Natural landslides are a common occurrence on inland slopes of Upper Carboniferous strata in south-west England. They range from shallow, translational slides to earthflows, and usually involve soils above and belonging to the zone of completely weathered mudrock. Solifluction was more active in periglacial conditions (due to softening by excess surface water) and formed widespread clayey head deposits found mainly on lower slopes. Translational sliding is more characteristic of temperate environments and locally may be initiated by artesian groundwater pressures on the base of silty clay soils of low permeability where these overlie pervious weathered rock. These slides may develop downslope into earthflows when seepage of groundwater produces softening. The clay soils do not have a high activity or plasticity on the whole, due to the high diagenetic grade of the underlying mudrocks whose non-swelling clay mineralogy they inherit. Measurements of the residual shear strength of the slipped soil at one site shows a curved strength envelope with ϕ′r falling to about 15° at high normal effective stress, but with values up to 30° at low stresses. The significance of the variation with normal stress of ϕ′r and c′r in the analysis of the stability of very shallow landslides makes it important to determine accurately both the laboratory strength and in situ effective stress at low overburden stresses. The direct observation of unstable or susceptible areas before the main ground investigation may be aided by electrical resistivity surveying. Stabilization can usually be achieved by drainage measures, but complete removal of unstable soils is sometimes necessary.
- © The Geological Society 1986