decrease, since a decrease in velocity may cause a deposit of suspended matter. Frequently, however, it is impossible to attain this in the design, since the flattest surface slopes are usually nearest to the outlet and the sewer grades are largely controlled by these.

Still more important than obtaining a constant or constantly increasing velocity is the keeping of the velocity within the limits given in Art. 13. If the ground surface is too flat to permit of obtaining this velocity by gravity, pumping must be resorted to. If the surface is steeper than is permissible for the sewer, the sewer grades can be broken by a drop made at each manhole, or by "flight sewers." (See Art. 38.)

A slight drop in the grade should be made at each manhole on flat grades to compensate for the obstruction offered by curves, etc., at this point, and for slight errors in measurement; 0.02 or 0.03 foot is usually sufficient. This is generally unnecessary in the case of masonry sewers or others with continuous inverts and straight alignment.

Of the above principles, the most important is that the velocity of the sewage shall be within the proper limits; then, that all basements and cellars to a reasonable depth shall be drained by separate sewers; also the depth of excavation should be kept as light as possible, and the principles outlined in Art. 34 should be regarded. The obtaining of the nearest possible approach to an ideal design will usually require many changes in, and rearrangement of, both lines and grades, since a change in those of one lateral may in some way affect the entire system.

The preliminary grades having been fixed according to the desirable depths and velocities of flow, the size of the sewer for each reach should be calculated or taken from the diagram and the velocities checked by accurate calculation. Additional changes, usually slight, will probably be required to obtain the best values for each interdependent velocity, size, and depth. The junctions and crossings of the sewer lines must be carefully examined and adapted to each other. It is a good plan to make a list of all the manholes, showing for each the

elevation at which each sewer enters and leaves it. Two sewer lines should never intersect each other, each having a continuous grade across the intersection; either one should discharge from both directions into the other or they should cross, the one above the other.

At junctions, the surface of the sewage in the contributing sewer should never be designed to be lower than that in the other; that is, the center of the tributary should not be below the center of the intercepting sewer if a factor of safety of 2 be used for both; if the larger is a main, the center of the tributary lateral should not be lower than a point two-thirds the diameter of the larger above its invert. It would be still better to place the invert of the tributary above the ordinary sewage-surface in the main when the former drains but a small district; but where the total available fall is slight, none of it need be utilized for this purpose.

Difficulty will sometimes be found in so arranging the comparative depths of storm and separate sewers that the houseconnections can pass under or over the former. In some cases this may be impossible, and it may be necessary to place a separate sewer on each side of the storm sewer, in which case the nearer both of these are to the building lines, the shorter and less costly will be the house-connections.

Reference to the data of locations and depths of gas- and water-pipes and other existing sub-surface systems should be made constantly and the sewers so designed as to interfere with them as little as possible.

On the profile of each sewer line the elevation of all sewers that cross such line should be indicated and a cross-section of the sewer shown. On the finished profile it is well to indicate the top as well as the bottom of the sewer and the thickness and material of the sewer-walls and of all manholes and other appurtenances. The materials may be indicated by colors, as red for brick, brown for sewer-pipe, etc. The size, grade and length of the sewer between each two manholes should be given in figures, and the exact elevation of the invert at each manhole and change of grade. (See Fig. 17.)

CHAPTER IX

DETAIL PLANS

ART. 38. THE SEWER BARREL

A SEWER may consist of a series of pipes joined together to form a continuous conduit, or may be built in place of bricks or blocks of different shapes, of stone masonry, or of concrete poured into forms in place in the trench. Sewers are built of hard-burned clay, stone, concrete, iron, and wood. The material should be practically indestructible under the conditions found in sewers. The construction should be such as to afford sufficient strength to resist all outside and inside pressures. The completed structure should be watertight and the inner surface free from unevenness and uniform in cross-section. The crosssection and grade should be designed with careful observance of the requirements of hydraulics and flow as previously outlined. Local conditions as to foundation afforded by the soil or by artificial support, the shape or dimensions of trench or tunnel that can be constructed most economically to receive the sewer, the head-room available, etc., should be carefully considered, and also the materials most readily available of which to construct the sewer. Large sewers are very expensive, and every effort should be made to keep the cost at the minimum that will give satisfactory service.

Pipes are used in practically all cases where the diameter of the sewer does not exceed 30 inches, and have been used up to 72 inches and possibly larger. For the sizes up to 24 to 36 inches, salt-glazed vitrified clay is the material most commonly used; the shape being cylindrical. Concrete pipe (also called cement pipe) also is used for these sizes, either round or egg-shaped in cross-section. Cast iron is used where the pipe is to be under

internal pressure, where water-tightness is of special importance or where for other reason unusual strength is required, as in unstable ground or outlets under water. Wood is seldom used for underground sewers, but is sometimes used for outlets, either entirely below low water, where there is little decay, or supported on piles, where some flexibility without leaking is desired; its absorption of sewage and liability to rapid decay

are objections to its use, but

both of these can be much lessened by creosoting.

For sizes larger than 24 or 30 inches, reinforced concrete is the material most commonly used for pipes. There are several patented methods of reinforcing, some involving special methods of securing tight joints between pipes. With few exceptions these

FIG. 24.-JOINT OF A REINFORCED pipes are made cylindrical.

CONCRETE PIPE.

The chief advantages of pipes are that they can be made outside the trench, by which more perfect construction can be insured; they can be laid (at least in the smaller sizes) more rapidly than a sewer built of blocks or other small units or of concrete poured in place and with less plant and at less cost; and they can be given a more exact shape than is practicable with construction in place. The chief objections are the difficulties of avoiding breaks in continuity of surface at the joints, and of securing water-tightness at joints. Also, if very large sizes are used, the great weight makes the handling of the pipes difficult and expensive.

Fifty years ago practically all large sewers were made of brick. Present practice uses concrete very largely for such sewers; also large circular sewers are built of specially shaped interlocking blocks of vitrified clay, so designed that all joints lap and there are no joints continuous through the sewer to cause leaks. These are called segmental block sewers. In some spe

cial cases large sewers are built with vertical side walls of brick or stone masonry and roofs of reinforced concrete flat slab construction, or concrete or brick arches between steel beams. The

arch is the cheaper and more common form of roof, however. The invert or bottom may be made of any form desired. Various forms will be discussed later.