Landslides at Folkestone Warren, Kent

The Folkestone Warren landslide (Figure 1) is a very large, deep-seated coastal landslide that is well known to geologists and engineers (see references). It is about 3 km wide, and up to 350 m in length. There are nine Surveys in the National Landslide Database IDs 1774/1 to 1774/9.

The village of Capel-le-Ferne is situated behind the landslide at the cliff top, and the wide undercliff is given over to a country park.

The Folkestone Warren Landslide (Photograph taken September 2002).
Figure 1 The Folkestone Warren Landslide (Photograph taken September 2002)

The Dover to London mainline railway (via Cheriton) passes through the entire length of the Warren, with the Abbotscliff Tunnel at the eastern end and the Martello Tunnel at the western end. Since its construction the railway has been regularly disrupted by landslide activity, and considerable expenditure has been, and continues to be, involved in remediation and monitoring. The old Dover road is situated close to the cliff edge. This has also been disrupted, and re-routed in parts, for the same reason.

The geology of the platform at beach level varies along the length of the Warren. At the eastern end the platform changes from Gault Formation to Lower Chalk (see below). The change occurs approximately in line with the eastern end of the concrete toe-weight apron (constructed in 1953), near Horsehead Point. Rock revetment has been placed to the east of the concrete apron by Network Rail. It is notable that westward of Horsehead Point the platform is displaced seaward by about 50 m due to movements of the various slide masses (Trenter & Warren, 1996).

Geology

The geology consists of the White and Grey Chalk subgroups overlying the Gault Formation and the Folkestone Formation; the principal slide surface being located at the Gault/Folkestone junction.

The upper part of the chalk sequence here is the White Chalk Subgroup comprising from top to bottom the New Pit Chalk Formation resting on the Holywell Nodular Chalk Formation. The lower part of the chalk sequence is the Grey Chalk Subgroup formed by the Zig Zag Chalk Formation resting on the West Melbury Marly Chalk Formation. Superficial deposits forming the Clay-with-Flints overlie the chalk .

Beneath the chalk sequence lies the Gault Formation, with the thin Chloritic Marl bed at their junction. The Gault Formation consists of between 44 and 48 m of over-consolidated clay. Beneath the Gault Formation is the Folkestone Formation (Lower Greensand Group) with the Sulphur Band at their junction. The Folkestone Formation consists of calcareous and glauconitic sands and sandstones with some argillaceous horizons (Trenter & Warren, 1996).

Landslide

The Folkestone Warren landslide is one of the largest on the English coast and is a classical example of a deep-seated (approx. 40 m below sea level) multiple retrogressive, compound mechanism, having translational, rotational, and graben features.

The Folkestone Warren cliffs are divided into the chalk 'High Cliff', essentially the overall landslide backscarp, and the sea 'cliff' consisting largely of landslipped material. The 'High Cliff' as such is not directly influenced by the sea, unlike the Folkestone Warren landslide as a whole.

The landslide has been extensively studied since 1915 (Figure 2), when a major re-activation occurred throughout the complex seriously disrupting the railway constructed in 1844 (Hutchinson, 1969; Hutchinson, et al., 1980; Trenter & Warren, 1996; Warren & Palmer, 2000). Included in this event were chalk slides from the High Cliff, in particular the 'Great Fall' landslide. Hutchinson et al. (1980) describe the three dominant sub-vertical joint sets within the Chalk and their influence on the form of the High Cliff and on landslides within it. In recent decades a large amount of instrumentation and remedial works has been installed and maintained by Network Rail within the Warren.

The age of the original landslide event is not known. Most landslide movements in recent times have been confined to the undercliff in the form of re-activations of existing deep-seated landslide slide masses. However, numerous significant slope failures of the High Cliff are believed to have occurred since the 1800s.

 

Massive rotational failure in 1915 affecting the Folkestone to Dover train (British Railways, Southern Region)
Figure 2 Massive rotational failure in 1915 affecting the Folkestone to Dover train (British Railways, Southern Region).

 

As part of a programme of work monitoring coastal erosion and landsliding at several sites around the coast of Great Britain, BGS has surveyed the Folkestone Warren landslide complex — see Terrestrial LiDAR Survey Techniques. Results from this work will be published soon.

Photo gallery

References

Warren, C D, and Palmer, M J. 2000. Observations on the nature of landslipped strata, Folkestone Warren, United Kingdom, Landslides in research, theory and practice, Thomas Telford, London, 2000, pp.1551-1556.

Hutchinson, J N, Bromhead, E N, and Lupini J F. 1980. Additional observations on the Folkestone Warren landslides. Quarterly Journal of Engineering Geology & Hydrogeology, Vol.13 (1), pp.1–31.

Hutchinson, J N. 1969. A reconsideration of the coastal landslides at Folkestone Warren, Kent, Geotechnique. Institution of Civil Engineers. London. Vol.19 pp.6–38.

Trenter, N A, and Warren, C D. 1996. Further investigations at the Folkestone Warren landslide. Geotechnique, vol 46 (4), pp.589–620.

Further reading

Acland, C. L. (1869), The late landslip on the Warren. Q. Jl Folkestone Nat. Hist. Soc. 3, 48-51.

