ENGINEERING GEOLOGY AND CONCRETE GRAVITY DAM STABILITY

A practical but critically important application of engineering geology is analyzing the stability of existing (and new) concrete gravity dams. The primary modes of failure for concrete gravity dams related to geologic conditions are: (a) failure along rock mass discontinuities, (b) generalized rock mass failure, (c) combined failure (i.e., failure along rock mass discontinuities and rock mass failure), (d) failure initiated by buckling of supporting rock strata downstream of the dam, and (e) failure along the concrete/rock interface. A geologic investigation is directed at determining the potential failure modes of the dam and estimating the appropriate strength parameters (shear strength of discontinuities and the concrete/rock interface and rock mass strength) for dam stability analysis. The investigation sequence typically consists of five principal steps, beginning with a review of project geological and geotechnical information previously developed for the dam including core logs, photographs, instrument data, and other reports. A field study of rock outcrops is then conducted in proximity to the dam with emphasis on the orientation and continuity of rock mass discontinuities, measurement of discontinuity profiles, and strength of the discontinuity surfaces. Drilling of boreholes through the existing dam also occurs to describe the geologic conditions underlying the dam and the condition of the concrete/rock interface, as well as to conduct packer testing and rock/concrete sampling. Next, laboratory testing is performed on rock and concrete samples via unconfined compressive strength, unit weight, and direct shear tests. Analysis of field and laboratory data follows, including the development of geologic cross-sections and analysis of rock mass discontinuities under the dam, to evaluate and determine potential failure modes. Finally, strength parameters are determined for the dam stability analysis. Hoek-Brown (2002) rock mass parameters are developed from the data and used to estimate the rock mass strength. The shear strength of potential sliding planes is estimated using the Barton (1973, 1976) shear failure criterion. As described in the above steps, the geologic investigation/analysis is an iterative process that will be detailed in this paper with the use of several case histories.