A pair of remote-controlled underwater jacking frames, an exceptionally deep level jack-up barge with pinpoint GPS aggregate placing , and a mini-submarine marine access vehicle, are among the innovative tools being used on the complex and difficult Busan-Geoje fixed link project currently under construction in South Korea.
The developments are a step forward for modern immersed tube tunnel techniques such as those used for the spectacular Oresund crossing between Denmark and Sweden in recent years. They build on that international experience to allow completion of an even more difficult project. Casting of the tunnel elements is also more complex, as undersea conditions are more difficult.
Korean contractor Daewoo Engineering & Construction, leading a seven firm consortium, has also developed new precast placing and positioning techniques for two cable stay bridges that make up the remainder of the 8.2km long crossing, again building on the Storebælt and Oresund experience in Europe. Work on the bridge sections is well underway and currently the whole six year long project is 79% complete, with a target completion for the end of 2010.
The immersed tube is perhaps the most challenging part of the project and has taken much of the attention of the contractor, designers and technical advisers.
After a slow start it is now going well. First of eighteen huge tunnel elements was placed early in early 2008 and after the planned summer months stoppage of immersion operations for the 2009 typhoon season, works recommenced in October with immersion of element E13, the deepest of the project, placed at a depth of 48m. The remaing 5 elements are now being taken from the dry dock to the temporary mooring area in readiness for transportation and immersion during the coming months with immersion completion planned for March 2010.
The current progress has given South Korean contractor Daewoo new confidence that it will complete on time a project which is not only one of the largest schemes in the country but among the world’s biggest schemes with a $1.8bn cost. The dual two lane motorway connection, being built in public-private partnership by concessionaire, GK Fixed Link Corporation, will provide a vital motorway link from the busy Busan city, South Korea’s second largest with a 3.6 million population, to important shipbuilding yards on the large islands off the southern Korean coast.
The connection also fills a crucial gap in the overall road transport network for the whole Korean peninsula, and will boost tourism in Geoje, a highly scenic island whose southern half is a national park. The immersed tube tunnel especially, is unprecedentedly complex, both significantly long at 3.7km total length and currently the deepest concrete roadway yet to be built by this technique, in up to 48m of water. This is more than twice the depth of the Oresund, and raises new challenges.
At such depth, water pressures are greater on tunnel walls, seabed preparation more difficult and immersion operations more extended. To add to difficulties the coastal site is also exposed to open ocean currents and to weather systems that include typhoon storms two or three times in most summers, with waves to match the howling winds which can lift and smash boats, topple and twist harbour cranes and wreck houses.
Such a challenging project is a first in Korea and for Daewoo. Though the company is South Korea’s largest contractor and already well-experienced in international work, in the Middle East particularly, it sees the project as an opportunity to step up further into a new league of technical skill and world level contracting capacity.
“People said to me ‘surely you should walk before you can run’ by doing a smaller immersed tunnel project” says Im-Sik Koo, a civil engineer with 30 years experience and project director for the overall scheme. “But we are proud to be taking this on and overcoming the challenges that it confronts us with.”
“It will give us many opportunities around the world on similar projects and other work,” he says. Eventually, in a longer term future, he believes it will also open up technical possibilities for even grander schemes, including very long undersea links to China and even to Japan.
To get a head start the company studied and absorbed the lessons of existing schemes, most of all the Oresund project but also others in Hong Kong, Japan, and Europe, through seminars and visits. It has also tapped into the skills built up previously, employing the Oresund bridge designer, Denmark’s consultant Cowi, for the overall project design and bringing in some highly experienced Dutch subcontractors for dredging and immersion work. UK consultant Halcrow was employed in JV with TEC of the Netherlands for technical advice and oversight on both the bridge erection and the tunnel elements. Halcrow and TEC also worked on the Oresund project where particularly large scale pre-cast works were used.
Precast is even more central to the Busan project, for both bridge piers, pylon bases and decks, and obviously for the tunnel, where the casting, and then placing, of the elements is the main feature.
