Conversion of a Concrete Slab and Buttress Dam to a Gravity Dam

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by P. M. Wimberly, III, GAI Consultants, Inc, Monroeville, United States,
F. B. Newman, GAI Consultants, Inc, Monroeville, United States,
S. G. Mazzella, GAI Consultants, Inc, Monroeville, United States,
S. J. Racketa, GAI Consultants, Inc, Monroeville, United States,

Document Type: Proceeding Paper

Part of: Geotechnical Practice in Dam Rehabilitation

Abstract: The 58-foot high, 425-foot long, Ambursen-type, concrete slab and buttress, Gorge Plant Dam, which has a 120-foot long overflow spillway near the center, was converted to a concrete gravity dam to reduce escalating maintenance costs, prolong the life of the structure, and upgrade it to modern design criteria. Eleven stability analysis cases were investigated for assumed existing and repaired structural conditions. The cases were evaluated for sliding, overturning and bearing pressures with and without the silt in the reservoir. Some cases were evaluated for both normal pool and Probable Maximum Flood (PMF) conditions. The analyses were complicated by the geometry of the dam's crossing buttress walls and typical cross-section. Cracked base conditions and uplift pressure redistribution effects were considered, as appropriate. Design, construction, and economic requirements necessitated a two-phase conversion. A reinforced concrete spillway overlay and reinforced concrete encasement of a 90-inch nominal diameter, steel penstock were required to rehabilitate the dam. Special drainage features, structural support systems, and controlled grouting were required to facilitate construction and to improve long-term performance. Rehabilitation was completed in December 1985, and the installed measures are currently performing well. Clark (1986) discusses the dam's overall history, project economics, and stages of rehabilitation, but does not discuss the technical analyses performed and details of the special design features, which are the subject of the current paper.

Subject Headings: Concrete dams | Gravity dams | Reinforced concrete | Drop structures | Rehabilitation | Spillways | Floods | Concrete slabs |

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