Proposed system to facilitate use of pedological information in preliminary stage geotechnical investigations

Abstract

According to the relevant legislation and standards, urban development in South Africa must be preceded by a preliminary stage geotechnical investigation (PSGI) in order to facilitate decision-making regarding site selection and the feasibility of the proposed development project. Geotechnical assessments are also required as specialist studies for Basic Assessment Reports (BAR) that form part of the Environmental Impact Assessment (EIA) process. The results of PSGIs allow classification and/or ranking of land parcels with regard to the cost and ease of urban development, thereby facilitating the conducting of cost-effective follow-up geotechnical site investigations. These desktop-type studies generally rely on information obtained from published sources, including regional geotechnical maps. In the light of the scarcity of regional soils data outside major urban centres, the use of readily available pedological information contained in the published land type maps and memoirs is proposed. This resource provides pertinent details on soil forms, soil thicknesses, underlying materials, stoniness of the soil-like overburden and the clay content thereof. However, the nature of the pedological information does not readily allow direct correlation with the regional geotechnical assessment parameters, and the regional soil mapping information is perceived by geopractitioners to be of importance only for agricultural applications. In this light, a soils effects grouping (SEG) system was devised to provide a relatively simple and scientifically based tool to convert pedological information into the relevant geotechnical parlance in accordance with industry-standard parameters suitable for use in preliminary stage geotechnical investigations. Proper application of the refined SEG system relies on the personal experience of the practitioner, thereby ensuring the accuracy of results to the benefit of decision-makers. Application of the refined SEG system on the hand of case studies, as well as in practice, indicates that this approach holds great potential in improving the accuracy and efficacy of desktop-level studies. However, some limitations do exist, with the depth limit of 1.2 m imposed by the use of the Binomial Soil Classification System on which the land type maps and memoirs are based, hampering accurate interpolation of geotechnical information at depth. Additionally, some inaccuracies with regard to the land type boundaries have been encountered. Conversely, it is envisaged that advances in pedological mapping will in future yield more detailed information, thereby improving the accuracy of preliminary stage geotechnical investigations utilising the refined SEG system.