A9 dualling: The biggest challenges facing one of the largest projects in Scottish history

When the road was last fully upgraded 50 years ago to bypass every town and village on the route, the epic feat included diverting the River Tay and building a viaduct on top of another road that was itself on top of a railway line at the Slochd pass.

This time, the project will be equally demanding - and not just in squeezing the widened road into the A9’s narrow corridor past Dunkeld.

Transport Scotland was unwilling to discuss potential hurdles for fear it would push up contractors’ prices, but it said tricky issues included the proximity of the Highland Main Line, since the Perth-Inverness railway hugs the A9 over much of its length.

The Scottish Government agency singled out blasting operations close to the line among such challenges as rockfaces are removed to provide space for the new lanes. It also said some bridges would be built at “sensitive environmental locations”.

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Civil engineering consultant Derek Halden said: “There is nothing particularly challenging technically about the A9, but the weather is a huge uncertainty. The main challenges are contractual, since remoteness means site establishment costs are a relatively higher proportion of the total cost of the road.

“On northern sections of the project, there are also challenges over water and ground conditions.”

Transport Scotland is also acutely aware of the potential impact of the work on people using the A9, since construction is scheduled to continue interrupted for a decade from spring next year until 2035.

Its spokesperson said: “The critical need to maintain traffic flow during construction will be vital for the remainder of the dualling programme.”

Following the start of construction of the next dualling section, Tomatin-Moy, south of Inverness, in the spring, the Tay Crossing-Ballinluig contract is due to be awarded in the summer followed by those for seven other sections over the next five years, some of which will be combined.

Those next two sections are scheduled to be finished in 2028, followed by Pitlochry-Killiecrankie two years later, Pass of Birnam-Tay Crossing in 2032, Crubenmore-Kincraig and Dalraddy-Slochd in 2033, and Killiecrankie-Glen Garry, Glen Garry-Dalwhinnie and Dalwhinnie-Crubenmore in 2035.