Rock Mechanics and Slope stability – a vast topic indeed. Slopes fall into two broad categories – those that fall into the realm of soil mechanics and the rock mechanics problems. They require completely different approaches in their analysis and it is beyond the scope of a web page to provide a detailed synopsis on how this is done. Suffice to say that GeoZone is au fait with both approaches and has worked on a number of stability problems and assessments around Southern Africa.
There is always a tendency to emphasise the challenges of the big jobs – the 30 metre high cuttings in weathered granite gneisses, or the special problems of Chapman’s Peak Drive. However here in KwaZulu-Natal much of our instability is due to movement along bedding planes within the Ecca Shales. This is an old problem and Durban Municipality for one has spent much effort and money in mapping and classifying the greater Durban area in terms of slope stability. The regional dip of the area is towards the east and southeast, and often during development cuts are made into the underlying shales to accommodate roads and housing platforms. If the shales are adversely orientated, to remove the supporting material allows movement to take place, and if there is clay gouge and water along bedding planes, down comes the slope.
There are no huge cuts here, nor massive projects involved; only a need to stabilise a slope behind a township house, or to prevent a slip of shale onto a highly trafficked bus route.
GeoZone has a solid grasp of the intricacies of slope stability assessments, and will characterise a rock slope in terms of the orientation of the discontinuities, i.e., the joints and bedding planes, and the shear strengths thereof. The orientation of the discontinuities within the rock mass controls the stability of the slope and by carrying out a 3-dimensional, stereographic analysis of the structural data, an assessment of its long-term stability can be made. These are powerful tools and often it is wise to apply them during a routine geotechnical investigation for an area underlain by mudstones and shales. The data and stability assessment will indicate which areas of a site should not be developed and where best to position roads and cuttings to prevent failure of cut slopes.
In terms of the analysis of soil slopes, we generally use Bishop’s Method of Slices, although we do make our lives easier by plugging the shear strength parameters of the soils into some very useful software.
So before you find yourself on the slippery slope, get GeoZone involved to assess the stability of your cuttings and earth embankments.