CHAPTER VIII.

DETAIL PLANS.

ART. 45. THE SEWER-BARREL.

SEWERS have been made of almost every conceivable shape and the walls built of all kinds of materials. A few shapes and materials are of almost universal applicability, others are adapted to peculiar circumstances only, and some are freaks of invention adapted to no circumstances.

The shape of cross-section is to a certain extent controlled by the material of which the sewer is constructed. The smallest sewers cannot be advantageously built of brick, but are usually composed of earthenware or metal pipes or of concrete. Earthenware sewers are made from 2 to 42 inches interior diameter. They are seldom made other than circular, owing to the liability of other shapes to become distorted in burning. Metal pipes are employed where the sewer will be under pressure, as in a siphon, or where there is a great deal of ground-water; also sometimes to better resist disturbing forces, as in made or treacherous ground or outlets under water or in shifting sands. The only metal commonly employed is iron. Metal pipes have always been made circular, although there are none but economic reasons why other forms could not be made.

Concrete and cement sewers are made of all sizes and shapes-circular, egg-shaped, rectangular, etc.-the smaller sizes being usually of cement, the larger of concrete

Wooden-stave sewer-pipe has been used in the West, and in the East to some extent. On the Los Angeles outfall sewer are 34, 100 feet of 36- and 38-inch pipe of this descrip. tion. The outlet sewers in New York and Brooklyn are many of them creosoted wooden-stave pipe of 3 or more feet diameter.

For all sewers the circle is the most economical shape, and generally the most desirable, if they are never to run less than

full, except that the use of platform foundations may modify the first statement. But if they are to be used as combined sewers the egg shape is to be preferred, or a form similar to Plate VII, Figs. 2 and 6.

In Brooklyn, N. Y., and a few other cities cement sewerpipe is used, and in general all sizes of this above 12 inchesin Brooklyn all sizes—are egg-shaped. are shown in Plate VI, Figs. 1 and 2. the pipe to prevent its rolling in the trench after being placed in position and to strengthen the bottom against crushing.

Sections of this pipe The flat base is giver.

In the case of large sewers, particularly those whose diameter exceeds 4 or 5 feet, it frequently becomes necessary to make the width greater than the height, because the depth of the invert is limited by sewer-grade requirements and the height of the arch by the street grade. A great number of shapes have been designed to meet these conditions. Some of the best are shown in Plate VI, Fig. 5, and Plate VII, Figs. 9 and 10. Plate VII, Fig. 4, shows a design for very low head-room, but the thrust of the arch is considerable and the side walls should be heavier than shown unless they are firmly backed by rock or solid earth. Plate VIII, Fig. 1, is a better design to employ where the head-room can be slightly increased.

The use of steel beams for supporting the roof, with vertical side walls, as shown in Plate VII, Figs. 9 and 10, is becoming quite common, and is probably the best construc

tion for soft ground with limited head-room. Fig. 10 is adapted to storm-water only, or to a flow of house-sewage never less than 15 inches deep. The egg-shaped sewer in Fig. 9 is intended for the house-sewage, the larger channels for storm-water.

Plate VIII, Figs. 2 and 3, show substitutes for egg-shaped sewers where the head-room is contracted. In Fig. 3 the semicircular invert should be sufficiently deep to admit of carrying the maximum house-sewage flow, that the sloping benches may not be fouled by it. Fig. 2 is especially adapted to an exceedingly variable house-sewage flow, as from a factory district whose Sunday and holiday flow is inconsiderable.

Plate VI, Figs. 5 and 9, Plate VII, Figs. 4, 5, and 10, and Plate VIII, Fig. 1, are best adapted to storm-sewage only, although they may be used as combined-sewer mains if the depth of the house-sewage flow is never less than 4 to 6 inches at the shallowest part, and the velocity is then sufficient. Plate VI, Figs. 1, 6, 7, and 8, are intended for house-sewage only. In Fig. 7 the flat invert is permissible owing to the constant depth of the sewage flow, which consists of intercepted house-sewage from a number of residence suburbs.

Plate VI, Figs. 2 and 3, Plate VII, Figs. 1, 2, 3, 6, 7, and 9, Plate VIII, Figs. 2 and 3, are intended to act as combined sewers. In Plate VII, Figs. 5 and 6, the side bench is horizontal, that it may serve as a sidewalk for sewer inspectors and cleaners.

The circular or egg-shaped form demands for strength a solid support under its invert. Where the soil is clay or firm loam, or a mixture of these with sand or gravel, or rock easily shaped, such a sewer may be built with walls of uniform thickness, the invert bearing upon ground shaped to receive it. If the ground is not firm, however, or cannot be readily shaped, the sub-invert spaces must be filled with concrete,

brick, or stone masonry, as in Plate VI, Figs. 3, 5, 6, 8, and 9. If the arch is of such dimensions that the horizontal thrust becomes more than the soil can receive without yielding, then the side walls must be designed to receive this thrust, as in Plate VI, Figs. 5, 6, 8, and 9. The general principles of arches apply, of course, to arched sewers, one of the most important being the necessity for stiffness of the haunches.

The circle, as has been stated, is the most economic shape for a sewer when the invert requires no backing. When this is necessary, however, the circle becomes an expensive shape, and the most economic is one with vertical side walls and bottom flat or conforming generally to the shape of the trench bottom. This is seen by an inspection of Plate VI, Figs. 6 and 8, Plate VII, Figs. 4 and 10. It is for this reason that most of the flat-bottomed sewers are built. Permanency of construction demands a covering for timber platforms, which are liable to abrasion and also to rotting away. This covering, forming the sewer bottom, is usually given a curved form, as in Plate VI, Fig. 5, or a sloping one, as in Plate VIII, Fig. 1, for two reasons: to concentrate small streams and decrease deposits, and to give strength to the bottom to resist the upward pressure which will exist when the soil is soft mud, quicksand, or similar material.

The materials of which sewers are commonly composed are brick, stone, and concrete masonry, cement and vitrified salt-glazed pipe, and, under special conditions, cast- or wrought-iron or steel pipe.

Stone and brick masonry is usually built up in cement mortar, and cement is always used for concrete. The stone masonry is usually rough, but compact and well-built, rubble. In arches brick is usually employed, as being cheaper and also stronger unless the stone are carefully dressed. The interior surface of the sewer, when this is built of stone, is usually