Construction crews have achieved full breakthrough of the A34 underpass at M3 Junction 9 near Winchester. For the first time, a continuous line of daylight is visible running through the entire 120-metre structure.
National Highways shared the milestone on its South-East Facebook page, describing it as a "big step forward" in the ongoing £290 million upgrade to the junction.
Breakthrough shows full underpass length
A breakthrough occurs when tunnelling from both ends meets, allowing the full length to be seen through. In this case, it confirms the two ends of the new A34 underpass now connect beneath the junction.
The structure still requires final finishing and integration with the surrounding carriageways before it opens to traffic.
Project aims and scope
The M3 Junction 9 upgrade will widen the junction and build two new gyratory bridges to create free-flowing links between the M3 and A34. One underpass carries the A34 while the other, also about 120 metres long, will carry the M3 beneath the junction.
Tom Beasley, National Highways senior project manager, describes the junction as “a key interchange for economic growth” connecting Southampton and Portsmouth ports to the Midlands and the rest of the country.
The scheme includes a dedicated northbound A34 freight link to improve journeys for vehicles travelling from the ports towards the Midlands.
Faster journeys and new connections
National Highways estimates journeys from the junction to the A272 will be nearly 30% faster during peak times once complete. Time savings of over 20% are expected on trips between the M3 and A34 at busy periods.
The scheme supports the delivery of 2,000 new homes in Winchester and features more than 8.9 hectares of restored chalk grassland. It also adds new routes for walkers, cyclists, and horse riders.
Progress to date and ongoing works
Work began well before the breakthrough. In February, two 48-metre steel beams were installed over weekend closures to form the new gyratory bridges. A new bridge on the roundabout's north side and a southbound off slip opened on 3 August.
Reconfiguring the roundabout will enable the M3 to expand from two lanes to four lanes as it passes under the junction, reducing congestion.
National Highways plans to complete the scheme by 2028. Initially estimated at about £210 million, government funding of £290 million was approved in March 2025.
Traffic impact remains
Despite the breakthrough, the northbound entry slip road to the M3 remains closed until winter 2027. Drivers currently cannot join the northbound M3 at Junction 9 from the roundabout.
The closure supports construction of the new northbound A34 freight link. National Highways directs traffic to use junction 11 southbound before travelling northbound to avoid the closure.
