Additional observations on the Folkestone Warren landslides

Abstract

Summary The Folkestone Warren landslides are first compared to the landslides at geologically related sites on the northern coast of France and the southern coast of England. Additional historical data is then presented on the Warren landslides, particularly that of 1915, and the influence on these of the interruption of littoral drift produced by the Folkestone Harbour works is examined. An indication is also given of other considerable mass shiftings which have taken place in the Warren since the early nineteenth century. A preliminary morphological study is made, the features being linked, where possible, with the historical data. The degree to which the frequent Chalk falls from the High Cliff are controlled by the joint pattern is briefly examined and the recent subsurface investigations in the Horse's Head area are reviewed and discussed. The existing back-analyses of the slips are critically re-examined in the light of the fresh historical and morphological data and additional back-analyses are carried out. Measurements of the residual strengths of the high and low liquid limit Gault are made, in two different ring shear machines, and compared with the combined results of the back-analyses.

Dicussion: Further investigations at the Folkestone Warren landslide

“…These results support those of the authors. (c) The range of ö9 r results obtained by Hutchinson et al (1980) for`high' and`low' liquid limit materials, respectively, was between 78 and 128, which was remarkably similar to those obtained by the authors (78±138; see Fig. 4).…”

“…After our investigations of the Warren in the late 1970s (Hutchinson et al 1980), we communicated to British Railways (Southern Region) our belief that falls from the High Cliff, particularly in its western parts, constituted the greatest potential hazard to the railway through the Warren. However, the authors' paper, which springs out of the additional investigations that followed in 1982±83, does not deal with this matter.…”

“…In the previous investigations of Hutchinson (1969) and Hutchinson et al (1980), no resources were available for borings or instrumentation, and, for the back-analyses, rather wide assumptions had to be made regarding the maximum and minimum groundwater pressures obtaining on the slip surfaces at failure. It is thus disappointing that, despite being able to carry out an extensive subsurface investigation, the present authors were unable to improve signi®cantly on the earlier state of knowledge on this important matter.…”

The residual shear strength of tropical soils

“…The residual shear strength of sedimentary soils and soils originating from the weathering of sedimentary rocks, as well as pure minerals and artificial soil mixtures, has been extensively described in the geotechnical literature. Well-known examples are overconsolidated clays, clays and clay shales from the United Kingdom and North America (Skempton and Petley 1967;Kenney 1967;La Gatta 1970;Bishop et al 1971;Townsend and Gilbert 1973;Kenney 1977;Hutchinson et al 1980;Lupini et al 1981;Skempton 1985;Mesri and Cepeda-Diaz 1986;Skempton and Vaughan 1993;Di Maio and Fenelli 1994;Stark and Eid 1994;Tika et al 1996;Trenter and Warren 1996;Anson and Hawkins 1998).…”

John Neville Hutchinson

“…The survey of coastal landslides covered many of the major slides that he later researched, at Herne Bay (Hutchinson & Hughes, 1968), Folkestone (Hutchinson, 1969;Hutchinson et al, 1980), the Isle of Wight (e.g. Hutchinson et al, 1991), Lyme Regis and elsewhere.…”