Hazard assessment for progressive, weathering-related breakdown of excavated rockslopes

D. T. Nicholson

Department of Environmental and Geographical Sciences, Manchester Metropolitan University, Chester Street, Manchester, UK. E-mail: d.nicholson@mmu.ac.uk Tel: +44 161 247 6232.

Abstract

There is currently no systematic approach for the evaluation of shallow, weathering-related breakdown of excavated rockslopes. The ad hoc approach often adopted to assess this type of rockslope deterioration gives rise to unplanned resource requirements, or alternately, results in resource wastage due to over-design or excessive and unnecessary monitoring and inspection. A field investigation of over 200 slopes in the UK found deterioration to be widespread in excavated road cuts and disused quarry slopes. Deterioration is manifest in a range of erosional processes which are classified according to constituent material size, relative velocity of movement and frequency of occurrence. These erosional processes correlate well with the type of rock mass in which they occur and result in a range of distinctive small scale erosional and depositional slope forms. A new, three stage approach to slope hazard assessment, Rockslope Deterioration Assessment (RDA), is proposed which addresses shallow, small-scale weathering-related erosion of rockslopes. In RDA stage one, rockslope susceptibility to deterioration is assessed via the application of ratings to intrinsic and extrinsic influences and controls on deterioration. In stage two, the nature of the likely deterioration hazard is qualitatively reviewed with reference to rock mass type and slope morphology. In stage three, guidance on appropriate mitigation is provided, based on the findings of stages one and two.