Investigation of a landslip in the Fuller's Earth Clay, Lansdown, Bath

Abstract

Summary Rubbish dumping into an abandoned quarry in the Great Oolite on a south facing slope of Lansdown, Bath, has provoked a series of slides which now sterilises 12 acres of grazing land. The first major slide in October 1969 was followed by two further movements by January 1971. They account for a total downhill movement of 210 m. Where access to the site made investigation possible, a distinctive shear zone was exposed, sandwiched between the base of the fill and top of the weathered Fuller's Earth clays. Adjacent to the slip, the natural ground succession showed a mixed stony soft brown reworked clay (solifluction) layer, overlying interbedded clay mudstone bands of the Fuller's Earth series. Drained triaxial tests on the brown clay gave shear parameters of φ′ = 17·4°, c′ = 0; and an estimate of residual strength of φ = 12°. The former value would accord with a high (perched) water table on the adjacent slopes at 12·7°–8·7°. Whilst accurate back analysis is not presently possible, due to absence of specific measurements of the surface ground water levels, it is concluded that a limiting stability exists on the slope generally, irrespective of the fill dumping.