1 March 1997
SEWER SYSTEM LAYOUT AND APPURTENANCES
a. Requirement. Manholes are required at junctions of gravity sewers and at each change in
pipe direction, size or slope, except as noted hereinafter for building connections.
b. Spacing. The distance between manholes must not exceed 120 meters (400 feet) in
sewers of less than 450-millimeters (18-inches) in diameter. For sewers 450-millimeters
(18-inches) in diameter and larger, and for outfalls from wastewater treatment facilities, a spacing
of up to 180 meters (600 feet) is allowed provided the velocity is sufficient to prevent
sedimentation of solids.
c. Pipe connections. The crown of the outlet pipe from a manhole will be on line with or
below the crown of the inlet pipe. Where conditions are such as to produce unusual turbulence
in the manhole, and especially where the size of the outlet pipe is to be smaller than the inlet
pipe because of the availability of a more favorable slope, it may be necessary to provide an
invert drop to allow for entry head, or increased velocity head, or both. Where the invert of the
inlet pipe would be more than 450 millimeters (18 inches) above the manhole floor, a drop
connection will be provided.
d. Frames and covers. Manhole top elevations will be set to avoid submergence of the cover
by surface water runoff and ponding. Where this is not possible, watertight covers will be
installed to prevent storm water inflow. Bolting or locking devices will be included on covers to
prevent unauthorized entry in areas designated by the using agency as secure. Frames and
covers must be sufficient to withstand impact from wheel loads where subject to vehicular traffic.
Covers of a nominal 30 inches or larger diameter shall be installed where personnel entry may
e. Design standards. Where suitable Federal, Army, or Air Force standard drawings and
specifications exist, they will be used for design of manhole structures, unless a special design is
required. The following construction practices will be required:
- Smooth flow channels will be formed in the manhole bottom. Laying half tile through the
manhole, or full pipe with the top of the pipe being broken out later, are acceptable alternatives.
- For manholes over 3.5 meters (12 feet) in depth, one vertical wall with a fixed side-rail
ladder will be provided.
- Drop connections will be designed as an integral part of the manhole wall and base.
- In areas subject to high groundwater tables, manholes will be constructed of materials
f. Materials of construction. The primary construction materials to be used for manhole
structures are precast concrete rings; prefabricated PVC, high density polyethylene, and
fiberglass units; and cast-in-place, reinforced or nonreinforced concrete. In the past, most