Evaluating hydraulic and mechanical properties of weathered intrusive and chemical sedimentary bedrock in South Africa

Abstract

Weathered rock profiles are frequently encountered during geological, engineering, hydrological, ecological and forestry projects in South Africa. The soil-end exists in the most decomposed portion of a weathering rock profile. This zone comprises residual soil and completely weathered rock, both of which can be excavated as a soil. In this study, completely weathered rock retains the parent rock fabric and macro-structure with secondary minerals forming in place of the primary rock-forming minerals. Residual soil forms through further intensive chemical weathering to the point where most parent rock structure and grain relations have been lost either through decomposition or volumetric strain or a combination of both. The study was conducted with the use of traditional geotechnical laboratory methods, of which the results were compared and analysed with advanced visual techniques. The laboratory testing included particle size analysis and Atterberg limits testing, chemical testing methods, compression testing in oedometers, and saturated permeabilities in a triaxial cell. The advanced visual testing comprised X-Ray Computed Tomography (XRCT) scan that allowed for the modelling of 2D and 3D porosity and density models. These models provide invaluable insight on the structural properties that govern the compression behaviour the completely weathered rock and residual soils. The soil texture of the completely weathered rock and residual soil can be determined from simple field tests that only require water and common stationery. These tests make use of well understood behaviour characteristic of fine-sand, silt, and clay at varying moisture to determine the plasticity of the material, and subsequently the soil texture. The process through which completely weathered rock and residual soil form is governed by the physical and chemical changes caused by chemical weathering. The degree of weathering decreases with an increase in depth into the ground profile. The state of weathering can be quantified with the use of chemical weathering indices. The close relationship between the chemical and physical states of completely weathered rock, allows for the determination of structural prominence, which characterises the degree to which the relict rock structure influences compression behaviour. The structural prominence decreases as weathering increases and, consequently, this increases the compressibility of the material, regardless of the material’s void ratio or dry density. The novel concept of structural prominence can be used to assist engineering geologists and geotechnical engineers to better understand the behaviour of completely weathered rock and residual soil and has been shown in this study to be a better predictor of soil compressibility than the dry density or void ratio.