William Kemp of Scott Wilson on construction of the 58km section between Lancaster and Penrith.
'Personal memories range from bridge design, together with 40 colleagues, supervised by bridges associate looking over our drawing boards several times a day, to soaking wet autumns and springs mired in sticky wet clay and freezing winter days on the fells when it was too cold to our concrete and there was frost in the beards grown by all the young engineers.
'The choice of route involved a major meteorological study, covering visibility as well as temperature and snowfall. The options lay between the direct route followed by the A6 and the Lune Valley route chosen by the Romans and the railway. The direct route would have involved a 2km tunnel and for significant periods of the year motorists driving south would have entered the tunnel in clear weather and emerged in cloud.
'This tipped the balance in favour of the Lune Valley route and the imperative during the detail design was to fit the road as closely as possible to the ground to minimise intrusion into the landscape.
'Carriageways were stepped for some 4km on the sidelong ground of the Peasey Beck Valley which minimised intrusion and earthworks, and again for almost 6km through the Lune Gorge where on the western side of the river the two carriageways are stepped above the railway with the realigned A685 county road further up the slope - a true transportation corridor.
'On the high moors above Tebay the carriageways were separated by up to 250m to improve landscaping and ease snow clearance. Sideslopes were designed to streamline airflow over the carriageway and minimise snow deposition.
'The surface geology is dominated by boulder clays which are particularly susceptible to moisture change. The high rainfall in the area mean that it would be difficult to keep excavated material dry enough for re-use in traditional embankments. The concept evolved was to use wet but otherwise suitable material by sandwiching it between rock drainage layers and then topping with a dry suitable layer compacted in the normal manner. A settlement period was allowed for the wet material to drain and consolidate before pavement construction commenced.
'The section has been open for almost 30 years and all those associated with the project still believe that they created the motorway to which all others should aspire in terms of harmony with the landscape and minimisation of environmental impact.'