Keeping the ocean out of underwater tunnels

Imagine a river or ocean above your head, its immense weight pressing down, trying to find any crack to flood a tunnel. This isn't a disaster movie; it's the daily reality for engineers building underwater tunnels. They don't just dig through the seabed hoping for the best. Instead, they often construct massive sections of the tunnel on land, seal them, and then submerge them. These colossal concrete tubes are then carefully floated into position, sunk, and connected like giant Lego blocks, often in pre-dug trenches on the seafloor.

The real trick is keeping the water out at the connection points. One ingenious method involves using temporary bulkheads at the ends of each tunnel section. Once two sections are joined, engineers pump out the water from the small space between the bulkheads. This creates a vacuum, and the immense external water pressure actually helps to push the sections together, forming a watertight seal. Special rubber gaskets, known as Gina gaskets, are compressed by this pressure, providing the final, crucial seal against the ocean. Once sealed, the bulkheads can be removed, creating a continuous, dry pathway.

Another technique, particularly for soft ground, is the tunnel boring machine (TBM). These gargantuan mechanical moles chew through rock and soil, simultaneously installing concrete segments to line the tunnel behind them. To prevent collapse and water ingress, TBMs use pressurized shields, often filled with slurry or earth, to balance the external pressure. This keeps the ground stable at the cutting face while the tunnel segments are erected within the safety of the shield, creating a protected, dry environment for workers even as they drill under water.