An Integrated Method of Engineering Geological Mapping for Large Foundation Excavations: The Katse Arch Dam Experience, Lesotho Highlands

Abstract

A method for mapping the steep foundation excavation of the Katse Arch Dam during construction is presented. The method is practical, adaptable and efficient, and field experience confirms that it overcomes problems often encountered with detailed mapping of large, steeply inclined, geometrically complex, foundation surfaces. The method accommodates on-going mapping activities with the progress of excavation and alleviates the problem of time constraints and limited access often experienced on sites. It involves setting out a 5 m-grid pattern of reference points directly on excavation faces as they become exposed. The reference points are then surveyed and located by elevation and the horizontal coordinate system of the project. Each differently orientated cut-slope surface, called a slope element, is treated as a separate entity. The reference points in an element are projected using the Pythagorean theorem onto a base map in the plane of the element, and the geology is then mapped relative to the plotted reference points providing a view that is always normal to a surface of the element. Mapping in the plane of the element permits accurate and detailed spatial location of discontinuities and lithologies as well as the gathering of appropriate geotechnical data needed for a) slope stability evaluations and support requirements, b) assessments of rock mass conditions for founding, and c) optimum orientation of grout holes. Because of the continually increasing expanse of the foundation excavation towards the river section, each slope element is identified based on its position with respect to the river, the dam center line, and 10 m elevation intervals. Slope elements are further divided into sub-elements in order to facilitate the mapping and tabulation of structural and geotechnical information. Entities of mapping completed in this way are then pieced together to produce composite maps to give the final “as built” record for the foundation excavation. A plan-view projection of prominent geologic data shown on the base maps is ultimately prepared, so that geologic contacts and traces of the major discontinuity sets can be presented on a plan view at a smaller scale. Geologic data presented in this way are very useful for slope and dam stability analyses and for designing optimal orientations for drilling during consolidation and curtain grouting operations.