Wad and ferroan soil developed in the dolomitic area south of Pretoria

Abstract

The geotechnical investigation of most sites located on dolomite invariably includes the assessment of the nature and properties of the residual weathering product of the dolomite, termed "wad". Ironically, much confusion previals concerning the geotechnical properties of this material leading to its potential behaviour often being grossly misinterpreted. This study attempts to improve the understanding of this comp~ex material, its origin, structure and geotechnical properties. Two principal types of weathering product of dolomite rock have been identified namely wad and ferroan soils. These respective groups of materials may be subdivided into laminated or ~assive wad and laminated or massive ferroan soil on the basis of the ma~r?fabric they display. Secondary features, such as bedding planes and joints superimposed on the original parent rock may be retained as residual features. The disturbance, displacement, slumping and reworking of the residual massive and laminated wad and ferroan soils, with the possible addition of external impurities, give rise to a material which may be classed as a totally reworked wad or ferroan soil. The geotechnical properties of.the different wad and ferroan spils have been ascertained through laboratory and field testing. Relationships between the nature, form and spatial arrangement of macrofabric features and laboratory measurements have been examined. The influence of secondary features or discontinuities imposed on these soils, are considered. Wad and ferroan soils are noted to possess both poor and very positive behavioural characteristics. The assessment of these materials, requires that particular attention is paid to fabric description and degree of reworking. Several additions to the MCCSSO system of soil profile description are proposed to embrace discriptors essential for the engineering geological appreciation of a site underlain by wad and/or ferroan soil.