Rock Structure in Relation to Concrete Dams on Granite

by Richard E. Goodman, Univ of California at Berkeley, Berkeley, United States,
M. Karaca, Univ of California at Berkeley, Berkeley, United States,
Y. Hatzor, Univ of California at Berkeley, Berkeley, United States,

Abstract: Granite is often considered to provide an excellent foundation for a concrete dam and a great many dams are thus founded on granitic rocks. Although granitic rocks lack the bedding of sedimentary rocks, they do often contain three or more sets of continuous and regular joints that divide the rock into blocks. Further, some granitic rocks contain foliation planes, with schistose zones. Almost all granitic rocks exhibit sheeting joints, and locations of deep decay, if not well developed weathering profiles. When these macro or micro joints contain water under pressure, blocks can move, as shown by tragic examples that resulted in loss of life and abandonment of sites. It can be said that most problems with dam foundations in granite derive from weathering effects and/or discontinuities. Existing concrete dams being reexamined with respect to revised flood or earthquake loading conditions, or both, need vigorous reassessment of rock structure in their foundations and abutments. In some cases, original site investigations were inadequate with respect to joint systems, and in others the geological information was imperfectly evaluated by engineering designers or engineering geologists. Techniques have radically advanced for mapping and describing rock discontinuities and for measuring their potential influences on dam stability and water tightness. A fundamentally important problem for civil engineering is the effect of high spillway discharge, or dam overtopping, on the stability of blocky abutment or foundation rock. For this question we have pursued model studies and stereographic projection analyses and suggest a potential method to mitigate this hazard. Two case histories will be discussed in which evaluation of the joint system was basic for establishing the dam safety under PMF (probable maximum flood) conditions.

Subject Headings: Rocks | Dam foundations | Concrete dams | Earthfill dams | Joints | Spillways | Water pressure | Floods

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