A total of eighteen tunnel elements 180m long are being used for the undersea section, certainly the longest and, at 48,000t each, among the largest units made anywhere. Each is 9.97m high and most are 26.46m wide, providing space for a two lane road in either direction separated by a central service passageway. Two of the elements are 28.40m wide; the extra width provides for an additional truck climbing lane as road gradient increases to join the first of the two bridges.
The tunnel is completed at either end with 170m long portal and approach sections built by cut and cover, one at the start point on the small island of Gaduk, just offshore from Busan city, and one on a tiny rocky outcrop island conveniently located halfway to the crossing’s destination, Geoje. Bridge connections complete the overall fixed link via one smaller island; there are also short sections of bored rock tunnel on these two small intermediate islands.
All three channels of the crossing will all be used for ship passage, but the tunnel is needed to allow the largest vessels and most frequent use. Plenty of ships need access, firstly from the constant output of the big shipbuilding yards in the area, with mid-size and large vessels in and out to open sea all day for trials and deliveries. There are also movements in and out to Busan, Korea’s second largest city which is currently expanding its container facilities with an ongoing Busan New Port development.
An immersed tube was selected because of the channel’s depth. The same steeply sloping hillsides which characterise Korea’s dramatic landscape plunge on at the coast to create clear deep water channels between the flooded island peaks. That is good for shipbuilding, and for local fish farming industries, but means bored tunnels would have to go very deep, which would require very long approaches.
The tunnel elements will sit in the seabed in an excavated trench formed by Dutch dredging giant Van Oord, using one of its large trailing suction hopper dredging vessels. That allows much shallower construction than a rock tunnel.
While the trench was being prepared however the focus of work has been on the casting and fitting out the tunnel elements. These are made in a dry dock facility created for the project at Anjeong, nearly 40 km across the wide bay formed by Geoje island and the curving mainland coast. Alongside ship yards and fuel landing facilities, an L shaped dock was excavated, 475m long and 11.5m deep, capable of holding four of the big tunnel elements at one time initially. This will then be further enlarged to allow five units to be made at once, to speed up the schedule once the second batch had been made.
Getting the casting work right is the key to the immersed tunnel integrity says Hyun Chil Lim, Daewoo’s General Manager responsible for all pre-casting works.
Don Fraser, Halcrow advisor to the project particularly for concreting and pre-casting works explains further. “This is massive civil engineering with large amounts of concrete to be poured, some 18,000m3 in each element, with all the problems that can cause such as heat cracking and shrinkage. But it is also the detail that matters very much; the little things such as joints, waterproofing of form tie holes, accuracy of embedded anchor plates for both permanent and temporary works.”
All these are critical because the tunnel design relies on the concrete structure for its watertightness. An alternative of cladding elements in steel, used frequently in Japan was ruled out by the design philosophy.
Using concrete means that the elements can be built up in separate segments, eight along the length of each element. That makes concreting work easier as it can be done one eighth at a time. But it is primarily intended to give the tunnel elements some flexibility along their length.
Each segment is match cast tight against its neighbours but is physically separate, which means that it can move relative to the others. Once the element is in position on the seabed it can behave like a eight segment “worm” to cope with possible seismic vibrations shifting the seabed, for shrinkage of the concrete, settlement and other effects, which would otherwise crack the units.
Such adjustment is small and to prevent movements going too far the segments are locked together with shear keys, restraining such flexibility to a tiny amount.
But there could be movement enough to let in water, and to prevent this happening, substantial and robust water stops are required at the joints between the segments, embedded in the concrete. Because of the depth of the tunnel there will be a double layer of seals, for the first time on any immersed tunnel. An outer “standard” rubber water stop with epoxy injection will be complemented on the inner face of the tunnel by a steel plate and rubber “Omega” seal.
Fitting these and ensuring their integrity is one of the key tasks during the overall concreting works. In turn the double seal affects the complexity of the concreting, most of all because the shear keys cannot extend the full wall width of 1.33m but must be narrower, in the centre.
However, first of all comes the build-up for the big concrete pour, some 2200m3 for a single segment. The segments, unusually, are formed in one pass, a decision which reduces the potential for seawater leakage problems from concrete construction joints, which could generate restraint cracking.
