(a) Location. Manholes for storm sewers shall be placed not more than 300 feet apart whenever possible. The maximum distance between manholes shall be 350 feet for sewers less than twenty-four inches in diameter, 400 feet for twenty-four inches to thirty inches, 450 feet for thirty-six inches to forty-two inches, and 500 feet maximum for forty-eight inch diameter sewers and larger. Manholes shall be placed at every change in grade, direction and pipe size and at the end of each sewer line. When convenient, manholes shall be placed at street intersections.
(b) Design. Manhole design shall be in accordance with Figure 1, following this chapter, and the following specifications:
(1) Diameter. The minimum inside diameter of all manholes shall be forty-eight inches.
(2) Wall thickness. The wall thickness of manholes constructed of various materials and at various depths shall be as follows:
Depth Brick Precast Concrete
(ft.) (in.) Concrete Block
(in.) (in.)
0 to 10 8 5 6
10 to 16 12 5 12
16 to 25 16 5 18
(3) Precast manholes. Precast concrete manholes shall conform with the appropriate ASTM specification according to depth. Brick or block shall be laid to a minimum height of twelve inches above the crown of the pipe. A minimum of one foot of concrete blocks or bricks shall be placed on top of the cone section of all precast manholes for adjustment purposes.
(4) Manhole bottoms. All manholes shall rest on a six-inch, 3,000 psi, concrete slab which shall protrude a minimum of six inches beyond the outside diameter of the manhole. Precast reinforced concrete slabs may be used and shall be bedded on a minimum amount of compacted and leveled fill sand over undisturbed earth.
(5) Manhole steps. Steps for block and brick manholes and for precast manholes shall be according to standard engineering practices as specified by the Division of Water.
(6) Manhole frame and cover. Manhole frame and covers shall be according to standard engineering practices as specified by the Division of Water. Manhole covers shall be set at finish grades whenever possible. Finish manhole cover grades shall be shown on the plans.