Physico-chemical effects on the behaviour of a coastal mudslide

Previous work has shown that the type of clay mineral, the valency state of adsorbed cations and the salt concentration of pore-water influences the residual strength of clays. Residual strength can therefore be modified through weathering reactions, clay mineral alterations and fluctuations in groundwater chemistry. This has implications for studies of the seasonal and long-term behaviour of clay slopes. The paper presents results from an investigation of a shallow coastal mudslide on the south coast of England which attempts to isolate the influence of physico-chemical processes on mudslide behaviour. Fluctuations in groundwater chemistry coincided with temporal movement patterns. Low pore-water ion concentrations were apparent immediately before seasonal mudslide reactivation or periods of high activity. Laboratory tests showed that low pore-water ion concentrations resulted in low residual shear strength; higher concentrations observed after movement events led to increases in residual strength.