Comparison of the engineering geological properties of near surface and deep sedimentary rocks of the Karoo Supergroup, South Africa

Abstract

The sedimentary rocks of the Karoo Supergroup cover approximately 75% of the surface area of South Africa. Major projects have been constructed on and in these rocks with a number of tunnelling projects such as the Lesotho Highlands Water Project and the Orange-Fish Tunnel. The mechanical properties of these rocks have been well studied and recorded through research and construction projects. During the exploration phase for shale gas, two deep boreholes were drilled in the southern part of the Main Karoo Basin, near Willowvale and Ceres. This provided the opportunity to access rock samples from deep formations. Various laboratory tests were done on selected cores to be able to determine the engineering properties and strength characteristics of the deep Karoo Supergroup rocks, and these engineering properties and strength characteristics are compared to the shallow or near surface rocks, which is presented in this dissertation. Five core samples from the Willowvale borehole (KWV-01) were retrieved for laboratory testing, which include samples of a dolerite sill, sandstone from the Pluto’s Vale Member, carbonaceous shale from the Whitehill Formation, massive shale from the Prince Albert Formation, and lastly, tillite from the Dwyka Group. Four core samples were retrieved from the Ceres borehole (KZF-01) and, included fine sandstone from the Tierberg Formation, carbonaceous shale from the Whitehill Formation, shale from the Prince Albert Formation and diamictite from the Dwyka Group. The density, porosity, water absorption, specific gravity, slake durability, free swelling and mineralogy of the different rock samples were determined and compared between the two boreholes (KZF-1 & KWV-1), as well as with those of the known properties of the near surface Karoo rocks published in literature, and also results from twelve surface samples collected during this research project. The Geodurability Classification system is used to classify the core samples, so as to determine its durability. The sandstone, shale and tillite show increased UCS, durability, density, and lower expansiveness when moving from surface deeper into the Main Karoo Basin rocks.