Pass of Killiecrankie
Lets get one thing straight right from the start, the Pass of Killiecrankie has absolutely nothing to do with the Krankies. It is much older than that, a geological feature that dates back tens of thousands of years to a time when ice was re-carving the landscape. Soon after the ice retreated, man returned to the north of Scotland and the Pass of Killiecrankie, with the River Garry running through it, would have become an important trade route as soon as such things were needed.
Jacobitism and General Wade
The Battle of Killiecrankie was fought in 1689, as one of the first skirmishes between the Highland supporters of the ousted King James and the red coat forces of the new King, William. Reports of the battle mention the road through the pass, and both sides using it.
Forty-odd years later, in the aftermath of the 1715 Jacobite Rebellion, General Wade built a road through the pass to help facilitate the movement of troops. It ran from Dunkeld north to Ruthven near Kingussie. This road can still be walked along, as it is now one of the paths linking the visitor centre at Killiecrankie to the car park at the Linn of Tummel under the Garry Bridge. Like so many of Wade's roads, however, it was built quickly and cheaply, meaning that when Thomas Telford came to survey it in the early 1800s, he chose a new route for his new road.
Telford
In 1802, Telford was given a commission to improve roads in the Highlands. This included repairing and improving the Military road network, and it would appear that he didn't think much of the road through the pass. Instead of repairing the road as it stood, he chose a new route, running higher up the hillside. This is the route now followed by the B8079.
The Viaduct
Even in the 1930s, Telford's road was struggling to cope with the demands of modern traffic, and many of his bridges were widened or replaced as the road was improved. By the 1970s, the road was no longer able to cope and a grand new scheme was plotted.
The result is the dramatic dual carriageway viaduct that carries the A9 through the trees ever higher up the hillside. Thanks to the dense forestry, the concrete is now almost invisible from most angles, even trying to see it from the old road just below can be tricky.
The viaduct is a concrete structure, or rather a pair of concrete structures as each carriageway appears to be carried independently. This may be to ensure that traffic flow can be maintained if one structure suffered a structural failure. Underneath, the forest of tree trunks becomes a forest of concrete piers with row after row of them supporting the road deck above. Each row has four piers - 2 for each deck - and there are in excess of 30 rows. Considering the steepness of the hillside that the viaduct is built on - the deck nearly meets the ground on the uphill side - this is quite an amazing piece of engineering.
Despite this being the shortest of the A9 improvements (2.5Km contracted) it was unsurprisingly the most expensive of all of them (£11.25million). This section was completed in 1986 and completed the A9 reconstruction programme, which began in 1972. Before the road was complete there were link roads at each end of the structure to join it the old road but these are now long gone.
To get a feel for the sheer scale of the structure, there is a footpath, part of the Pitlochry - Killiecrankie 'blue' route which actually runs under the viaduct before dropping down onto the B8079 below.
The construction and engineering of the viaduct and the A9 reconstruction project is featured in this Highways magazine http://www.ciht.org.uk/download.cfm/docid/91EA4FAF-8685-475C-9FC7D767EB968C77
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