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Modern Viaduct To Replace Historic Timber Structure

Rendering of new modern, concrete and steel viaduct which is being constructed in Wales, United Kingdom
The new viaduct that is being constructed in Wales, United Kingdom, to accommodate increased rail traffic will contain dual tracks atop a continuous steel and concrete composite deck. The deck will be supported by piers located 36 m apart. © Network Rail

A concrete and steel viaduct is being constructed adjacent to a historic timber structure in Wales, United Kingdom, so that during a 360-hour rail line closure it can be slid into place along the same alignment.

November 13, 2012—A modern concrete and steel viaduct is being constructed alongside a 160-year-old historic timber structure in Wales, United Kingdom, so that it can be slid into place along the current alignment of the South Wales Mainline rail system. Even though the new 220 m long viaduct will ultimately replace the existing structure—which has been designated by Welsh authorities as historic—the rail line closure will last a only 360 hours.

“The most challenging aspect of the design process was achieving a structure that was buildable within the constraints of the environment and the local area, [while] maintaining an operational railway,” said Rosie Majer, a scheme project manager with Network Rail, who wrote in response to written questions posed by Civil Engineering online.

The new structure, known as the Loughor Viaduct, is being built alongside the existing 18-span, 220 m long steel-deck and timber-trestle viaduct, which crosses the Burry Inlet, the tidal estuary for the River Loughor in southwest Wales. Built in 1852, the bridge was listed in 1998 as a Grade II structure by CADW, the Welsh government’s official governing body for the historic built environment. Grade II is the most common category of historic structures in Wales, and indicates that it is a structure of special interest but not one of particular importance or exceptional interest, according to an explanation on the CADW website

 Historic timber viaduct

The historic timber viaduct that the new crossing will replace was
originally built in 1852 and crosses
the Burry inlet, the tidal estuary
for the River Loughor, in southwest Wales. © Network Rail

The busy rail line on which the viaduct is located could not be shut down for any significant length of time for the work to be completed, so Network Rail had to develop a solution that caused minimal disruption. “The challenge has been met through close liaison between the designers, the contractor, Network Rail, and the train operating companies to achieve a proposed structure that sits well within its surroundings,” Majer said.

Because the rail alignment needed to remain the same, the design team decided to use self-elevating platform barges so that piling rigs could be located on either side of the timber structure, according to Majer. The modern replacement will accommodate two rail crossings rather than the single crossing provided currently, so the piles were placed outside of the current viaduct’s footprint in order to accommodate the new, wider deck.

A total of 12 permanent 1.2 m diameter piles were placed around the existing foundations of the structure. Crosshead shells were then slid under the viaduct and concreted into place to create the necessary concrete beams for the new viaduct’s deck.

The new design uses a continuous steel and concrete composite deck supported by piers located 36 m apart, three times the distance of the existing timber trestle piers, according to Network Rail.

New abutments—spanning the width of both rail line crossings and situated behind the existing stone abutments—will also be built, according to Majer.

Temporary piles—a total of six—and crossheads were built alongside the viaduct to hold the new deck in a temporary position parallel to the historic structure. The 8 m wide steel bridge deck is being bolted together and painted on-site, in an excavated area west of the structure. As it is built, the deck is being launched across the estuary. The concrete, ballast, and dual tracks will be placed on the deck while it is on its temporary piers. A temporary support beam installed next to the permanent piers will allow the finished deck to be slid into its final position along the existing rail alignment.

Rendering of 220 m long concrete and steel deck for the new bridge, constructed alongside the 160-year-old timber viaduct

A 220 m long concrete and steel deck for the new bridge is being
constructed alongside the 160-year-old timber viaduct. The historic
structure will be dismantled and the modern deck slid onto its new
piles—and into the existing alignment—within the span of just
360 hours. © Network Rail

The project is operating on a strict timetable and the rail line will be closed between two nearby stations for 360 hours—from March 23 to April 7—at which point the historic viaduct will be demolished via equipment located atop the new deck. Once that work is complete, the team will slide the new deck into place atop the previously placed piles and crossheads.

Network Rail is proposing to build a monument to the replaced structure in the excavated area that is currently being used to construct the new deck, according to Majer. Additionally, four of the existing timber trestles will be kept in place to memorialize the historic structure, while trestles removed from the site will be distributed to a variety of local heritage and public interest groups for display as part of their collections.

The project is expected to cost £20 million (nearly $U.S.32 million) and is part of a broader Welsh government initiative—costing an additional £28 million (nearly $U.S.45 million)—to increase the entire 5.5 mi line from Swansea to Gowerton from one lane to two; the line was reduced from two to one in 1986 to reduce costs, according to Network Rail.

The 40 m long, 8 m wide first section of the new viaduct was launched across the estuary in late October; three more launches are required before the new deck will fully cross the estuary, according to Network Rail.

Chepstow, South Wales-based Mabey Bridge Ltd. is performing the construction work for the new viaduct. The company traces its roots in railway bridge construction in Wales back to 1849, according to its website.



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