Fig. 46.—Tide Gate.
Outlets are protected against wash and the impact of debris by the construction of deep foundations and heavy protecting walls. Although the construction of an outlet in a slow current or a back eddy would avoid danger from wash and debris, the discharge of the sewage into the most rapid current possible aids in the prevention of a local nuisance. A row of batter piles on the upstream or exposed side of the sewer is desirable, or it may be necessary to construct a break-water to prevent the wash of the current from dislodging the pipe. These break-waters are low dams of wood or broken stone, more or less loosely thrown together. The backing up of water into the sewer can be prevented by constructing the sewer above the outlet on a steep grade. Where this is not possible the use of tide gates will be helpful. A tide gate, one form of which is shown in Fig. 46, is a special form of check valve placed on the end of the sewer.
Fig. 47.—Sewer Outlet on a Trestle.
Eng. News, Vol. 49, p. 9.
Sewer outlets are sometimes constructed on long trestles in order to reach deep or running water. Such a trestle is shown in Fig. 47. In Boston the outlet sewers are submerged under the harbor and discharge through outlets well out in the tidal currents. The sewage is discharged under pressure and the pumps are operated at some of the stations only at such times as the tidal currents will carry the sewage away from the harbor. It is not always necessary in a combined sewerage system to carry the storm flow to a distant submerged outlet. A double outlet can be constructed as shown in Fig. 48 in which the dry weather flow is carried to the channel in a submerged sewer and the storm flow is discharged on the bank.[44] Cast-iron pipe should be used for submerged outlets as the sewer is subject to disturbance by the currents, anchors, ice, and other causes.
Fig. 48.—Dry Weather and Storm Sewer Outlet at Minneapolis, Minnesota.
Eng. Record, Vol. 63, p. 383.
66. Foundations.—Sewers constructed in firm dry soil require no special foundation to distribute the weight over the supporting medium. In soft materials the lower half of the sewer ring may be spread as shown in Fig. 22, and in rock the pressures on sewer pipes are evenly distributed by a cushion of sand. In wet ground such as quicksand, mud, swamp land, etc., a foundation must be constructed if the water cannot be drained off.
The permissible intensities of pressure on foundations in various classes of material allowed by the building codes in different cities are given in Table 25. These figures are based on the assumption that the material is restrained laterally, which is generally the condition in sewer construction. In the softer materials it becomes necessary to spread the foundations not only to reduce the intensity of pressure, but also to care for the thrust of the sewer arch. The arch thrust may be found by one of the methods of arch analysis, and the haunches spread to care for this, or the sewer invert may be transversally reinforced to assist in caring for this action. Some sewer sections in hard and soft material are shown in Fig. 22 and 23.
| TABLE 25 | |
|---|---|
| Allowable Bearing Value on Soils in Various Cities | |
| From Proc. Am. Soc. Civil Engrs., Vol. 46, 1920, p. 906 | |
| Quicksand and alluvial soil | ½ to 1 ton per sq. ft. for Providence, R. I., ½ ton per sq. ft. for 6 cities |
| Soft clay | 1 ton per sq. ft. for 27 cities, ¾ ton per sq. ft. for New Orleans, 2 to 3 tons for Providence, R. I. |
| Moderately dry clay and fine sand, clean and dry | 2 tons for 7 cities, 1¾ to 2¼ for Chicago, 2½ tons for Louisville, 2 to 4 tons for Providence, 3 tons for Grand Rapids and Los Angeles |
| Clay and sand in alternate layers | 2 tons for 19 cities, 1¾ to 2¼ for Chicago, 3 to 5 tons for Providence |
| Firm and dry loam or clay, or hard dry clay or fine sand | 3 tons for 24 cities, 2½ tons for 2 cities, 2 to 3 tons for Atlanta, 3½ tons for Philadelphia, 4 tons for 6 cities |
| Compact coarse sand, stiff gravel or natural earth | 4 tons for 25 cities, 3½ tons for Buffalo, 3 to 4 tons for Atlanta, 4 to 5 tons for Cincinnati, 5 tons for Denver, 4 to 6 tons for 3 cities, 6 tons for Rochester, N. Y. |
| Coarse gravel, stratified stone and clay, or rock inferior to best brick masonry | 6 tons for 3 cities, 5 tons for 2 cities, 8 tons for 1 city |
| Gravel and sand well cemented | 8 tons for 5 cities, 6 tons for 2 cities, 8 to 10 tons for 1 city |
| Good hard pan or hard shale | 10 tons for 4 cities, 6 tons for 2 cities, 8 tons for 1 city |
| Good hard pan or hard shale unexposed to air, frost or water | 8 tons for 1 city, 10 to 15 tons for 1 city, 12 to 18 tons for 1 city |
| Very hard native bed rock | 20 tons for 5 cities, 15 tons for 1 city, 10 tons for 1 city, 25 to 50 tons for 1 city |
| Rock under caisson | 24 tons for Baltimore, 25 tons for Cleveland |
On soft foundations such as swamps or for outfalls on the muck bottom of rivers the sewer may be carried on a platform. For small sewers 2–inch planks, 2 to 4 feet longer than the diameter of the pipe are laid across the trench, and the sewer rests directly upon them. For large sewers imposing a heavy concentrated load, a pile foundation should be constructed. The foundation may consist of piles alone, pile bents, or a wooden platform supported on pile bents. The load which can be carried by a pile is determined with accuracy only by driving a test pile and placing a load on it. Where piles do not penetrate to a firm stratum the load they will support can be determined by any one of the various formulas, either theoretical or empirical, which have been devised. Probably the best known of these formulas are the so-called Engineering News formulas one of which is:
Reference should be made to engineering handbooks for other forms of pile formulas. The accuracy of all of these formulas is not of a high degree.
The piles are driven at about 2 to 4 feet centers, to a depth of from 8 to 20 feet, unless hard bottom is struck at a lesser depth. The butt diameter of the piles used for the smallest sewers is about 6 to 8 inches. If bents are used, 2 or 3 piles are driven in a row across the line of the sewer and are capped with a timber. For brick, block, pipe, and some concrete sewers, a wooden platform must be constructed between the pile bents for the support of the sewer.
67. Underdrains.—The construction of special foundations can sometimes be avoided by laying drains under the sewers, thus removing the water held in the soil. The laying of the underdrains facilitates the construction of the sewer and reduces the amount of ground water entering the sewer. The underdrains usually consist of 6– or 8–inch vitrified tile laid with open joints from 1 to 2 feet below the bottom of the sewer as shown in Fig. 1. If the sewers are large, parallel lines of drains may be laid beneath them. An observation hole should be constructed from the bottom of the manhole to each underdrain. This hole usually consists of a 6– or 8–inch pipe, embedded in concrete, connected to the drain and open at the top. It is too small to permit effective cleaning of the underdrains, which are usually neglected after construction, and which as a result clog and cease to function. Since the principal period of usefulness of the drains is during construction, their stoppage after the work is completed is not serious. The hollow tile used in vitrified block sewers serve as underdrains after construction, but are of little or no assistance to the draining of the trench during construction.