make it impassible first. The tunnel itself might well last a century or so, but if there's no electric power for as little as a few weeks the tunnel will be closed by water seepage that can't be pumped out.

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Necessary repairs and permanent resiliency improvements are being implemented at the Holland Tunnel as part of the Holland Tunnel Sandy Repairs and Resiliency Improvements project. To accommodate this work, overnight closures will be required, Sunday through Friday, until the end of 2025.

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“The first infrastructures of this type were built at the end of the 19th century, and at present there are nearly 200 underwater tunnels built worldwide. Although some are used for water and electricity supply, most are used to house road or rail connections.

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Norwegian road infrastructure operator Statens Vegvegsen has officially opened world's deepest subsea tunnel, the 14.4km Ryfylke tunnel near Stavanger. The tunnel reaches a maximum depth of 292m below sea level.

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The Laerdal Tunnel in western Norway is the longest road tunnel in the world. It takes about 20 minutes to pass through the tunnel. Brightly colored lights placed every six kilometers help drivers stay alert.

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In terms of underwater tunnels in general, though, the top honor goes to the Channel Tunnel, which connects England and France by rail. Its submerged portion runs for 37.9 kilometers, the longest of any underwater tunnel in the world.

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Proper ventilation for sewers and water tunnels is essential to provide proper airflow, containment of fumes, and dispersion of gases into the atmosphere. Fans are used to exhaust or supply air and are exposed to a multitude of harsh chemicals, temperatures and mild abrasives.

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