3d. The arch culvert is employed for the large openings where appearance is of more importance than the question of cost, as the construction of arches is more costly than plain rectangular work. In very large culverts, however, the arch is somewhat more economical in material for a given area of waterway.

The larger culverts, both box and arch types, may be reinforced and it is good practice and economical to do so, as it enables much lighter sections of concrete to be employed.

The concrete for culvert construction may be a 1:3:6 mix, a somewhat richer mixture, however, being employed about the reinforcement. Otherwise the construction follows the usual method

of concrete work. The particular point to be mentioned about culvert work is, the protection of the inlet to and outlet of the waterway against any scour by the flowing water. Water finding its way underneath the floor or around the ends will either undermine the culvert or erode the banks and both of these must be prevented. The method of prevention consists in a substantial stone or concrete pavement laid on the floor of the culvert to confine the water to its proper channel, and parapet walls to prevent erosion of the embankment.

The construction of culverts may become a difficult matter when a large amount of water is to be taken care of. The best method of procedure is to excavate a temporary channel or provide

a temporary flume near the culvert site and divert the course of the stream through the temporary waterway. The culvert can then be constructed in the dry, and when completed, the stream diverted into the culvert, the temporary channel being removed.

Imperviousness of Sewers and Conduits.-Sewers, particularly in the separate system, should be as impervious as it is possible to make them. There are three important reasons for this, as:

Ist. The necessity for excluding ground water from the sewer. The infiltration of ground water is a serious matter where the sewage must be purified before being disposed of, and records show that millions of gallons of ground water find their way into leaky

and defective sewers, entailing a great burden and expense on the purification plant for which they were never designed. Furthermore, the leakage of sewage through the lining of sewers has a contaminating influence on the ground, is very unsanitary and may indirectly give rise to epidemics of disease. Another important reason for imperviousness of sewers is the protection of the concrete from possible destructive action of sewer gases which has been discussed in Chapter IV. The importance of waterproofing treatment in extensive sewer projects is beginning to be recognized and in one of the largest projects, the Bronx Valley Sewer, in New York State, the entire length has been protected by an exterior shield

of 2-ply coal tar felt and pitch, following the method described in the chapter on waterproofing. A dense concrete, properly reinforced, and to which has been added a small percentage of a good waterproofing compound, or the interior surface of which has been treated to two coats of a durable and impregnating waterproof paint, will answer the purpose very well.

Water-carrying conduits likewise must be impervious, as otherwise there will be not only a large loss of water, but ground water, often polluted, may filter in and cause trouble. When expense is a secondary consideration, a coat of waterproof cement may be applied to the interior surface in accordance with the specifications for the Integral Method as given in Chapter XXX. Impervious concrete may be obtained by scientific proportioning of materials, as described in Chapter VI, but a good waterproofing treatment is usually advisable.

TABLE XXVI.-AMOUNT OF MATERIALS FOR ARCH CULVERTS.†

MATERIALS FOR CULVERT FOR 10-FOOT ROADWAY EXTRA MATERIAL FOR EACH ADDITIONAL FOOT WIDTH OF ROAD.

* A double load of sand or gravel is taken as 40 cubic feet or about 1 cubic

yards.

† From "Concrete Construction About the Home and on the Farm," published by Atlas Portland Cement Co.

CHAPTER XXVI

CONCRETE TANKS, DAMS, AND RESERVOIRS

Uses of Concrete Tanks.-How to Build Tanks.-Reinforcement for Tanks.Concrete Dams.-Small Reinforced Concrete Dams.-Concrete Reservoirs.

THE Construction of waterworks has received a new impetus with the development of concrete, plain and reinforced. Its durability, adaptability to any condition, and economy have made possible the erection of any number of works which would have been impossible if more expensive and less permanent material had to be employed. Concrete has thus contributed not a little to improved sanitary conditions in water supplies.

In the collecting and storage systems, as well as with distributing systems of all modern waterworks, concrete plays an important part and will continue to do so more and more as its many advantages over other constructive materials become better known.

In the smallest of wells, springs, and watering-troughs, as well as in the largest tanks, reservoirs, dams, and conduits, concrete can be advantageously employed and a volume alone could be written on this branch of the subject. The smaller structures used about the farm are discussed in that chapter, and the question of pipes and conduits has also been discussed in other parts of the book. We must therefore confine ourselves here to the discussion of such typical structures as tanks and water towers, reservoirs, and dams.

CONCRETE TANKS*

Various Uses.-Concrete tanks have been built as receptacles for such a variety of substances that it is impossible to name them all. We naturally think first of a tank as a receptacle for water,

*For complete directions see "Concrete Tanks," published by American Association of Portland Cement Manufacturers, Land Title Building, Phila., Pa., from which this description is partly condensed.

but this is only one liquid for which a concrete tank is suitable. Manufacturers of oil, wine, milk, molasses, pulp, glue, and a variety of other materials are now using concrete in the construction of their tanks (or vats), both for the finished product and in the course of manufacture. Vegetable oils are said to have a deteriorating effect upon concrete, but through the use of the very excellent waterproofing compounds now available, concrete can be used in the construction of tanks for these oils. Very naturally, the use we will discuss most fully is that of water, as probably nine-tenths of the tanks built are for the holding of water. For other substances, where the use is a new one, careful experiments should be made to determine the chemical effect upon the concrete of the substance to be held. Concrete tanks are also extensively built to hold dry materials, such as sand, stone, coal, and grain.

Choosing the Location, Size, and Shape.-Tanks may be generally divided into two classes: viz., those above the ground surface, and those below, and in choosing the proper design the tank location must be first selected.

The next step is to decide the shape of the tank. Tanks are built in many shapes, but the convenience of use usually decides the shape selected.

How to Build the Tank-Rectangular Tanks.-Laying Out the Ground. After the size has been decided upon, select a site near the water-supply if possible, and mark off the ground. In selecting the size, remember that 7 1/2 gallons make one cubic foot, and that a barrel holds from 49 to 54 gallons.

Put four nails in the ground in the shape of a rectangle, to mark the outside line of the tank walls, and stretch strings from nail to nail.

Excavate inside the space thus marked to a depth of 6 or 8 inches. If the soil is good stiff material, the bottom of the tank may be placed directly on this ground.

If the ground is soft, dig a trench just inside the strings one foot deep and one foot wide to secure additional foundations.

The ground under the proposed tank should be thoroughly tamped (beaten down), with as heavy a tamper as one or two men can handle. A block of wood, square or round, 12 or 14 inches across, with handles for lifting, makes an excellent tamper.