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Full control of tunnel elements

Immersing the tunnel elements is a complex yet highly controlled process. Contractors will use GPS, echo sounding, ballast tanks and several sets of cables and winches to guide the element into place.

Immersion of a tunnel element is very controlled. When the green light is given for immersion, the tunnel element is towed from the holding area to the exact place it needs to be in the tunnel trench.

The tunnel element is held fast and guided by two advanced immersion pontoons. Their position is controlled by mooring cables that are connected to anchors on the seabed.

Tunnel element readied for immersion

The actual immersion begins by partially filling the tunnel elements’ ballast tanks with water. This makes the element heavy enough to allow it to sink to the bottom under its own weight. An extensive set of cables still suspends the element during the immersion pontoons at sea level.

The manager responsible then commences the lowering operation using the cable winches to slowly lower or ease the element down into the tunnel trench. Using cables, winches and ballast tanks, the increased weight makes it possible to guide the element towards the bottom under full control.

When the tunnel element is safely below the seabed, approximately a metre of concrete is run into the bottom of the tunnel tubes. This concrete ballast makes the tunnel heavy enough to remain firmly on the seabed.

Immersion of a tunnel element
A tunnel element during immersion: cables, winches and advanced measuring equipment guide the element to its exact position.

GPS and echo sounding guide the tunnel element

GPS is used to locate the exact place where the tunnel element is to be lowered. This is accurate to within a few centimetres. When the tunnel element is under water, the contractor uses advanced echo sounding together with a positioning system to guide the element down to its exact position.