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Royal Engineers hail success at Workington Bridge
In November 2009, record rainfalls were recorded in Cumbria County, England, as rivers, streams, and becks poured into area homes, roads, and businesses. Cockermouth and Keswick were among the worst affected, but many other communities across Cumbria felt the impact of the deluge.
Rarely has a disaster like the Cumbrian floods demonstrated with more clarity the benefit of designing working platforms and bridge abutments to tested performance specifications, as opposed to slavishly following accepted, but often less-efficient, design norms.
One case in point was the installation of a temporary bridge crossing the River Derwent through Workington, Cumbria, after the 2009 flood isolated the two sides of the town. In a project designed and led by the Royal Engineers’ Capt. Caroline Livesey, a replacement bridge was installed in just 13 days.
The installation was based on a temporary working platform and bridge abutments, constructed from Highways Agency (HA) Type 1 sub-base aggregate that was mechanically stabilized by geogrid layers. This provided the load-bearing base carrying all the associated plant for deploying the 52-m span of the bridge, without requiring excavation of the wet topsoil, which saved time and materials.
Craig Roberts, Tensar’s area civil engineer, designed the supporting geogrid structures. The design was based on the improved layer stiffness resulting from the mechanical stabilization of the aggregate due to the interlocking and confinement provided by the geogrids. Full third-party testing programs had shown that load-bearing structures could be achieved with significant savings in aggregate compared to the default design approach based on tensile strength. The bridge deployment was a well-deserved success for the Army engineers.
After 15 months and with repairs to one of the damaged bridges completed, the temporary bridge was removed. Capt. Livesey published a paper on the project in the Institution of Civil Engineers (ICE) publication Civil Engineering*, which offered the opportunity of examining geogrid performance in an actual application by a third party.
Captain Livesey commented (courtesy ICE): “The platform performed very well throughout the life of the bridge. During the construction phase, there was very little deformation observed despite heavy point loads and extensive trafficking from plant.
“Of particular note was the abutment area. During construction, the maximum temporary loading conditions were not reached as the bridge landed on the north bank rollers before the center of gravity had moved forward of the southern abutment rollers. However, no movement was recorded.
“Numerous surveys were then conducted during the life of the bridge and just before its recovery. Only 5mm settlement was noted in the abutments during the 15-month life of the bridge.”
These observations demonstrate the importance of relying on design methodologies derived from empirical data drawn from appropriately comprehensive performance trial programs. By using state-of-the-art design methods based on such data, civil engineers are reducing the costs and enhancing available sustainability benefits.
*(Civil Engineering, 164 May 2011; Pages 81–87; Paper 1000023)