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Tunnels: Support network

In South Korea precast concrete arches are in demand as tunnels are built over roads to create new space for development. Mark Hansford reports.

Creating tunnels using precast concrete arches can be a risky business, as the spectacular June 2005 collapse of the Gerrards Cross railway tunnel showed (NCE 7 July 2005). There, a major disaster was narrowly prevented by the quick reactions of a train driver spotting the collapse up ahead.

While the technique is almost certain to be absolved of all blame - the construction sequencing is suspected to have been the culprit (see box) - the accident certainly cooled enthusiasm for the technique in the UK. But around the world its use continues and South Korea, in particular, is especially keen. Freyssinet, whose Techspan arch was deployed at Gerrards Cross, has seen its system used on 37 projects in the country since 2003.

Rapid installation

It is the speed of installation and the (in theory) minimal disruption to traffic while the tunnel is created that is the key attraction; indeed between 15m and 20m of tunnel a day can be installed “easily”, says Freyssinet director for Korea Jae-Hyung Kim. There is, he says, “minimal traffic interference” during construction.

South Korea is creating new city districts on or near major transport arteries feeding the capital Seoul. Home of electronics giants Samsung and LG and car giants Hyundai and Kia, Seoul has become a major business hub and is the true definition of a megacity - with a population of over 10M it is the largest city proper in the developed world and the Seoul National Capital Area, which includes the surrounding Incheon metropolis and Gyeonggi province, is the world’s second largest metropolitan area with over 25M inhabitants. Almost a quarter of South Koreans live in Seoul, with half living in the metropolitan area along. More than are 275,000 international residents.

“This is the biggest project using Techspan in the world”

Olivier Caplain, Freyssinet Asia

All of which means that traffic jams are chronic despite a bewildering array of eight, 10 and 12 lane highways slicing through the metropolis, one of the world’s busiest metro systems and a high speed rail network linking it to every other city. Cars are cheap and fuel cheaper,so living near a good road close to the capital is highly desirable; and actually living on top of it is perfect.

So creating tunnels over roads to create space for vast residential satellite cities is a booming business. One of the latest is the Homesil Housing Complex, south of Seoul on the route of the under-construction Suwon-KwangMyeong Expressway. The project is pushing precast arch technology into new territory - with a 3km long tunnel largely being created using Freyssinet’s Techspan system.

“This is the biggest project using Techspan we have in the world,” says Freyssinet Asia director Olivier Caplain.
The tunnel is part of a 27.6km long road project, costing £703M in total and with a construction cost of around £395M. Majority funded by private finance, with the South Korean government putting up £84.5M, on time and on budget construction is demanded and it was this desire for speed - and the cost savings offered through the need for less raw materials - that were the deciding factor in going for a precast arch solution over a
more conventional reinforced concrete box.

Cost savings

“Saving in the cost of raw materials was important,” says Kim. Freyssinet calculated that it needed 37,000t of steel reinforcement and 210,000m3 of concrete to build its arch, versus the 50,000t of reinforcement and 270,000m3 of concrete that would be needed to build a conventional box.

Coupled with a much shorter construction period - 36 months against 46 months - the precast arch solution came out at £4.7M cheaper. In total, Freyssinet, which is acting as main contractor for the tunnel itself, is earning £18.9M from the project.

Construction began in January and is now well underway, with work advancing in stages. First a large pad foundation is poured, then a series of precast arch sections are placed, before a crown beam is cast insitu to stitch both sides of the arch together. A waterproofing layer is then added and the arches are carefully backfilled by another contractor.

“The main thing is to take care in the backfilling,” says Caplain,” referencing Gerrards Cross. “You must do both sides at the same time, with the same amount of compaction.” It is also wise to stick to the backfilling specification provided by Freyssinet. Here, fill depths are between 1m and 13m.

“It just requires good quality control on site,” adds Kim.

The road that will pass beneath the arches is a four lane highway, so twin arches are being used, each spanning 12.6m and creating a tunnel with a total span of 27.8m. Maximum height is 10.5m. Of the 3.1km long tunnel, 2.6km is being built using the Techspan system and this will demand 3,666 arch segments in total, each 3m wide and 400mm thick.

The thickness of the arch is significant. “In Korea, we are more stringent on this,” says Kim. “The thickness of the segments is more than would be used in other countries. We have a bigger margin of safety, so this one is 400mm thick - at Gerrards Cross it was just 220mm thick.”

Moulded segments

Freyssinet is producing segments using eight moulds located near the tunnel site. It has completed nearly a third of the 3,666 units, casting an average of seven arch segments per working day.

The arches sit on a 1.2m thick cast insitu pad foundation. “It is a thick slab because there is a high uplift from the high water table,” says Caplain. “We suggested a thinner slab using post-tensioning, but the concessionaire did not want that.”

Upstands, cast at either side and at the centre of the slab for the arches to connect in to, are thicker still at 1.4m. It is robust stuff.

” The cast insitu pad foundation is a thick slab because there is a high uplift from the high water table.

Olivier Caplain, Freyssinet Asia

Despite the desire for speed, land acquisition problems and other snags beyond Freyssinet’s control have delayed progress on the tunnel by “some months,” says Kim. Nevertheless, more than 100m of tunnel had been completed and backfilled when NCE visited in June, while another 150m of base was being prepared for the next advance.

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