The engineering geology and stability of the rapidly alternating limestone and mudrock sea cliffs of Glamorgan
Processes, 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 assessement 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
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
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.