Slope Stability Assessment of a Major Trunk Road at Scrabster Harbour, Scotland, UK

In 2017 two landslides resulted in temporary closure of the main access road to Scrabster Harbour, located in the north of the Scottish mainland. A slope stability assessment was commissioned to investigate the occurrence, causes and mechanisms of historical landslides and their associated consequences to inform future landslide hazard potential. Within Scotland, most slope stability studies are undertaken using qualitative rather than quantitative methods, largely due to insufficient historical data. This paper presents a case study where a semi-quantitative risk assessment was used to assess the stability of coastal slopes above the A9 Trunk Road at Scrabster Harbour. A database of historical landslides and slope characteristics was compiled and used in a semi-quantitative risk assessment to provide the client with targeted information on which areas of the slope can be stabilised most effectively. This was based on ranking the slopes in terms of relative risk, thus providing the road operator and maintenance contractor with an indication of those slopes presenting a higher risk so that these areas could be prioritised for remedial works. The analysis showed that surface water drainage intersecting the slopes and locally over-steepened slopes were primary controls for the observed landslides.