M6 Extension

Client Highways England/Carillion

Since its first section – the ‘Preston Bypass’ – was opened by then Prime Minister Harold Macmillan in 1958 to become the first part of the UK’s motorway network, the M6 has been hailed as the “Backbone of Britain”, providing a cultural and economic link between Scotland and England.

However, there was a small part ‘missing’ - a six mile gap from Carlisle to Gretna. This so-called ‘Cumberland Gap’ prevented the M6 from linking up with Scottish motorways on the other side of the border.

Nevertheless, thanks to a massive £174m project that involved a mixture of new road and the upgrade of the existing A74, as well as two major bridge structures, the M6 gap has now been closed, providing a seamless motorway network between Glasgow and England’s South Coast.

Our teams' work on the project began in 2003 when we were commissioned by contractor Carillion Roads (working on behalf of client the Highways Agency) to provide design and construction supervision services on the scheme.

The new motorway can be considered as two separate sections, each with its own specific challenges. The first – the southern section - called for the upgrading and widening of 6km of dual two-lane dual carriageway to three-lane motorway standard. Here, we focused on reusing as much of the existing infrastructure as possible to keep costs down, as well as arriving at a design that minimised the impact on the 45,000 vehicles who used the existing road each day.

However, it was the creation of 2km of off-line motorway at the northern end that was to prove particularly challenging. The team had to stiffen boggy ground to provide a solid foundation for the 8m high embankment that carries the road over the West Coast Railway. Of course, although it initially sounds relatively unremarkable, it becomes something of a complex engineering feat when you consider the embankment sits on 5700 piles and 18500 band drains, and is within just six metres of live railway lines that remained open throughout the construction period.

By working in partnership with the Highways Agency and Carillion throughout this huge project, we were able to play a pivotal role in ensuring that the project was delivered ahead of schedule and under budget while, perhaps most importantly, minimising disruption to road users, rail passengers and even the local wildlife.