AbstractProcesses, mechanisms and parameters significant to instability have been identified from an investigation which has included:
1) Geomorphological mapping and appraisal. 2) Field Monitoring. 3) Quantitative assessment of rock mass geometry, mineralogy, engineering properties and physical/physico-chemical behaviour.
The north coast of the Bristol Channel is exposed to severe marine attack. Weak lithologies are exploited and the cliffs undermined. Principal failure modes recognised were toppling and vertical translation.
Thermal gravimetry indicated allotropy of sulphide minerals. The least stable forms were identified in the more calcareous mudrocks, and a laboratory leaching experiment demonstrated that weatherability was greatest in such mudrocks. Pyrite oxidation was shown not only to enhance carbonate leaching, but to degrade the clay minerals and induce rehydration of the double layer.
Uniaxial compressive strengths of representative mudrocks were determined for a range of moisture contents. Mean values varied between 5 and 64 MPa. An investigation into deformation anisotropy of a clay shale is also recorded. Meso and micro scale carbonate filled discontinuities are apparent in the limestones. Hoek & Brown's criterion was used to characterise results of a programme of strength testing. Average uniaxial compressive strength of the intact limestone was 234 MPa.
In the field, many major discontinuities were identified as tensile in origin. Results from shear tests along limestone tension fractures could not be adequately represented by Barton's equation. A power relationship between shear strength and normal stress gave good correlation.
Leaching, moisture and temperature movements, freeze-thaw and pressure release all act to impair stability. Extreme weather was shown to trigger failure.
Limiting equilibrium analyses of toppling showed that torsional shear strength mobilized along the failure surface in the plane of toppling contributed significantly to forces resisting toppling.
Both local masonry protection and pre-split blasting are recommended as viable expedients in protection and stabilization works.
|Date of Award||Apr 1986|