“No matter how carefully it is done, if you pour one level of concrete for the base, and then the walls later perhaps, the concrete in the first section will already be behaving differently, starting to shrink as it cools and cures. That will lead to cracking so to avoid this either a carefully controlled cast in pipe cooling system is used or the full cross section is done as a single casting as we are doing” explains Fraser.
Segment formwork, and the heavy reinforcement within it, is therefore built up over some weeks for an eventual 24 hour pouring operation. First a plywood base is laid down on a painstakingly grooved concrete foundation, which will allow water flow under the unit when the drydock is flooded. Working platforms go around the sides and base and half the wall reinforcement is setup. Inner formwork units, devised by specialist company Doka, are moved into position on a traveller frame and the rebar finished, with ties through to the outer forms at the top.
During this work comes much of the important detail that Fraser is politely insistent the crews get precisely right. Exact positioning of various cast-in anchor points is required, for both permanent and temporary works for example. Most important perhaps are cast in anchors for two steel beams at the top of the primary end segment, which will be used during immersion positioning; for fixing temporary water ballast tanks used during float out; for detachable brackets which support the steel bulkheads which seal the units during the floatout; and most critically for the steel mounting for the Gina gasket at one end of the unit, and the facing plate at the other end of the element against which the next element’s Gina will press.
“The Gina is a compressible seal for the immersion joint between two elements,” explains Bo Hyun Yang, Daewoo general manager in charge of immersion work. Famously named after the rounded figure of Italian actress Gina Lollobrigida, the gasket seal must touch precisely around the perimeter and onto the facing plate during immersion to allow dewatering. It must be set with millimetre accuracy.
Pouring of the concrete is also critical. A special mix for the elements was developed at Daewoo’s large construction research campus outside Seoul and then tested and modified at site. “We finally devised a mix that, while not totally self-compacting has very good flow properties which takes some of burden off the vibrator guys if they miss a little” says Fraser.
Getting concrete compaction right was a major struggle during the casting of the first batch of four tunnel elements. “There was a fairly long learning curve” says Fraser. Some surface defects were apparent in trials and the first units though “nothing that was not repairable,” he says.
Concrete quality is now “second to none” he says. Work crews have learned how to get inside the walls while the concrete is rising to vibrate it well, and a high strength grouting top up system has been developed to ensure the difficult male shear key projections are completely concreted, in between the waterstops.
A well choreographed pouring operation has developed for fast work on segments concluding with a plastic covering for the concrete faces inside which steam is injected. “It is not steam curing as such but temperature balancing to make sure the gradient between high internal curing temperature, up to 65°C and ambient which can be very cold, is never more than 20°C” explains Fraser. Heat detectors in the concrete monitor what happens.
This measure ensures there is no thermal cracking, which could cause another possible leakage path.
Segments have been cast in an alternating pattern, which eases the difficulties and allows easier fixing of joints later. Once cast, the rubber for the Omega seals has to is installed inside and other finishing done. The Gina gasket must be installed too, another precise operation.
The second batch of casting has seen elements produced in just eight months against 17 for the first difficult batch. The current operation is going better than that, with exemplary quality. With the yard enlarged to allow five elements to be made at a time and with hard work, the casting is on track for the placing operations.
Several units are currently available for placing, moored to the seabed a little way out from the yard where they are taken by tugs when the yard is flooded and the finished units floated up and carefully inched through the dock gate. From here they are eventually taken on an overnight journey, 37km to the site by subcontractor Mergor of the Netherlands.
But before immersion the new seabed trench floor has been under preparation. That involves, firstly, ground improvement works on the marine clay of the seabed, from which the trench was excavated. A 30m deep layer lies over gravels and then hard rock granodorite and andesites.
Because of differential settlement risks the main section of the tunnel has been treated with cement deep mixing (CDM), injecting and mixing cement into the soft clay ground in a pattern of columns. “A ‘floating layer’ is created” says Bong-Hyun Cho, the Daewoo deputy general manager for planning and engineering.
