Three-dimensional stability analysis of a coastal landslide at Hanover Point, Isle of Wight

The applicability of 3-D slope stability analysis is discussed in the first part of the paper. A conclusion is drawn that 2-D analysis is often deficient in one or more respects, and often fails to model properly the true mechanics of landslides. The paper describes the application of 3-D slope stability analysis using Hungr's Method of Columns to a small but active landslide in Wealden strata at Hanover Point on the SW coast of the Isle of Wight. The basal slide surface of this very small landslide follows a bedding plane. This bedding plane has components of dip both across the landslide and also into the slope. The shape of the slide mass is affected by the shape of the basal shear, but as the slide is also clearly prevented from moving in the direction of steepest dip (inland) by the confining effect of the embayment within which it is developing, the slide is a strong candidate for 3-D analysis. Systematic analyses of the slide have been made using the Method of Columns, which is essentially a 3-D variant of the 2-D Method of Slices. By changing the assumed movement direction by a small amount between analyses, and covering all possibilities, it is demonstrated that there is a vector direction of movement that exhibits the least Factor of Safety. This vector direction is comparable to the actual movement direction observed in the field. The paper draws conclusions as to the applicability of 3-D methods of analysis, and also concerning the evolution of bedding controlled landslides with dips into or out of the slope, when subject to strong marine attack and cliff retreat.