Reactivation of landsliding following partial cliff stabilization at Barton-on-Sea, Hampshire, UK

The historically unstable eastern side of the Barton Clay coastal outcrop was subject to stabilization works during 1967-1968, the principal part of which was a 1450 m long filter drain with a sheet pile cut-off aimed at the seepage-prone Barton Sand-Barton Clay junction. Subsequently four reactivated landslides occurred and by 2008 amounted to c. 46% of the total length of the filter drain. Two of these landslides were massive failures of the whole undercliff. This paper examines the relations between the landslides and the stratigraphy, including the presence of the known preferred surfaces of shearing, and the characteristic geomorphological modes of degradation in the Barton Clay undercliffs. It is shown that although the original drainage design reflected the influence of the stratigraphy, there was insufficient consideration of the hazards posed by the natural patterns of degradation and their geomorphological expression. The earlier two of the landslides were followed by remedial measures but the latest two are continuing. Although the cliff toe is well protected by a robust rock armoured revetment, the western end of this area is now reverting to the state that existed before the stabilization works were installed.