RICHARD BRADY & ASSOCIATES, INC-A-2020-064

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1188 R. C. RENMIR lottr.AWWA tures. In the design of such walls, the weight of the water over the footing is usually the largest single factor pre- venting overturning of the wall. Therefore, if through lack of under - drainage of the subgrade, the hydro- static uplift pressure is equal or nearly equal to the water pressure on the inside of the reservoir, the beneficial effect of the weight of the water over the wall footing is lost. Under this condition, overturning of the wall may result, depending upon the depth of earth backfill against the outside face 12 in.a1« I rn« 2H designs have been used, some of which are illustrated in Fig. 9. For many years it was common practice to construct underdrains using an envelope of pea gravel around the drain tile bound together with about 1} sacks of cement per cubic yard. Ex- perience has shown that the cement leaches out of the pea gravel, which can cause plugging of the drain tile. Therefore, in current practice no ce- ment is used in the filter media around underdrains. 4. Construction joints. Generally In. It fn.X2 It In. 11 Plan Dimensions 187 HX397 H Fig. 6. Reservoir 68, Pomona, Calif. The capacity of this reservoir is TO mil gal. of the wall and whether the wall foot- ing is tied structurally to the floor of the reservoir. In any case, at least some structural damage to the wall is likely to occur. 3. Design details. An underdrain frequently consists of a shallow trench filled with 1-in. crushed rock with or without drain tile. Consequently, with no filter provided, a bad leak could cause fines from the subgrade to be washed out through the under - drains. This eventually might cause localized failure of the structure. The author has observed several failures of this sort, A good underdrain must be designed so as to filter out fine material if leak- age water or high ground water en- ters the underdrain. Many effective Slope 1 per cent speaking, construction joints in con- crete reservoirs are contraction joints rather than expansion joints. Excep- tions to this are certain joints in roofs of large reservoirs that are exposed to the weather. In reservoirs, as in most hydraulic structures, true expansion joints should be avoided if possible because movement in a concrete struc- ture creates a potential maintenance problem. In the design of hydraulic struc- tures, construction joints usually are spaced at shorter intervals than in nonhydraulic structures. A crack be- tween joints in a hydraulic structure is of considerable concern, whereas it might be of little consequence in a nonhydraulic structure. Although many satisfactory hydraulic structures RICHARD BRADY & ASSOCIATES, INC. F-57