697. Although the choice of the material with which to construct a reservoir may in some cases be varied by local conditions, it is found that under ordinary circumstances concrete offers the greatest advantages for a minimum Cost. For the side walls of small reservoirs, concrete furnishes the requisite strength and water-tightness with a moderate thickness; earthen embankments and floors may be made practically impervious with concrete and mortar, combined with asphalt when considered necessary; while for the roofs, groined arches or beams and slab construction, with supporting piers, all of this material, make a neat, permanent, and altogether satisfactory covering, at a smaller expense than would be required for brick or stone masonry.
In the walls and floors, water-tightness is a prime consideration, and this is best attained by a layer of mortar on the inner surfaces or between two layers of concrete.
As in floors, walks, etc., the necessity of providing for expansion and contraction will depend upon the extremes of temperature to which the surface is to be subjected. In covered reservoirs which are to be almost constantly filled with water, or in very equable climates, the blocks may be large, say twenty feet square, while under more severe conditions the blocks may not contain more than twenty square feet. The joints between the blocks may well be wide enough to be filled with asphalt. This furnishes an elastic joint which is compressed as the blocks expand, and swells when the blocks again contract.
One of the principal difficulties experienced in the construction of floors is from settlement of the foundation. The floor should, therefore, have strength enough to bridge any small irregularities in the foundation that may result from inequalities in settlement. For a similar reason, it is not well to make the blocks too large, as smaller blocks with compressible joints will more readily conform to an uneven surface without permanent injury. In order that the reservoir shall not leak even if the foundation settles, the concrete and mortar may be covered with one or more layers of asphalt.
In building the floor lining, alternate blocks are sometimes placed first in molds and the intermediate blocks built in later. In other cases the blocks are laid consecutively. The advantage of the former method seems to lie principally in the ease of construction, as access may be had to all sides of the block.
700. In hard clay soil not liable to settlement, four inches is sufficient thickness for the floor, the concrete to be covered before it has set with a half-inch layer of rich Portland mortar, troweled to a smooth surface. If the reservoir when empty will be subjected to hydrostatic pressure from without, the floor must be designed to resist this pressure. In this case, if seepage from without into the reservoir is objectionable, a layer of mortar may be placed over the first layer of concrete and protected by the concrete laid upon it. This outside pressure may be provided for in a covered reservoir by making the floor of inverted arches between piers, the weight of the floor, piers, roof, and earth filling over the roof, being made sufficient to balance the upward pressure on the floor. If there is no objection to the water from without being led into the reservoir, a porous layer of broken stone or gravel beneath the floor may be connected with the interior of the reservoir through pipes provided with check valves, and the outside pressure be thus removed. Where it can be accomplished, it will usually be better to lead this ground water through a pipe to a sewer or a lower level rather than into the reservoir.
The thickness of the wall is determined by methods similar to those used in designing a retaining wall or a dam according as the pressures are greater from the embankment without or the water pressure within. In the case of a covered reservoir, the thrust of the roof arches may convert any vertical section of the wall into a beam, the earth pressure from without being supported by the floor at the bottom and the roof at the top. Or in case there is no back pressure from earth filling, the thrust of the roof may be added to the inner water pressure. In circular covered reservoirs the arch thrust is usually taken by steel bands laid in the concrete and encircling the reservoir near the top of the wall. In narrow reservoirs rectangular in plan, tie rods may be used, or the wall may be buttressed to take the roof thrust. Concrete side walls are usually built vertical, or nearly so, on the inside, and with a batter on the outside.
Linings of sloping earthen embankments are laid the same as the floors, and similar precautions are required. There is greater danger of settlement of embankments than of the floor foundation, and the blocks, therefore, may well be made smaller. Some difficulty may be experienced with laying horizontal asphalt joints on a sloping face, and some sliding of the lining may be expected under ordinary conditions, the asphalt joints being compressed. For this reason it would seem to be better to use asphalt in the inclined joints only, and a mastic in the horizontal joints. Another method which would probably prove satisfactory is to lay first a tier of blocks next the floor, and when these have set, apply a very thin coat of asphalt to the upper edges of these blocks, following with another tier, and so on.
Where it is necessary to cover, a reservoir, either to prevent the formation of ice, or the growth of algae, or for other reasons, the groined arch is an excellent design for the roof on account of the small amount of concrete required, the clear head room given, and the ease of ventilation. The extending use of reinforced concrete will also probably enter this field to a greater extent in the future than it has heretofore.