The geotechnical character of some South African dolerites, especially their strength and durability

Doleritic intrusions in Natal and adjoining areas of South Africa are very numerous, and are associated with the break-up of Gondwana which took place some 187 to 155 million years ago. These dolerites are either tholeiitic o r olivine dolerites. Plagioclase and pyroxene represent their two major min eral components. A series of tests was carried out to determine the physica l and mechanical properties of dolerites from a number of localities in and around Natal, in addition to the determination of their chemical and miner alogical compositions. Some dolerites in Natal have been found to deteriora te within a short space of time when exposed and when used as construction materials. These have been designated rapidly weathering dolerites, Such di sintegration is due to the presence of chlorite and clay minerals. Frequent ly, the clay material is smectitic, in which case breakdown results from th e expansion that occurs when such clay minerals swell on absorption of wate r. Unfortunately, however, such dolerites cannot be distinguished readily f rom sound dolerite. As a consequence, several durability tests were underta ken. The results from the fatter were compared with those of the physical a nd mechanical properties, and certain petrological indices derived from the mineralogical character of the dolerites to see if there were any signific ant relationships that were diagnostic in terms of recognizing rapidly weat hering dolerite. Rapidly weathering dolerites can possess high strengths an d ultrasonic velocities. Generally they can be detected by ethylene glycol soak testing and perhaps may be recognized by their petrological weathering index.