Pont Dawn

Pont Dawn is a single span bridge carrying an unclassified carriageway over the river Afon Dulas located in the village of Dawn, Conwy, Wales. The superstructure consists of 5 No. rolled steel joists and transversely spanning in-situ unreinforced concrete. The substructures consist of stone abutments and stonewall wingwalls at each corner of the bridge. The parapets consist of metallic handrails with two horizontal flat rails and circular vertical infill. The original structure was built at an unknown date.

Scope of services

Nuttall Bowser undertook design surveys to inform feasibility, outline design and detailed design with the aim of increasing the structure capacity from the current 3T weight restriction to 40T capability. Nuttall Bowser undertook geometric and condition surveys of the existing assets, topographical survey and ecology surveys. The feasibility report compared options including deck replacement only with retention of existing abutment and footings and whole replacement options including both in-situ and precast concrete portals and box solutions.

Challenges

There were no archive record drawings depicting the shape and founding level of the abutments and foundations. Coring of the abutments was undertaken to establish the new clear span of the selected option which comprised of in-situ reinforced concrete portal situated behind the existing stone abutments.

The lack of archive information meant that an assessment of the existing abutment and footings was not viable beyond a qualitative assessment at the current level of loading only – circa 7.5T. An quantitative assessment would have required substantial investigation with associated disruption and costs which may not have led to an assessed capacity at the required 40T rating.

The design solution needed to at least meet or improve upon the current capacity against scour whilst any construction solution building within the watercourse could have been rejected by Natural Resources Wales – leading to associated adverse programme and cost impact.

The proposed solution improved upon existing weight limitation, improved upon the current scour protection by building behind the existing substructure whilst removing the risks associated with rejection by NRW by avoiding works within the watercourse.

Value added

Nuttall Bowser provided in depth feasibility reporting including: performance, residual asset life, technical risk, constructibility, safety, initial cost, whole life costs, environmental impact, carbon impact and disruption. We provided estimated carbon use, within whole life performance, to inform carbon impact option scoring for each solution considered.

Nuttall Bowser recognised that various options provided differing benefits and risks. The retain abutments solution would be preferred if the substructure could be cost effectively proven as adequate for 40T and scour design criteria, whilst a precast option would provide the best technical solution but provided significant risk of rejection by NRW.

The option evaluation showed clearly that several options were worthy of consideration whilst there was no outright preferred option unless existing risks could be completely mitigated.

The open minded and fair reflection of options enabled swift transition between project options and thus avoiding programme and costs delays. Taking advantage of the in-situ option Nuttall Bowser design enhanced laps to add design flexibility in the event the existing geometry was slightly greater than expected.