A French subcontractor is using a patented precast technique to build one of the world's longest precast-concrete arch-tunnel sections at the site of a residential complex along South Korea's new Suwon-Kwangmyeong Expressway. Paris-based Freyssinet Group claims its TechSpan technique is resulting in significant cost and time saving but acknowledges careful backfilling is key to maintaining safety during construction.
The twin-bore precast tunnel, some 60 kilometers south of Seoul, forms a 2.6-km-long section of the roughly 26-km-long expressway, which a large consortium, led by Korea Development Corp., is constructing under a build-operate-transfer contract.
Freyssinet’s $30-million subcontract entails building the tunnel in an open trench. The technique, patented by its subsidiary Tierra Armada S.A., involves assembling precast-concrete tunnel arches in halves and backfilling to finished ground level. The three-pin arches support the earth above in compression. Cast in three-meter lengths, the 40-centimeter-thick arches rise nearly eight meters from the tunnel's 1.2-m-tick reinforced-concrete base slab and span 12.6 m each. Once placed, the opposing arch halves abut at the crown, propping each other in position.
Freyssinet's local chief executive officer, Ji-Yeong Kim, claims the precast system on this project costs 20% less than a more conventional rectangular cast-in-place system and uses 16% less rebar. Further, by cutting 10 months off the schedule and reducing construction time to three years, the technique contributed to an overall cost savings of about $7 million, he adds.
Starting this January, the subcontractor is producing segments using eight molds located near the tunnel site. Casting seven arch segments per working day on average, Freyssinet has completed nearly a third of the 3,666 units it needs for the job.
Once Freyssinet has placed the segments, another contractor is responsible for backfilling up to 13 m above the crown to ground level. However, to ensure the arches remain stable in their temporary state, Freyssinet engineers have produced a back-filing specification that covers the quality and compaction characteristics of the material to be used and the maximum depths of progressive backfill layers, says Kim.
"The main thing is to take care during backfilling," adds Olivier Caplan, Freyssinet's Asia operational manager. Unevenly backfilling such tunnels can introduce unsymmetrical loads on the three-pin arches, which could lead to an accident like the one that felled a TechSpan tunnel in Gerard's Cross, England, six years ago (ENR 7/11/05 p. 16). In that case, while being backfilled by a local contractor, a 30-m-long section of the 320-m precast arch tunnel collapsed but caused no injuries.
At the time, backfilling was thought to have contributed to the failure. With the official investigation by the Health & Safety Executive (HSE) still pending, the cause remains uncertain. The tunnel finally was completed by another contractor, using reinforced concrete cast on the original precast segments as formwork. The investigators' report is due soon, says an HSE spokesman.
On the Korean project, land acquisition problems and other snags beyond Freyssinet's control have delayed progress on the tunnel by "some months," says Kim. Nevertheless, more than 100 m of tunnel has been completed and backfilled, while another 150 m of base is being prepared for the next advance.
