Section 350. Manholes

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.

Historical Notes

Former 12VAC5-581-410 derived from Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-350, Virginia Register Volume 20, Issue 09, eff. February 12, 2004.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.