ELG. 9. DOUBLE STORM CHANNEL AND HOUSE SEWER; BRUSSELS.

UNDER RAILWAY TRACKS.

FIG. 8. OLD LONDON "SEWER

OF DEPOSIT."

lined with a 4-inch ring of brick, because a brick surface can be more easily made smooth than can stone masonry (see Plate VI, Fig. 9). If much wear is anticipated smoothdressed granite or trap blocks are frequently used as invertlining (see Plate VI, Fig. 8).

Where the foundation is yielding a concrete base is frequently used under the sewer, as in Plate VI, Fig. 8, Plate VII, Fig. 9. But if it is soft a platform or even piles should

be used under the concrete.

Sewers built entirely of concrete have been used in Europe very extensively and are coming into use in this country. In many localities concrete is cheaper than rubble or brick masonry. If well made a concrete sewer is both stronger and tighter than a stone or brick one, and can be made more durable than many kinds of stone or brick. The wearing-surface should be given a smooth coat of rich Portlandcement mortar inch to 2 inches thick, or a lining of hard brick, which is probably better owing to the liability of cement coatings to separate from the body of the concrete (see Plate VI, Fig. 6; Plate VII, Figs. 1 and 2).*

If arches of small radius are built of brick-work laid with radial joints much cement is used, the arch is often weak, and the inner surface a polygon in section rather than a curve, unless brick especially shaped are used. If laid well such arches are also expensive in labor. To meet these objections, which apply particularly to inverts in egg-shaped brick sewers, invert-blocks of vitrified clay have been used. There are objections to these, the principal of which is that a joint entirely through the sewer is made, and where the hydrostatic head is greatest, which is almost sure to permit the leakage of water into or out of the sewer. They are also rather expensive, and are but little used now. A section of such a block

is shown in Plate VI, Fig. II.

A better plan for constructing short-radius inverts is by *The use of expanded metal with concrete has recently been adopted with success for many large sewers.

the use of concrete or brick, lined on the inside with vitrified sewer-pipe split into thirds, which is approximately the arc of the small invert-circle in the egg-shaped sewer. Such a construction is shown in Plate VIII, Fig. 2. This construction is also well adapted to such sewers as are shown in Plate VII, Figs. 2, 6, and 7, Plate VIII, Fig. 3.

Whole vitrified pipe are used for lining to circular sewers up to 42 inches diameter, when the pipe is not used alone on account of the additional strength or tightness of joints required.

There is no fixed rule for the thickness of sewers, which depends upon the shape and diameter of bore, the material, the pressure received from the surrounding soil, and other circumstances. Brick sewers less than 30 inches diameter are frequently made but one ring-4 inches-thick; from this up to about 60 inches, 2 rings or 8 inches thick; from this up to 120 inches, 3 rings or 12 inches thick. This applies to the arch more particularly, unless the surrounding ground is very firm, when the invert may be made of equal thickness, or even 8 inches thick only when the arch is 12 inches or more thick. Some engineers never use less than two rings of brick in a sewer-arch; some use one ring up to diameters of 3 feet or The latter may give sufficient strength against crushing, but is hardly stiff enough to resist distortion except under unusually favorable circumstances.

more.

The thickness of the side walls, when these are vertical, must be such as to enable them to withstand the pressure of the soil without or of the water within the sewer when it is full; also to receive the thrust of the top arch when the soil is not capable of doing so.

When two sewers intersect one or both should be curved in the direction of flow of the other. If one or both are small the curve may be made in a manhole (Plate VIII, Fig. 5). If one is many times larger than the other the curve may be

omitted, the branch making an angle of 45° with the main sewer at the junction. Where they are each larger than 30 to 36 inches diameter the intersection should be made by bringing the two barrels gradually into one. This will require considerable skill in both design and construction when the tops and inverts are both arched. When the top is a girder construction the plan is much simplified, and still more so if the bottom also is flat. The crown of the sewer a short distance below the junction should be as low as that of the lower of the two sewers a few feet above it. A plan of a junction of two circular sewers is shown in Plate VIII, Fig. 6. If the head-room is limited the plån shown in Fig. 7 may be used. In Wilmington, Del., the junction of two 6-foot and a 10-foot sewer forms a chamber which is roofed with countergroined arches.

ART. 46. PIPE SEWERS.

Pipe is ordinarily used for sewers up to 18 or 24 inches diameter. Above this up to 42 inches vitrified clay pipe is sometimes used, but many engineers are doubtful of the strength of the larger sizes against crushing. The smaller sizes up to 18 or 24 inches, when made of good clay well burned, are sufficiently strong for ordinary locations, although the "double-strength" pipe (having a thickness of shell the diameter) is recommended rather than those of the standard thickness, which is less than the diameter by a difference which increases with the diameter. It has so far been found impracticable to make good, sound, symmetrical clay pipe with shells much thicker than the diameter. It is probable that if this thickness be maintained the largest sizes of pipe are amply strong for ordinary circumstances.

In many instances where vitrified clay pipe has been crushed in the ground it has been found that this was probably