The Jurassic Coast of the southern shores of England form a beautiful UNESCO World Heritage Site that is reminiscent of the California coastline with relatively high, near vertical slope faces above the beach. These shores are frequently exposed to aggressive coastal erosion and subsequent deep seated global stability failures, which pose a major hazard to the public enjoying the shoreline below. A massive slope failure occurred in August 2020 near Burton Bradstock that dropped approximately 9,000 tons of soil and rock onto the shore below.
A common misunderstanding of slope stability results from confusing erosion and deep-seated global stability failures. Erosion is where the slope face is gradually washed away when the slope face material isn’t adequate stabilized with vegetation or armoring such as rip rap (large rock). Deep-seated global stability failures occur when the shear strength of the soil or rock within the slope is exceeding by the shear forces coming from the weight of the material within the top of the slope.
Typically, erosion of unstabilized cliffs such as the Jurassic Coast of England or the Big Sur shoreline in California leads to the slope getting gradually steeper over time after rain events until the slope is eventually so steep that a large volume of material suddenly slides down the slope, typically termed as a landslide.
Geotechnical engineers are most concerned with preventing deep seated global stability failures as they can occur suddenly and can move very large volumes of material quickly, which poses imminent danger with little or no warning. Considering the great length of these shorelines, they will likely never be fully stabilized to prevent accidents, so if you see a sign that says not to lay out your beach chairs too close to the cliff side, make sure you follow these warnings! – Mike Batten, Civil Geotechnical Instructor
