Processes of slope degradation in the Barton clay cliffs of Christchurch Bay

Abstract

Research into the processes of slope degradation has primarily concentrated on the factors which influence the dominant processes on a degrading slope, Bromhead (1979), Hutchinson (1967), Brunsden and Jones (1976) and the mechanics of individual processes of slope degradation, Hutchinson (1970), De Freitas and Watters (1974) and Hutchinson and Bhandari (1971). No published research has quantified the rates of movement, the depths to shear surfaces and the volumes of material transported by each process in a degrading coastal clay slope. A detailed two year field study, involving regular surveying and monitoring, into the nature and pattern of degradational processes present in an actively eroding stretch of the Barton Clay cliffs have been carried out. Seven processes contributing to the slope degradation have been identified. An area of 25,520m² has been monitored by 154 survey pegs, 11 inclinometers, 24 slip indicators, 10 spalling rods and 8 piezometers. Subsurface investigations and field observations located 11 active shear surfaces. Surface investigation identified three phases of annual movement consisting of a summer, a surge and a winter period. Between July 1981 and July 1983, a total of 9,598m³ of soil debris was removed from the study area, which represented 12% of the total colluvial volume in July 1981. Stability analyses have shown that the Influence line approach Hutchinson (1977, 1984) is a good method to quantify the effect of a cliff top slump block on the stability of the degrading slope.

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