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Analysis of the occurrence and importance of slot development (grykes) within shallow dolomite zones in a selected type area on the Eccles Formation

Council for Geoscience, 280 Pretoria Road, Silverton, Pretoria, 0001, South Africa

Patrick Eriksson

Department of Geology, University of Pretoria, Pretoria, 0002, South Africa e-mail: louis.vanrooy{at}up.ac.za; pat.Eriksson{at}up.ac.za

A better understanding of the geology and dolomite stability of sites to be developed is crucial in the quest for releasing land for development that is deemed safe from a dolomite risk perspective. A geological setting often considered to be particularly problematic is shallow dolomite bedrock. This paper presents an analysis of the occurrence and importance of slot development (grykes) within shallow dolomite in a selected type area.

Geophysical exploration forms the initial phase of a ground stability investigation. A Bouguer gravity anomaly contour map was produced from the data set for the type area. The interpretation of the residual gravity map was used as the basis for planning the subsequent phase of investigation, in which further rotary percussion borehole drilling was carried out. Over 750 boreholes and trench-mapping results were available to test the interpretation against. Various attempts showed just how problematic it is to produce a gravity map that represents a good estimated depth to dolomite bedrock. The shallow dolomite areas were firstly delineated using only the residual gravity results and incorporate the gravity high plateau where the average bedrock head is predicted to be shallower than 8 m. An analysis is given of the conditions related to these shallow bedrock areas.

Shallow dolomite areas associated with some stratigraphic horizons are notorious for instability, however, to the layperson the presence of rock on a site signifies stable conditions to build on. Upon closer investigation dolomitic rock is found to present problematic founding conditions, which are not always easy to overcome by engineering design.