Thoughts on: Northern Skye - Beautiful but problematic in equal measures.
Plate 1: The Quiraing
It makes a refreshing change for me to have the opportunity to see and work with such a huge variety of rock types. As is often the case in the Civil Engineering industry, bedrock is generally obscured by thick drift deposits, but the Isle of Skye features many different rock types exposed in such dramatic landforms. These impressive landforms nevertheless can present complex engineering challenges.
One of the most striking landscapes, located less than 10 miles from our head office is the Quiraing, which forms the northern scarp slop of the Trotternish ridge. The Trotternish ridge is part of the largest landslide in the UK, and one of the largest in Europe, extending over a length of approximately 45 km and a maximum width of 6.5 km.
Plate 2: Trotternish ridge, from the Quiraing
The geology of the ridge comprises weak Jurassic shales and sandstones at its base, overlain by basaltic lava and intruded by doleritic sills and dykes. These were formed as North America and Greenland began to separate from Europe during development of the Atlantic Ocean. Once the volcanic activity had ceased, continued plate movements tilted this part of Skye up, towards the east, which in turn created north to south trending faults along the peninsula.
At the Quiraing, the eastern scarp face is reported to have formed as a series of rotational, toppling, planar sliding, rockfalls, mudflows and gravitational spreading failure blocks. The boundaries between adjacent landslide blocks and the zones of different modes of failure have been shown to be demarked by pre-existing structures, including fault zones, dykes and major joint sets, with the basal slip plane at depth, within the Jurassic sediments.
Although the Quiraing is largely currently stable, the toe slopes exhibit low rates of movement, which present an ongoing hazard to local infrastructure and development. As recorded by Transport Scotland, rotational slips and mass movements of debris flows and slides have occurred on repeated occasions over recent years, especially around Flodigarry, leading to road closures that require regular road maintenance.
Plate 3: Flodigarry Road affected by the slip.
The question now is – Can anything be done about this?
The landslip itself has, of course, created the dramatic scenery that is so popular with locals and tourists alike, and Skye would not be the same place without it.
However, if the areas where most of the movement occurs are to be considered for development, or the vital infrastructure protected, then something needs to be done. Whilst it may be possible to slow down ground movements via traditional slope stabilisation techniques, this will only help with the shallow failures and will not stop them or the deeper ones all together.
At present, the only economically viable solution is to adopt a regular maintenance regime of the road and services. Although there may not be an easy or affordable engineering solution to fully manage this condition, would it be possible to reduce the frequency of maintenance required by mitigation incorporating geogrid reinforced mattresses, with flexible drainage and an observational monitoring regime?
What are your thoughts on the future of this part of Skye’s iconic landscape ?
If you have a project that you would like to get going in the Highlands and Islands, then please do not hesitate to contact us for a free, confidential discussion.
- Richard Izatt-Lowry