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2 Feasibility Re\lort for Dam Safety Modifications to DeWeese Dam April 22, 1997 face of the dam with manually-operated lifts at the dam crest. The dam is classified by the Colorado State Engineer's Office (SEO) as an Intermediate, Class II dam. As early as the 1970s, reports of arumal inspections of the dam by the SEO have noted significant deterioration of the concrete and a generally poor observed condition of the two crest raise sections. Recent SEO inspection reports (1994 to present) have stated that storage restrictions will be imposed if the Company does not take appropriate action to improve the condition of the two crest raise sections. Independent inspections by GEl Consultants, Inc, (GEl) and others [1, 2] have resulted in conclusions consistent with the SEO inspections. The principal dam safety issues to be resolved concern the generally poor condition of the concrete in the two crest raises and the ability of the raise sections to safely withstand all reasonable loading conditions, The concrete in the crest raise sections is exhibiting considerable deterioration, generally in the form of spalling and pattern cracking, This deterioration is probably the result of freeze-thaw weathering and erosion due to overtopping flows. These effects may have been initiated and accelerated by poor concrete quality and poor placement techniques, Presently, the surface deterioration in some areas extends as deep as 6 inches or more below the original surface of the concrete, exposing steel reinforcement in Some areas. The lower crest raise section is in generally poorer condition than the upper crest raise section, probably due in large part to lower quality concrete. Deterioration of the concrete in the lower crest raise has been reported to extend as deep as 18 inches, In addition, nUmerous areas of efflorescence can be seen on the downstream surfaces of the concrete, indicating water seepage through the crest raise sections. This seepage could be the result of cracks extending through the concrete, relatively porous concrete, or poor bonding of the raise concrete to the original dam. The structural stability of the crest raise sections is not adequate under all loading conditions, In general, the stability of existing concrete gravity dams, such as DeWeese, is evaluated under three primary loading conditions: usual, unusual, and extreme. Required factors of safety for these loading conditions are established by engineering practice and are set forth in the SEO's Rules and Regulations for Dams in Colorado [6]. The usual loading condition assumes that the reservoir level and the tailwater level are at their normal water surface elevations and requires a computed factor of safety 3,0 for sliding and compressive stresses, The unusual loading condition assumes that the reservoir level and the tailwater lever are at their design flood water surface elevations and requires a computed factor of safety of 2,0, The extreme loading condition assumes reservoir and tail water surface levels consistent with the normal loading condition, together with loading from the design earthquake. The computed factor of safety required for the extreme loading condition is 1.0, Previous analyses of the existing crest raise sections at DeWeese Dam have indicated that the crest structure does not meet the stability requirements for the usual loading condition or the unusual loading condition and does meet the ~ GEl Consultants, lnc, 96006\R.EPORTS\FEAS\TEXT.NEW I I I I I I I I I I I I I I I I I I I