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12 Feasibility Reponfor Dam Safety Modifications to DeWeese Dam April 22, 1997 reservoir levels. Our estimated costs for this alternative conservatively assume that the demolished concrete will be disposed of off-site, Drainage/Seepage Provisions Uplift (pore) pressures within concrete dams play an important role in their stability, The relief of pore pressures and the control of seepage along any horizontal lift surface or construction joint through a concrete dam is an effective way to reduce net internal tensile stresses in the concrete and to reduce uplift forces detrimental to sliding stability. Accepted design practice [5], allows for consideration of a reduction of uplift pressure if a drain system is present. Assuming proper maintenance, the proposed drain system for Alternative No. I would relieve uplift pressures and control seepage at the base of the replacement crest section. The proposed drainage/seepage control system for Alternative No. I would consist of the following components: Expandable Waterstop: Construction of a new crest section on top of the original dam concrete would make the effective installation of a conventional PVC waterstop difficult. However, there are several expansive-type waterstops available, which are effective in controlling seepage through concrete joints, Drains: The drainage system for Alternative No. 1 consists of the installation of 3-inch- diameter, half-section PVC drain pipe at the upstream heal of the new crest s,,:ction. The; drain pipe would extend along the entire length of the crest and would include 3-inch-diameter, half- section outlet pipes provided at approximately 20-foot spacing along the crest of the dam. The outlet pipes would allow access for routine maintenance of the drainage system. New Concrete As discussed in Section 1.1, a key component to the sliding stability of a concrete dam is the bond between the concrete surfaces at horizontal construction joints, A significant amount of resistance to lateral load can be developed along any horizontal plane within a concrete dam, provided that even a relatively small amount of bond is developed. Adequate bond between the new crest section concrete and the original dam concrete can be obtained through a combination of proper design, well-conceived and well-written technical specifications, quality materials, and quality workmanship by the Contractor. An explicit and rigorous technical specification will be prepared during final design regarding the preparation of surfaces upon which new concrete will be placed, This specification will include proven requirements for concrete placement [8] relative to surface preparation and concrete placement techniques, ~ GEl Consultants, Inc. 96006\REPORTS\FEAS\TEXT.NEW I I I I I I I I I I I I I I I I I I I