Modelling joint parameters to understand the influence of a pegmatite vein on excessive seepage at the De-Hoop Dam South Africa

Abstract

Water supply dams are essential infrastructural components, contributing to water security, ensuring economic growth. The construction of De Hoop Dam in South Africa is no exception. Water supply needs to keep abreast with the rapidly growing district in Limpopo. Prior to construction, detailed ground investigations are conducted that ultimately inform the design and construction methods. Certain water losses are anticipated, irrespective of conducted studies. Seepage levels are capped at certain flow rates, provided no safety concerns arise. External from these flow rates, leakage and excessive seepage is assumed. This is primarily identified through Lugeon testing. Several features were identified. This study focusses on the pegmatite feature. Through back analysis, the study examines the influence of joint parameters, aperture and roughness, comprehending the excessive seepage that the pegmatite vein exhibits. This analysis is achieved through a single fracture plexiglass model, based on parallel-plate assumptions. Appropriately sized strips are used to simulate the applicable aperture. An altered plexiglass plate and two diverse rock samples with varying uneven surfaces, are casted to examine the influence of uneven surfaces on the fluid flow. Possible flow regimes or flow structures associated with the varying aperture and roughness can then be identified, deducing favourable discontinuity surface conditions, inciting a pronounced flow. From the collected data, minute differences between the average joint roughness coefficient values, for variously grouped fracture faults grouped from the dam site, denote that a higher roughness coefficient value does not imply a pronounced fluid flow; joint roughness coefficent can be misleading. The visual experiments from this study, illustrate that an interplay between gravitational and capillarity forces, influences liquid flow migration. No specific aperture and roughness conditions leads to a pronounced flow. These experiments do not identify the occurrence of any distinct flow structures. Localised preferential fluid flow migration, with variable aperture or roughness conditions, either promoting or inhibiting flows, are observable. The excessive seepage recognised at De Hoop Dam, cannot be attributed to a single or a definite joint parameter, resulting in distinct flow structures. A combination of variable fracture parameters, resulting in preferential flow pathways, are indicated. This is a favourable condition for a pronounced flow.