Arup’s designers, engineers and technical assurance experts played a major part in completing the Welsh Government’s £1.4bn commitment to transform the A465 Heads of the Valleys Road. This key transport route links former mining valleys to south and west Wales, the Celtic Sea ports, and the English Midlands.

In one of the UK’s largest highways projects, the A465 has been upgraded to a dual carriageway, better connecting residents and businesses across South Wales as well as supporting jobs and economic regeneration.

We were the lead designer for Section 3, tackling engineering challenges to create new sections of road and upgrading the structures and junctions along the route. For Sections 5 and 6, our technical experts were part of a joint venture that acted as the authority’s site representative on behalf of Transport for Wales. 

We checked that construction work met the standards set out in the contract, offering expert advice to ensure that the road would withstand the expected traffic, as well as verifying that it met safety standards and followed industry best practice. 

The final section of the upgrade programme opened in May 2025, bringing safer, faster travel, and more reliable journey times for an estimated 45,000 vehicles each day. 

Integrated expertise for challenging conditions

Originally built in the 1960s as a three-lane single carriageway, the A465 had become notorious for congestion and safety issues. After the Welsh Government commissioned a comprehensive upgrade programme to create a dual carriageway along the 40km route, Arup’s roads and streets experts worked on two key sections of the project.

As lead designer for Section 3, stretching 7.8km from Brynmawr to Tredegar, our integrated team worked with the contractor to design bridges, retaining walls, underpasses and new junctions. Technically challenging and highly varied ground conditions on the site added complexity. We encountered subsidence in areas of historic mine workings and the area’s high rainfall meant we had to manage run-off carefully. 

A diagram of a house.

BIM model visualisation of Cemetery Retaining Wall. We used BIM to communicate the design, construction process and the impact of the new road – helping the scheme progress through public local inquiry.

Throughout the project, we looked for ways to reduce costs, programme timescales and whole-life carbon. This included making extensive use of BIM (building information modelling), which was then in its infancy, to streamline the design process – and identifying and mitigating clashes on the construction site. We also used 3D modelling to communicate the design, the construction process and the impact of the new road. This helped the scheme progress through the public local inquiry with very few objections.

We replaced a planned viaduct across the Carno Valley with an embankment, which reduced the risk to construction workers, sped up the programme and cut costs. This single design change lowered emissions from constructing Section 3 by 10%. To minimise in-use carbon emissions, the new road has shallower ascents and descents and grade-separated junctions that let vehicles maintain a constant speed.

Section 3 was completed on time and under budget, and is an exemplar of collaborative, sustainable infrastructure delivery.

A group of men standing next to a large pipe.

Carno Embankment during construction. We replaced a planned viaduct with an embankment, the choice reduced the risk to construction workers, sped up the programme and cut costs.

A large white building with a tunnel.

Carno Embankment post construction. The choice to build an embankment rather than a viaduct lowered carbon emissions from constructing Section 3 by 10%.

Using our technical expertise for asset life extension

Sections 5 and 6 between Dowlais Top and Hirwaun formed the final and most complex phase of the A465 project. The Welsh Government developed the mutual investment model (MIM) to finance and deliver this 17.7km stretch, the first time this form of private investment with a public share was used for an infrastructure project in Wales.

Drawing on the relationships we had built during Section 3, as well as our technical expertise as asset management specialists, we acted as the authority’s site representative on behalf of Transport for Wales in a joint venture with Arcadis. We were entrusted with assessing whether work met the Welsh Government’s requirements, as well as making sure it complied with quality standards and industry best practice.

Our team of experienced engineers undertook technical assurance role undertaking design reviews and on-site construction monitoring for 30 bridges, 28 retaining walls, and 38 culverts.

A city with roads and buildings.

A470/A465 Junction during construction. The old junction struggled with high peak-time traffic and also high accident rates. The new layout, pictured at the top of this webpage, was introduced to make journeys smoother and safer. 

A high angle view of a construction site.

Taf Fawr Viaduct during construction. Our onsite construction monitoring team ensured that this three span post tensioned viaduct was built in accordance with the standards and quality required by the contract. 

A highway with cars on it.

Taf Fechan Viaduct during constriction. We worked closely with the contractor and client to ensure the existing structure could be widen and strengthened, activity vital to the schemes success.

At Taf Fechan, we worked closely with the contractor to revise their plans to widen and strengthen the concrete open spandrel arch viaduct. Our input made sure the existing structure was integrated into the scheme, reducing disruption for road users during the works and minimising embodied carbon. The remodelled viaduct will support predicted traffic loads for decades to come.

When the Covid-19 pandemic struck, we adapted to keep the project moving and to uphold the quality of the scheme in a very uncertain time for the industry. Sections 5 and 6 ultimately opened on time, with construction taking just four and a half years as planned.

A legacy for local communities

The A465 upgrade scheme generated significant social and environmental benefits, including thousands of new jobs and hundreds of apprenticeships. Our team monitored progress against community benefit and environmental targets on Sections 5 and 6.

The environmental impact of the scheme was minimised, measures have been put in place to improve and enhance biodiversity in the area, including safe passages for wildlife. This included planting more than 120,000 trees and shrubs, the creation of 20 ponds to clean and attenuate rainwater and prevent flooding, and the generation of 345,000 square metres of habitats for wildlife.

The scheme enables social inclusion by reconnecting the Merthyr, Cynon and Rhondda Valleys, reducing community severance. It improves access to key public services such as Prince Charles Hospital, Pen-y-Dre High School and local transport hubs.

Thank you for your hard work, positive attitude and collaborative spirit throughout. Arup’s expertise and dedication were instrumental in the successful completion of the A465 upgrade. Their collaborative approach ensured the project was delivered on time and to the highest standards, significantly benefiting the local community and economy.

Geoff Ogden

Chief Transport Planning and Development Officer, Transport for Wales

Over 14km of new walking, cycling and horse-riding routes link neighbouring valleys, encouraging active travel and healthier lifestyles. Of the £400m invested in Wales, more than £230m was spent in the Heads of the Valleys region through small and medium-sized enterprises.

Our work on transforming the Heads of the Valleys Road contributed not just to the successful completion of a major engineering project but also to enhancing lives in the communities it connects. 73% of the workforce came from Wales, with 44% from the Heads of the Valleys.

The works created 2,200 new jobs, including 765 for disadvantaged workers and 114 for the long-term unemployed. Arup members contributed their skills, running STEM careers sessions for local schools and colleges.

Partners & collaborators

Arcadis