Case study: Bigsweir Bridge repainting

Bigsweir bridge is a Grade II listed cast iron bridge which carries the A466 over the river Wye between Monmouth and Chepstow in the Forest of Dean.

Bigsweir Bridge following repairs. See below for high resolution image gallery.

The route is particularly popular with tourist traffic to the Forest of Dean and is a landmark structure in the local area. Following routine bridge inspections, the structure’s paint system was found to be in a very poor condition.

Gloucestershire Highways (GH) (a partnership between Atkins and Gloucestershire County Council) were commissioned to undertake a full repainting scheme to the structure and repair any significant structural defects.

Minimising disruption

The bridge had an existing weight restriction of 17 tonnes, and permanent traffic lights due to its narrow single lane width. As the bridge carries a main route, it was necessary to keep the bridge open throughout the duration of the works.

To enable the bridge to safely support the additional weight of the temporary works while remaining open, a detailed review of the previous assessment was undertaken and the following restrictions were determined:

A temporary reduced weight restriction of 7.5 tonnes was imposed on the structure – this required a diversion route to be put in place for overweight vehicles. Due to repeated violations of this restriction at the start of the scheme, manned traffic lights and cameras were employed to stop overweight vehicles using the structure. Additional analysis and controls were put in place to allow limited continued access by emergency vehicles and gritting lorries
The painting works needed to be carried out in five distinct phases to limit the loading imposed on the structure from the temporary works. Carrying out the works in this way had significant consequences both in terms of cost and programme
Work was undertaken outside of summer months, to reduce disruption to the large number of coaches using the bridge over the main tourist season. This presented an increased risk of low temperatures and high river levels during winter flooding, so additional control measures were put in place to reduce the impact of these issues on the works

Discovering defects

A roped access inspection was carried out to identify where there were visible structural defects which required repair, and confirm the details of the existing paint system.

This highlighted a number of cracks to the existing cast iron members, which led to discussions with specialist contractors in order to determine the optimum method of repair, while meeting the requirements of the Conservation Officer.

A number of environmental surveys were undertaken to verify the presence of any protected species in the vicinity of the works, and mitigation measures to minimise the disturbance to bats. Approval for the works was obtained from Environment Agency, Natural England, Wye Valley AONB, and consultation completed with local stakeholders.

All aspects of the scheme design, inspections, consents, consultation, procurement and supervision were completed by the Gloucestershire Highways team.

A specialist painting contractor, Jack Tighe Ltd, was awarded the subcontract by Atkins to complete the works, with Atkins acting as the principal contractor for the scheme.

Repainting to reopening

The structure was repainted using a four coat system (1 primer, 2 MIO Epoxy undercoats, and one Polyurethane finish coat). The paint colours were chosen to match the existing bridge colours. Before this could be applied the existing paint system along with any corrosion was grit blasted off.

As the existing paint system was lead based and the works were over a SSSI watercourse the blasting and painting works needed to be fully encapsulated, to prevent contamination of the river below. Extensive preparation of the cast iron surfaces and joints was also required prior to repainting.

Bigsweir Bridge reopened in June 2011 following the works which will greatly extend the life of the 180-year-old structure.