Current through Register Vol. 41, No. 3, September 23, 2024
A. Location.
Manholes shall be installed at the end of each line of eight-inch diameter or
greater; at all changes in grade, size, or alignment; at all intersections; and
at distances not greater than 400 feet for sewers 15 inches or less in diameter
and 500 feet for sewers 18 inches to 30 inches in diameter, except that
distances up to 600 feet may be adequate in cases where adequate modern
cleaning equipment for such spacing is provided.
1. Slightly greater spacing may be utilized
in larger sewers.
2. Terminal
cleanouts may be acceptable in place of manholes, on lines eight inches in
diameter or less, on a case-by-case basis. Cleanouts may be used in lieu of
manholes for collection of settled sewage. Manholes are required where four or
more sewers intersect, or where two or more sewers intersect at depths greater
than eight feet. Cleanouts shall be installed at distances not greater than 400
feet for settled sewage systems.
B. Materials. Manholes shall be constructed
of materials that will maintain structural integrity throughout the design life
of the sewer. Manhole wall and bottom construction shall be such as to ensure
water tightness and the Virginia Department of Labor and Industry, Occupational
Safety and Health Administration (VOSH) requirements may also specify design
requirements. Confined space entry restrictions are to be met. For those
manholes and vertical sections of pipe tees used for maintenance access, safety
slabs or platform benches should be provided at depth intervals of 10 feet or
less as required unless adequate access lifting devices are provided in
accordance with VOSH or other recognized standards. The use of sections of
reinforcing bars as access steps is not recommended for safety
considerations.
C. Features. The
base inside diameter of manholes and vertical pipe tees used for maintenance
access shall be a minimum of 42 inches. The clear opening in the manhole frame
shall be a minimum of 24 inches. Larger base diameters are preferred.
1. The manhole foundation shall be adequately
designed to support the manhole and any superimposed loads that may
occur.
2. The flow channel through
manholes shall be of such shape and slope to provide smooth transition between
inlet and outlet sewers and to reduce turbulence. Benches shall be sloped to
the channel to prevent accumulation of solids.
3. When the flow direction or horizontal
deflection of a sewer line varies significantly, elevation changes may be
necessary to provide for head losses. The minimum vertical change in elevation
from upstream to downstream should provide for a head loss of up to 3 inches or
more, when ninety degrees of deflection is specified.
4. Watertight manhole covers or watertight
manhole inserts shall be used whenever the manhole tops may be flooded for
several hours or more. As a minimum, watertight manhole covers or watertight
manhole inserts shall be used when the manhole top is below the elevation of
the 100-year flood/wave action.
5.
Masonry manholes of brick or segmented block and the nongasketed joints of
precast manholes shall be waterproofed on the exterior with suitable coatings
(e.g., cement supplemented with bituminous).
6. Inlet and outlet pipes shall be joined to
the manhole with a gasketed flexible watertight connection or any watertight
connection arrangement that allows differential settlement of the pipe and
manhole wall to take place without destroying the watertight integrity of the
line connections.
7. Ventilation of
gravity sewer systems shall be provided where continuous watertight sections
greater than 1,000 feet in length are incurred.
8. In accordance with this chapter and
standards contained in this chapter, frames, and covers shall be of suitable
material and designed to accommodate prevailing site conditions. Ventilation,
safety lines, hoist arrangements and other requirements, as necessary for
material maintenance access, should be provided in accordance with VOSH
requirements.
9. A drop pipe should
be provided for an upstream sewer entering a manhole at an elevation of 24
inches or more above the manhole invert unless sewer pipe crowns match
elevations, or as may otherwise be required to conform to the use of standard
fittings in the drop pipe construction. Where the difference in elevation
between the incoming sewer and the manhole invert is less than 24 inches, the
invert shall be filleted to prevent solids deposition. A drop pipe shall be
used when the upstream to downstream invert difference exceeds 24 inches and
the sewer deflects horizontally at a manhole. The drop through the manhole
should be a maximum of four inches for a 90° horizontal
deflection.
D. Leakage
testing. Manholes may be tested for leakage at the same time that gravity sewer
lines are being hydrostatically tested for leakage. For manholes greater than
four feet in depth whose entire depth was not included in the hydrostatic
testing of the sewer line, the manholes shall be tested by exfiltration.
Inflatable stoppers shall be used to plug all lines into and out of the manhole
being tested. The manhole shall be filled with water to the top of the rim. A
maximum 12-hour soak shall be allowed. Leakage shall not exceed 0.25 gallon per
hour (gph) per foot of depth.
1. If air
testing of sewer lines is employed, the manholes shall normally be tested by
exfiltration. Inflatable stoppers shall be used to plug all lines into and out
of the manhole being tested. The stoppers shall be positioned in the lines far
enough from the manhole to ensure testing of the untested portions of the
lines. The manhole shall then be filled with water to the top of the rim. A
maximum 12-hour soak shall be allowed. Leakage shall not exceed 0.25 gph per
foot.
2. Air testing or vacuum
testing of manholes for leakage may be considered on a case-by-case basis. It
is important that the entire manhole from the invert to the top of the rim be
tested.
Statutory Authority
§ 62.1-44.19 of the Code of Virginia.
