The tunnel elements on the Busan-Geoje link are large: an external width of 26.5m incorporates two lanes of traffic in each direction and a central services/escape tunnel. The 3.24km length of the tunnel comprises 18 elements that are 180m long.
Each element is being cast in eight segments in the Anjeong precasting yard – a massive dry dock on the mainland to the east of Geoje Island. This can accomodate five 180m long tunnel elements at a time. Each segment is cast in a continuous 24 hour operation.
There are twin batching plants on site because, as Halcrow technical advisor Don Fraser says, the last thing you want is to be running out of concrete. "We don’t want cracks so each segment is cast as one section," says Fraser. "Also we don’t want early age cracking so thermo sensors are cast in and the heat development is monitored. We don’t use steam curing as such but steam is used to heat the external environment. Concrete generates heat so we need to keep the immediate environment warm."
Keeping the exterior surface of the concrete warm reduces the thermal gradient across the segment, helping to limit the risk of early age thermal cracking. The watertightness of the finished elements is also critical. "Because this will be the [world’s] deepest roadway tunnel of this type, there is a big concern on water tightness," says Fraser. "European tunnels have only one water stop at segment joints, here we’ve doubled up. "The Marmaray tunnel under the Bosphorus is deeper but it is a Japanese style steel box with no segment joints. Even in shallow tunnels in Holland they get leakage in segment joints which rely on one seal using an injectable waterstop. The design philosophy [for this tunnel] erred on the side of caution."
The joints between the segments have two waterstops. The first is an injectable waterstop – a rubber seal stop with the facility to inject epoxy resin which can make its way into small flaws or depressions in the concrete. The second is an omega joint, which is a rubber and nylon membrane that arches over the joint, clamped to the tunnel element either side of the joint, able to accommodate movement caused by temperature changes, shrinkage, creep and earthquakes.
At either end of each element is a temporary steel bulkhead which seals the tunnel section when it is floated out from the casting yard. The steel bulkheads can be removed when the tunnel sections are sealed together on the seabed and can then be reused in further elements. There is a lip around the edge of each element end on which the main seal will be achieved. The lip at one end has a flat steel plate and the other has a 360mm thick rubber Gina gasket fixed onto it.
The Gina gasket forms a watertight seal against the steel end plate of the next element. The elements are temporarily post tensioned to help them withstand the extra forces encountered when they are floated and sunk. When five elements have been cast, the dry dock is flooded, the elements float and are towed to a mooring area ready to be sunk onto the seabed. The dock is dewatered ready for the next five elements.
Preparing the Ground
A 12.5m deep trench for the immersed tube was created in the seabed by dredging through soft clay along most of the tunnel alignment and then blasting in the areas of bedrock at each end. The elements at either end of the tunnel sit on bedrock but the ground conditions for the central sections are poor – around 30m of very soft mud and clay sitting on top of gravel and rock.
Ground improvement in the sea bed was carried out using partial cement deep soil mixing, known as CDM. Here augers were used to mix cement with insitu soil to form soilcement columns. The partial CDM columns finish 3m above the gravel and rock layer. This allows the tunnel some degree of flexibility and movement, but also strengthens the seabed.
Taking the columns all the way down to the rock layer, would have created rigid support points or "hard spots". This could cause problems in an area prone to earthquakes where some flexibility needs to be maintained. Two elements needed extra support and preloaded sand compaction piles were used to improve the ground there. Sand compaction piles work by drawing in water from the surrounding soils make them stiffer.
Before the immersed tube tunnel can be placed in the trench, a gravel bedding layer is laid down to ensure even distribution of loads from the tunnel elements to the ground. Grout bedding layers have been used in previous immersed tube tunnel projects, as they give an accurate and level bed. But the process is time consuming as the grout needs time to set. As a result, Daewoo chose to use a gravel bed for this project as it is quicker to place but special equipment had to be developed to place the gravel with high accuracy under the open sea. "We have developed a jack up barge and our gravel laying techniques over a period of 10 months to give us the tight tolerances needed," says Daewoo immersed tunnel engineering manager Bong-hyun Cho. "It’s the first time a jack up barge has been used [for laying an immersed tube tunnel gravel bed in deep sea conditions]."