Arber, M. A. (1940), The coastal landslips of south-east Devon. Proc. Geol. Assoc. London. 51, 257-271

Barton, M.E. & McCosker, A.M. (2000), inclinometer and tiltmeter monitoring of High Chalk Cliff, Landslides in research, theory and practice, Thomas Telford, London, 2000, pp.127-132.

Bell, F. G., Cripps, J. C., Edmonds, C. N. and Culshaw, M.G. (1990), Chalk fabric and its relation to certain geotechnical properties, Chalk, pp. 187-193.

Bishop, C. H. (1973), Folkestone: The story of a town. Headley Brothers Ltd., London & Ashford.

Bromhead, E. N., Hopperm A. C. & Ibsen, M. -L. (1998), Landslides in the Lower Greensand escarpment in south Kent, Bulletin of Engineering Geology and the Environment, Vol 57 (2), pp. 131-144.

Bromhead, E. N. & Ibsen, M. -L. (2004), Bedding-controlled coastal landslides in Southeast Britain between Axmouth and the Thames Estuary, Landslides, vol 1., pp.131-141.

Drew, F. (1864). The geology of the country between Folkestone and Rye, including the whole of Romney Marsh. Memoir of the Geological Survey of Great Britain, H.M.S.O., London.

Folkestone Chronicle (1859). 10th December, 1859.

Folkestone Chronicle (1965), 11th February 1856.

Folkestone Chronicle (1877), 20th January 1877

Gentleman's Magazine (1806), Country News, (June), 76: p. 575

Haselock, P. J. (1998), The Folkestone Warren landslip: a model for the decay of fault scarps?: Abstracts : Tectonic Studies Group 19th annual meeting, December 1988 / convenors - Nigel Woodcock and Bryan Storey

Hutchinson, J. N. (1965), A survey of coastal landslides in Kent. Building Research Station Note No 35/36.

Hutchinson, J. N. (1969), A reconsideration of the coastal landslides at Folkestone Warren, Kent, Geotechnique. Institution of Civil Engineers. London. Vol.19 pp.6-38.

Hutchinson, J. N. (1986), Cliffs and shores in cohesive materials; geotechnical and engineering geological aspects. In: M.G. Skafel (ed). Proceedings of the symposium on Cohesive shores, May 5th-7th 1986 Burlington, Ontario. pp.1-44.

Hutchinson J.N., Bromhead E.N. and Lupini J.F. (1980) Additional observations on the Folkestone Warren landslides. Quarterly Journal of Engineering Geology & Hydrogeology, Vol.13 (1), pp.1-31.

May, V. J. (1964) A study of recent coastal changes in southern England. MSc thesis (unpublished) University of Southampton, Southampton, SO9 5NH.

McDakin, J.G. (1900). Coast erosion: Dover cliffs. Dover.

McDakin, J. G. (1911), Abbotscliff (Warren) Chalkfall Geological Notes, pp.15, East Kent Scientific and Natural History Society Reports and Transactions, Vol.16

McGowan, A., Roberts, A. G. & Woodrow, L. K. R., (1988), Geotechnical and planning aspects of Coastal landslides in the United Kingdom In: C. Bonnard (ed). Landslides Glissements de Terrain Vol 2.

Morgenstern, N. R. & Price, V. E. (1965), The analysis of the stability of general slip surfaces. Geotechnique. Vol 15 pp79-93

Muirwood, A. M. (1971), Engineering aspects of coastal landslides. Proceedings of the Institution of Civil Engineers, Vol.50, pp.257-276

Osman, C. W. (1917), The landslips of Folkestone Warren and thickness of the lower chalk and Gault near Dover, Proceedings of the Geologists Association. London vol 28, pp.59-84.

Phillips, W. (1821), Remarks on the Chalk cliffs in the neighbourhood of Dover, and on the Blue Marls covering Green Sand near Folkestone, Trans. Geol. Soc. 5: 16-46.

Price, F. G. H. (1877). On the beds between the Gault and Upper Chalk near Folkestone, Quarterly Journal of the Geological Society, vol 33, pp.431-448.

The Railway Magazine (1919), Railway landslides: engineering problems at Wembley and the Warren, (July), 45: 23-31

The Times (1859), 14th December 1859. p12.

Toms, A. H. (1946), Folkestone Warren landslips: research carried out in 1939 by the Southern Railway Company, Institute of Civil Engineers, Railway paper no 19.

Topley, W., (1893). The landslip at Sandgate, Proceedings of the Geologists' Association, vol 13 (2): 40-47.

Trenter, N. A. and Warren, C. D. (2001), Further investigational at the Folkestone Warren landslide, Geotechnique, Vol.51 (2), pp.189-192.

Viner-Brady, N. E. V. (1955), Folkestone Warren landslips: Remedial measures 1948-1954 Proceedings of the Institution of Civil Engineers, Railway Paper, Vol.57, pp.429-441.

Warren, C.D. & Palmer, M.J. (2000), Observations on the nature of landslipped strata, Folkestone Warren, United Kingdom, Landslides in research, theory and practice, Thomas Telford, London, 2000, pp.1551-1556.

Wood, M. M. (1955), Folkestone Warren landslips: investigations 1948-1950, Railway Engineering Division Meeting, 22 Feb. 1955, Railway Paper No.56.

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