Assessment of Geology as it Pertains to Modeling Uplift in Jointed Rock: A Basis for Inclusion of Uncertainty in Flow Models

摘要

Uplift is one of the major forces affecting stability of rock-founded concrete dams. Problems occur in determining the magnitude and distribution of uplift pressures within rock foundations and in extrapolating uplift pressures to reservoir levels above the pool of record. Uplift is controlled by the flow regime within the rock foundation. The flow regime is a function of the geology. The goal of research in uplift uncertainty and probabilistic modeling is to develop a methodology, analytical procedures, and software to assess uplift pressures and forces within rock foundations for use in the assessment of the reliability of rock-founded concrete gravity dams. The objectives of this study were to identify geological factors affecting the prediction and modeling of flow and the development of uplift pressures in rock foundations; to identify the kinds of uncertainty in uplift prediction resulting from geological investigations, particularly in testing and description of rock discontinuities; and to select a case history for assessing the uncertainties associated with geological and uplift analysis of a large dam. Much uplift prediction uncertainty is caused by insufficient investigation or treatment of rock discontinuities, or by deformation of discontinuities with resulting changes in discontinuity aperture and in the flow regime caused by stresses imposed by the dam and reservoir. Tight discontinuities, those with mechanical apertures less than about 250 microns, may cause a nonlinear response in uplift pressure with rising headwater. Apertures can be estimated from foundation borehole pressure tests using the cubic law, which relates flow rate to joint aperture. There is uncertainty in determining the persistence of aperture and condition of discontinuities through the foundation rock mass.