A treatise on concrete, plain and reinforced : materials, construction, and design of concrete and reinforced concrete; 2nd ed. . gn, by Freeman C. CofBn in Journal Association Engineering Societies,July, 1899, P- I- RESERVOIRS AND TANKS 697 strong enough to resist the upward water pressure from the underlying soilwhen the reservoir is emptied. Mr. Coffin* considers a thickness of 3 or4 inches sufficient when the soil is so compact that there is no danger, whenempty, of pressure from without. In pervious earth he suggests 6 inchesof concrete for heads as great as 20 feet. Inverted groined arch
A treatise on concrete, plain and reinforced : materials, construction, and design of concrete and reinforced concrete; 2nd ed. . gn, by Freeman C. CofBn in Journal Association Engineering Societies,July, 1899, P- I- RESERVOIRS AND TANKS 697 strong enough to resist the upward water pressure from the underlying soilwhen the reservoir is emptied. Mr. Coffin* considers a thickness of 3 or4 inches sufficient when the soil is so compact that there is no danger, whenempty, of pressure from without. In pervious earth he suggests 6 inchesof concrete for heads as great as 20 feet. Inverted groined arches for the floor not only distribute the pressure ofthe piers, but also present increased thickness of concrete around the pierswhere there is most danger of unequal settlement, give a minimum vol-ume of concrete, and afford channels for the passage of the water when thereservoir is emptied. The groined arches are laid in alternate diamonds jjefore the piers arebuilt, so that each pier will rest upon the corners of four diamonds. Themethod of laying the floor arches at the Albany Filtration Worksf isillustrated in Fig. Fig. 227. — Reservoir Floor. {See ) Before the concrete has set, the surface may be covered with a grano-lithic or mortar finish, as in sidewalk construction (see p. 600), or it maybe simply troweled. Methods of treating joints between blocks and othermeans of waterproofing are discussed on page 346. *See second footnote on p. 696. •fAllen Hazen in Transactions American Society of Civil Engineers, Vol. XLIII, p. 262- 698 A TREATISE ON CONCRETE Reservoir Roofs. Groined elliptic arches* are especially suited toreservoir roofs because requiring the minimum volume of concrete tosupport their ov^^n weight and the weight of the earth above them. Mr. Cofl&nf says that the cost per cubic yard of groined arches ofconcrete is about one-half that of brick masonry. Although the centeringcosts more than brick because a tight surface is necessary, the brickwor
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