Towards the development of a modified indirect method of estimating the shear strength properties of shales using shales of the Pietermaritzburg formation as a case study.
Urban sprawl in the Durban metropolitan area has necessitated for the development of areas underlain by shale which were once deemed unsuitable due to instability and the adverse degradable nature of shales when used in construction. Shale is a highly degradable material which weathers easily when exposed to the physical environment. It is thus extremely difficult and costly to obtain representative samples for laboratory testing especially in the determination of their shear strength properties. Representative samples of both fresh and weathered shale material of the Pietermaritzburg Formation were sampled from four localities within the Durban-Pietermaritzburg area. The fresh and weathered shale samples were subjected to simple index tests such as the jar slake test, the slake durability test and the Point Load Strength as well as the large scale shear box test. Geochemical analyses were also conducted on the fresh and weathered shales to determine the mineralogy of the shale samples. Results from the geochemical analyses revealed that the common minerals found in the shale samples are quartz and muscovite mica and these samples contain a low percentage of clay minerals whereby illite is the dominant clay mineral. The results from the index tests and the large scale shear box tests were used to develop a modified method of indirectly predicting the shear strength properties of shales. A shale rating system was subsequently modified and developed which uses simple index tests such as the Atterberg limit test, the slake durability test, the jar slake test and the Point Load Strength test to rate shales and indirectly predict the shear strength properties of shales. The rating system divides shales into three main categories based on a range of index properties and shear strength parameters. This study revealed that by performing simple index tests, the shear strength parameters of shales can be predicted using such a rating system. The shale rating system will be very useful to geotechnical engineers in the construction industry as they are bound by financial limitations and time constraints. Such a rating system will be very useful as it will enable the use of simple index tests to predict the shear strength properties of shales thus reducing costs and the time spent on performing large scale shear tests that are very laborious.