The gravel is dispensed via a tremie pipe into the prepared trench. Instead of laying the gravel across the trench, it is laid in a zig zag pattern where a 1.8m wide strip of gravel is laid across the 30m wide trench, the strip then continues along the trench for 1m and then traverses back across the 30m wide trench at 90 degrees before continuing for another 1m and so on.
The 1m gap between the lateral strips ensures that successive strips across the trench do not disturb previously laid ones while allowing space for excess gravel. There is a sensor in the tremmie pipe to ensure that the right amount of gravel is delivered. "Along the 180m length, the element should be flat," says Cho. "If we fail [with the gravel bed machine], the whole project fails."
Getting the details right
The tunnel is made up of 18 largely similar elements – offering opportunities for continuous improvement as the project progresses. "We did the first batch [of four] in 17 months, but the second in eight months," says Halcrow technical advisor Don Fraser. "It’s the benefit of having a production line. In the first batch we were looking at constructability. Now we’ve reached the stage where there’s not much more we can do to improve."
The concrete mix was crucial. Quality was important, as was the need for the concrete to be workable so it could get into shear key details and corners. "We’ve spent lots of time on site trials," said Fraser. "We did full scale trial testing. We wanted to take concrete from lab into reality to see how it performed. We made changes to workability, concrete properties and aggregate coarseness. The mix design is not self compacting, but it’s close, as it is workable, pumpable and user friendly. Transferring this scale into the dry dock wasn’t right the first time, but the mix was continually improved. "It’s on a huge scale but has got to be water tight. We need to find the balance of looking at the big stuff and the finer detail."
An accurate view of the weather ahead is critical to placing tunnel segments on the sea bed. Daewoo has utilised a special weather forecasting system for the construction site. Elements are not positioned during the typhoon season which runs between July and September.
Once the all-clear is given, an element’s journey starts between 8pm or 9pm. It reaches its final location by 5am or 6am the next day. Each element has six water ballast tanks with a combined capacity of around 6,000t. These are used to alter the weight of the element during float out and positioning.
When an element is ready to be taken down to the sea bed, the water ballast is added to increase the weight of the element and sink it. During the sinking process, it is guided by 14 anchors attached to two immersion pontoons.
Once on the sea bed the element is guided into its final position using a specially designed external positioning system (EPS). "The difficulty is that our site is facing open sea," says Cho. "We used a special forecasting system. Weather was the main changing factor. We also developed an EPS to minimise the immersion period and ensure correct alignment."
The EPS consists of a set of 800t vertical capacity jacks which are used to reduce the friction between tunnel bed and tunnel during final positioning. There are also two sets of 200t capacity horizontal jacks. When an element is in position, the nose of the Gina gasket touches the steel frame of its neighbour creating a seal for the chamber between the two bulkheads.
The water in the chamber is pumped out, creating a pressure imbalance between the chamber and the extermal water pressure. This imbalance sucks the new tube element on to its neighbour and compresses the 368mm thick Gina gasket by about 178mm to 190mm. The water ballast is replaced by 8,000t of concrete ballast poured onto the tunnel floor.
Once the tunnel elements are connected, it is a race against time to backfill the tunnel walls to protect the tunnel against the next period of inclement weather. The first layer of locking fill comprises crushed stones which are smaller than 80mm diameter. Once they are in place, the tunnel is protected from waves smaller than 3m. Then a further layer of stones with diameters of less than 300m is placed on top to protect the tunnel from 10m high waves.
Interlocking concrete elements of the type used on sea defences have been placed on top of the tunnel in shallower depths where there are larger breaking waves and there is a risk of ship impact. These have to be carefully installed to ensure that they lock together so they can function correctly. "There are sensors to show these elements have the right position and direction," says Cho. "Each weighs between 36t and 25t and had to be placed by crane. It took three months to get them in the right position."