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tJUu.?82- Dam Terminology (cont) Thickness or Width of Dam. - The thickness or width of a dam as measured horizontally between the upstream and downstream faces and normal to the axis or centerline crest of the dam. Base Thickness (Base Width). - The maximum thickness or width of the dam measured horizontally between upstream and downstream faces and normal to the axi s or centerline crest of the dam, but excluding projections for outlets, etc. In general, the term thickness is used for gravity or arch dams, and width is used for other dams. Top Thickness (Top Width). - The thickness or width of a dam at the level of the top of dam (excluding corbels or parapets). In general, the term thickness is used for gravity and arch dams, and width is used for other dams. Mass Concrete. - Any 1 arge vol ume of concrete cast-i n-p 1 ace , generally as a monolithic structure. Dimensions of the structure are of such magnitude that measures must be taken to cope with the generation of heat and the resulting volume changes and cracking. Axis of Dam (Concrete). - A vertical reference surface coincident with the upstream face at the top of the dam. Cutoff Trench. - An excavation in the foundation of an embankment (earth or rockfill) dam, usually located upstream of the dam axis or centerl ine crest which extends to bedrock or to an impervious stratum. The excavation is backfilled with impervious material to reduce percolation under the dam. Cutoff Wall. - A wall of impervious material (e.g., concrete, asphaltic concrete, timber, steel sheet piling, or impervious grout curtain) located in the foundation beneath the dam and which forms a water barrier and reduces seepage under a dam or spillway. Top of Dead Capacity. - The lowest elevation in the reservoir from which water can be drawn by gravity. Streambed at the Dam Axis. - The elevation of the lowest point in the streambed at the axis or centerline crest of the dam prior to construction. 2