The Engineering Geology of Slopes in the South Cotswolds

  • Kevin Duncan Privett

Student thesis: Doctoral Thesis


This thesis presents the results of research undertaken in the south Cotswolds. Emphasis is placed on the engineering geological aspects of planning and construction on hillslopes, many of which have been superficially disturbed by cambering and landslipping. Special attention is paid to the geotechnical properties of the mudstone/clay horizons of the Fuller’s Earth and the Rhaetic, with emphasis on a study of the residual shear strength parameters.

During the examination of numerous gulls in the Blue Lias at Radstock, a simple classification scheme was devised for the various extension styles produced in the interbedded limestone and clay sequence. The engineering significance of these phenomena is discussed. At Radstock cambering has taken place over the Rhaetic, especially the highly plastic Cotham Beds; these horizons have been investigated at this and other localities.

Superficial structures are not well depicted on conventional geological maps. An attempt is made to determine to what extent they may be identified by engineering geomorphological mapping. Only disturbances which rupture the ground surface are clearly discernible and hence reliably depicted by this type of mapping; moreover it is shown that land use can obscure evidence of past instability.

Engineering geomorphological mapping has been used as a preliminary study in the realignment of the A46 north of Bath. The proposed routes are discussed and some of the potential problems highlighted.

It is demonstrated that false colour infra-red photography can be of considerable use in helping to identify areas of ground disturbance.

A study of the geotechnical properties of the Fuller’s Earth shows an increase in “clay fraction”, expanding lattice clay minerals, plasticity and oxidation as weathering proceeds, with a corresponding drop in shear strength, bulk unit weight and calcite percentage. The presence of thin limestone bands modifies this profile. Calcite is found to have a significant effect on the plasticity of the Fuller’s Earth.

The use of the Bromhead ring shear apparatus to determine residual strength has enabled many samples to be tested at normal loads up to 600 kPa; a number of advantages over the conventional shear box are discussed. The parameter φ’r is found to be stress dependent and as a result previously published correlations with other data are critically assessed and the choice of φ’r in engineering practice is considered.
Date of AwardNov 1980
Original languageEnglish
Awarding Institution
  • University of Bristol
SupervisorA. Brian Hawkins (Supervisor)


  • Engineering geology
  • Superficial structures
  • Landslides
  • Fuller's Earth Formation
  • Cambering
  • Gulls
  • Geomorphological mapping
  • Residual shear strength
  • Clay minerals

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