Page 547 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

JWCL344_ch14_500-554.qxd 8/7/10 8:56 PM Page 505

14.2 Gravity Sewers 505

Backfill
Backfill
Concrete
9˝ clearance 90° cradle
(minimum) 3 d
4
(a) Pipe in earth trench (b) Pipe in cradle

Sheeting
Backfill
6˝ gravel
(minimum)
6˝ gravel
(minimum)

Underdrain
(c) Pipe in rock trench (d) Pipe with underdrain

Natural
ground

Access manholes
for large sewers
Terminal cleanout
Figure 14.6 Sewer Trenches, Access Manholes, and Terminal Cleanouts.
Conversion factor : 1 in. 25.4 mm

The smallest public sewers in North America are normally 8 in. (200 mm) in diameter.
Smaller pipes clog more frequently and are harder to clean. Vitrified clay and plastic are
the material of choice for small sewers, prefabricated lined concrete and fiberglass for
large sewers. Vitrified-clay sewer pipe, 4 to 36 in. (100 to 900 mm) in diameter, is ordinarily
3 to 6 ft (0.90 to 1.83 m) long. Unreinforced-concrete sewer pipe, 4 to 24 in. (100 to 600 mm)
in diameter, is generally 2 to 4 ft (0.60 to 1.22 m) long. Preformed joints made of resilient
plastic materials increase the tightness of the system. To reduce the infiltration of ground-
water, sewers laid without factory-made joints in wet ground must be undertrained, or
made of cast iron, plastic, or other suitable materials. Cast-iron and plastic pipes are long
and their joints are tight. Underdrainage is by porous pipes or clay pipes laid with open
joints in a bed of gravel or broken stone beneath the sewer. Underdrains may serve during
construction only or become permanent adjuncts to the system and discharge freely into
natural water courses. Sewage seeping into permanent underdrains may foul receiving
waters. As stated before, 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 m/s). Very large sewers are