Page 379 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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10.3 Collection of Sanitary Wastewater 339
distances are not too great. Maxima of 300 or 400 ft (90 or 120 m) for pipes less than 24 in.
(600 mm) in diameter are generally specified, but effective cleaning is the essential crite-
rion. For larger sewers, distances between manholes may be upped to as much as 600 ft
(180 m). Sewers so large that workmen can enter them for inspection, cleaning, and repair
are freed from these restrictions, and access manholes are placed quite far apart either
above the center line or tangential to one side.
The introduction of flexible cleaning devices has encouraged the construction of
curved sewers of all sizes, especially in residential areas. On short runs ( 150 ft or 45 m)
and temporary stubs of sewer lines, terminal cleanouts are sometimes substituted for
manholes. They slope to the street surface in a straight run from a Y in the sewer or in a
gentle curve that can be rodded out. In very flat country and in other unusual circum-
stances, sewers are laid on flat grades, in spite of greater operating difficulties, in order to
keep depths reasonably small and pumping reasonably infrequent.
The smallest public sewers in North America are normally 8 in. in diameter. Smaller
pipes clog quickly and are hard to clean. Vitrified clay or plastic is the material of choice
for small sewers (Fig. 10.5); prefabricated concrete or fiberglass for large sewers. Grit or
other abrading materials will wear the invert of concrete sewers unless velocities are held
below 8 to 10 ft/s (2.4 to 3.0 m/s). Very large sewers are built in place, some by tunneling.
Hydraulically and structurally, they share the properties of grade aqueducts.
Sewers are laid deep enough (a) to protect them against breakage, by traffic shock, for
example; (b) to keep them from freezing; and (c) to permit them to drain the lowest fixture
in the premises served. Common depths are 3 ft (0.90 m) below the basement floor and 11 ft
(3.3 m) below the top of building foundations (12 ft or 3.7 m, or more for basements in
commercial districts), together with an allowance of ⁄4 in. per ft or 2 cm per m (2%) for the
1
slope of the building sewer. In the northern United States, cellar depths range from 6 to 8 ft
(1.8 to 2.4 m) and frost depths from 4 to 6 ft (1.2 to 1.8 m). A 2-ft (0.60-m) earth cover will
cushion most shocks. The deep basements of tall buildings are drained by ejectors or
pumps.

Figure 10.5 PVC (Polyvinyl Chloride) Plastic Sewer Installation
(Courtesy Wikepedia)
http://en.wikipedia.org/wiki/Image:Sansewer.jpg