Birse Civils' timber project over the River Dee

When the Manchester, Sheffield and Lincoln-shire Railway Company constructed the line over the River Dee near Queensferry, north Wales, between 1887 and 1889, a swing opening mechanism was incorporated to allow vessels to access the industrial areas, and Chester, further upstream.

The 752t, 287ft span was the largest (by 36ft) of its kind in Britain at the time. The total cost of the bridge was £71,000.

Although in its early days of operation it was necessary to open the Hawarden Swing Bridge about 11 times a week, river traffic fell to such a degree that in 1971 the swing mechanism was decommissioned and the bridge closed.

Two of the four masonry support piers sit in the water, with the pier housing the swing mechanism having a timber fender pier protection. Extending each side of the pier with a length equal to that of the moving part of the bridge, the pier protection, a trestled structure, serves to reserve an area in the river to stop waiting vessels berthing in the swing area, which could have had disastrous results.

Listed bridge

The bridge and the pier protection are Grade II listed, and much of the surrounding area is either a Site of Special Scientific Interest (SSSI) or Special Area of Conservation (SAC), so all maintenance and repair works have to follow strict guidelines. Weather, river water, and 'slaters' (a local term for marine wood-boring insects) have all taken their toll on the timbers, and the damage caused by decay and infestation has necessitated considerable timber reinstatement.

Client Network Rail chose Birse Rail for this work, which in turn tendered the installation of the new timbers to specialist marine subcontractor Kaymac.

Bruno Martin, Birse Rail's project manager, explains that the early and continual involvement of key stakeholders - the harbour master, Flintshire County Council (for the Grade 11 listing element), the Countryside Council for Wales (CCW) (for the SSSI aspect) and the Environment Agency - was "a key factor to successful delivery".

Martin admits there was more to the work than first thought. "One of the main issues we had was the condition of the existing structure. When we first came on site it had deteriorated more than we had expected, so we had to rescope the works accordingly." Earlier diving surveys had determined the satisfactory condition of the piles that go through to the river bed.

The River Dee opens to intertidal sand-flats, mud-flats and saltmarsh, with the estuary, feeding Liverpool Bay being one of the most important wetland sites in Wales. "The CCW was involved in how the works could potentially affect the river itself, the SSSI element area is the salt marsh in the estuary, not the actual river," continues Martin.

These restrictions, together with the river having a tide speed of seven to eight knots and being three to four metres between high and low tides called for some unusual assessments and methodology, and some 6am starts.

Access problems

Access from the shallower bankside held its own problems, as Rob Ayre, site manager for Birse Rail, describes. "At the (river) bottom there are lots of rocks and pebbles, which would have damaged the underside of the boat, so most of the time we were restricted to being on the deeper riverside rather than the bankside," he says. "If that was totally smooth and sandy we could have put it down there every time the tide came in, and got even better productivity, but we were restricted with the tides."

The location and variable depth of the water, as well as dictating that the compound was some 200m from the bridge, caused some site access difficulties too. "A 24m steel workboat was chosen for its minimal draught (to avoid scouring the river bottom), and was fitted with a 4.8t Hiab crane and a two-man man-riding basket," says Ayre.

Replacing and repairing the timbers used both shuttering and poured timber concrete grout, and replacing others with hardwood greenheart timber from an approved source.

Nowadays, the river traffic tends to be the harbour master's craft, a survey boat, some small paddle boats and a dredger. The other river user is the DRC (Dee River Craft), the vessel that transports the wings for the Airbus A380 from upstream Broughton's Aerospace factory to Mostyn docks. And passengers on the train are given no indication of the piece of history they've just rolled over.

Project fact file

Official name of project: Hawarden Swing Bridge

Client: Network Rail

Main contractor: Birse Rail

Subcontractor: Kaymac

Value of works: £621,000

Start date: 25 February 2008

Finish date: 11 April 2008