For the end elements, which rise from the seabed to land, there was rock excavation required for the foundation. For two rising elements on the transition island of Jungju there were sand columns installed in the clay, to release pore water pressure and accelerate settlement. Heavy rock surcharge was left in place for 18 months, eventually forcing nearly 3m of settlement.
The additional measures were needed because these rising sections of the tunnel sit partially exposed as the tunnel rises from the seabed and require extra rock protection against potential ship collision. The protection is a substantial extra load on the foundation.
Once the ground was prepared a gravel layer is needed in the trench base on which the elements will sit, a system similar to that used on Oresund. It is an alternative to grouting the elements which adds significantly to the immersion time.
The bed needs to be highly accurately placed, a significant challenge in depths of water up to 48m. Daewoo worked with subcontractor Eunsung Foundation to develop a jack-up barge capable of working in such water. A key element is a large diameter vertical tremmie tube to place a 1.6m wide gravel “trail” on the seabed which is built up into a maze like pattern, forming the foundation. The tremmie is precisely positioned and also has a fine control hopper release at the end, adjustable by small hydraulic jacks. Positioning is controlled by total station sightings on the tremmie for the nearshore coastal elements, and by a specially developed high accuracy GPS system for the rest.
“When we had run successful trials for the barge I was able to sleep again at night” says Cho. “The foundation bed is critical for the success of the placements.” Once everything is prepared, the elements can be positioned. This is done only during the winter season when weather conditions are calm. Even then detailed weather observations from locally installed wave height buoys and a specialist weather service based in Ireland, are needed to ensure a five day calm window for the operation.
“We need less than 0.4m wave height to move and immerse the elements,” says Cho.
Once the conditions are right, two special frames are fitted to the next element close to the back and the front. This is the so-called EPS or external positioning system, specially developed for the project. Each has vertical and horizontal jacks on its “feet” and is stressed by cable to the element.
Over these go two pontoons which are connected by winch cables to the elements and EPS frames. The whole assembly is then tugged 37km to site overnight, ready to go with immersion in the morning.
A series of lateral and longitudinal anchor cables to the element and its immersion pontoons allow fine position control of the element as it is made ready. The entire “sinking” is then controlled by pumping ballast water in six 1000m3 temporary tanks fitted inside the element, to alter the buoyancy.
“We add just 2% to its overall weight initially” says Cho. Nine winches on the pontoons then control the slow descent of the element to sit close to its final position. Divers hook up 150t jacks which pull the new unit close. Using guide beams on the element’s top and corresponding guide receivers on the existing unit, helps position it.
It is here that the EPS system comes into place too, with its 200t horizontal jacks able to move the unit end for fine tuning. Its vertical jacks also lift the unit slightly to reduce friction on the seabed.
“We use an EPS here because of the depth and the possibility of sea current movement From the open ocean” explains Cho.
Once the Gina is lined up within plus or minus 10mm tolerances, and touches the end plate of the existing section, a new chamber is formed by the now joined spaces in front of the elements’ bulkheads. This can be dewatered, and as it is pumped out, reducing internal pressure, the sea pressure forces the new element against the old, compressing the Gina by almost 50%.
To ensure total water tightness an inner Omega seal is also fitted across this joint, once the unit is firmly in position.
More ballast water settles the element tight and then a “locking fill” can be deposited around it filling about half the trench height. Above this there is then a backfill and over that goes a rock fill layer which protects the now buried tunnel against impact should a ship sink over the tunnel line, for example.
These fills add to the load on the tunnel but to ensure that it is weighted down an additional 3000m3 of concrete is poured in the tunnel floor to build up the road bed, adding more than 7000t load to the element. Ballast tanks are then removed, the bulkhead dismantled for reuse and tunnel fit out work proceeds.
Precast units are used extensively on the project The immersed tube section is 3.7km of the 8.2km crossing Bridge connections and rock bores are needed to complete the link Each segment is poured in a single casting using 2200m3 Daewoo worked with Ensung Foundation to develop the jack-up barge
