Robot spiders help place immersed tubes at Busan Geoje

In deep waters off South Korea, undersea workers got help positioning immersed tube sections from robotic aids.

Jackie Whitelaw
23 April 2015

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Immersed tube tunnels are created by sinking built-offsite rectangular elements into a dredged trench, most commonly at the bottom of rivers, estuaries and harbours. They are often perceived as difficult and expensive to build and remain relatively rare; there are only around 200 in the world.

Despite the general assembly process being well understood, there is no one conventional method of building these structures. The depth of water, soil type and crossing length affect how buildable or cost-effective such a solution may be.

Project Busan-Geoje transport link Location: South Korea 

Situation Linking four offshore islands between Gaduk (Busan) and Geoje 

Tunnel length: 3.2km Tunnel elements: 18 box sections, 180m long, 26.5m wide, 48,000 tonne displacement 

Water depth: Up to 48m

The deeper the water, the more difficult it is to manoeuvre the individual tunnel elements accurately and the more hazardous it becomes for the divers who must oversee the underwater construction. Offshore locations are particularly challenging, as operations can be affected by strong currents and poor visibility.

On the US$1.6bn Busan-Geoje fixed link project, in South Korea, Mott MacDonald provided expert advice to Korean construction supervisor Yooshin Engineering Corporation.

Comprising two cable-stay bridges and an immersed tunnel, the link connects the port city of Busan with shipbuilding and tourism centres on Geoje Island. It includes the country’s first immersed tunnel.

A thick layer of weak marine clay ran along the alignment, which required strengthening for the tunnel foundation. This was achieved by using deep cement mixing and sand compaction piling, pushing the limits of previous applications of these ground improvement techniques offshore.

When it came to installing the tunnel elements, to safeguard construction workers in the deep, offshore location, the contractor – a consortium of Korean companies led by Daewoo Engineering & Construction – used an innovative ‘external positioning system’ solution from Dutch specialist Strukton Immersion Projects. These ‘robot spiders’ greatly reduced the input required from a subsea workforce, improving worker safety and speeding the schedule.

■ Precast tunnel elements were sealed with steel bulkheads, floated and towed 35km from the casting yard to site. They were then lowered by winches to the seabed, guided by steel cables and anchors, and laid in a dredged trench.

■ Lateral tolerance between elements was just 30mm and tolerance on the tunnel alignment was 50mm, which is hard to achieve in deep water or offshore locations using cables alone.

■ In an innovative solution, each descending element was turned into a ‘robot spider’ by fitting it with guides and steel frames with hydraulic jacks, like legs and feet. Once on the seabed, the guides aligned the element with the adjacent section, the feet pushed it sideways to the correct position and the jacks pulled the sections together. Rubber gaskets between the elements created watertight seals.

■ Once complete, the trench could be backfilled and the steel bulkheads removed.