Page 354 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SITE UTILITY SYSTEMS

6.76 CHAPTER SIX

Manholes can be constructed of brick, precast concrete sections, poured (cast-in-place)
concrete, and concrete block. The most common method of construction is with precast
sections called risers. The typical precast manhole consists of a base, riser sections, and a
conical top section ready for installation. The sections fit together with tongue-and-groove
joints with gaskets used in the joints to make them watertight. Gaskets should be installed
in accordance with manufacturers’ recommendations regarding lubricants, cements, and
other special installation requirements. Risers are available in a large variety of sizes and
shapes.
Poured-in-place or precast reinforced concrete structures must use concrete with a com-
pressive strength of 4000 psi after 28 days in accordance with ASTM C-478. Manhole steps
should be installed into forms. A concrete base should be provided under the structure.
Rebars must conform to ASTM A-615, grade 40, and wire fabric, to ASTM A-185.
Brick and block manhole walls should be constructed 8 in (20 mm) thick for up to 8 ft of
depth, and 12 in (30 mm) for greater depths. The outside of the manholes should be parged
(coated) with at least one coat of cement mortar at least 1/2 in thick for protection against
deterioration. Two coats are often used. For additional protection in wet soils, a coating of
coal tar epoxy is often applied over the parging after it has dried for 30 days.

Steps
Manhole steps are generally constructed of cast iron or aluminum. Common step diameters
are three-quarters in (19 mm) or 1 in (25 mm). Width is 16 in (40 cm) if it is desired to have
both feet on one rung; 12 in (30 cm) rungs are the most common size. Steps are spaced 12 in
(30 cm) to 16 in (50 mm) apart, with 12 in the most common. Good practice is to have a
distance of approximately 6 1/2 in (16 mm) from the wall to the inside of the rung.

The Base and Channel
Structures are built on concrete bases or footings. For precast manholes a recessed center is
provided to receive the male tongue and groove joints of a riser section. The invert channel
is formed either by shaping poured concrete after the drainage lines have been installed or
by a cast iron pipe that has the top cut away after installation.
A channel is the lowest internal part of the manhole and should be a smooth continua-
tion of the pipe. It provides a U-shaped open flow path for the liquid. Its height should be
at least one-half the pipe size for 12-in (30-cm) pipe and three-quarters the height for 15 in
(38 cm) and larger.

SIZING THE SANITARY SEWER

The sanitary sewer pipe size is selected after calculating the peak flow rate, determining the
slope of the pipe, and selecting the sewer pipe material.

Peak Flow Rate Determination
Four factors must be considered when calculating the peak flow:
1. Peak sanitary discharge and flow from fixtures
2. Peak nonsanitary discharge, i.e., drainage and estimated leakage flow from all process,
utility, and manufacturing sources, as well as blowdown and similar sources

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