Preliminary site investigations indicated the possible presence of a partially degraded landslide along the proposed route of the A9 Perth-Inverness Trunk Road Improvement through Killiecrankie Pass in Scotland. The steep natural topography and the congested nature of the site, with an existing trunk road and railway line, necessitated the design of relatively deep cuttings through the landslide area.

To investigate the ground conditions in the area and to enable designs for the cuttings to be prepared, a programme of site investigations was implemented, consisting primarily of engineering geomorphological mapping, seismic refraction surveys and the excavation of trial trenches and pits.

The investigation revealed the presence of a sequence of glacial tills, outwash deposits and ancient mudflow deposits overlying a steeply sloping and undulating rock-head. Spring sapping was identified as the only major mechanism controlling natural slope instability. Preliminary designs for the cutting slopes were proposed incorporating the need for extensive drainage of the slopes cut in the glacial deposits.

This paper describes the scope of the investigations carried out, the interpretation of the ground conditions and an outline of the approach adopted towards design of the cut slopes.

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