Questions from the Field is a selection of questions NPCA Technical Services engineers received from calls, emails and plant evaluations.
Maverick Asks:
Can you provide some extra guidance and an example on how to use the Manhole Sizing Recommendations Resource Guide?
NPCA technical experts answer:
Proper sizing of precast concrete manholes is necessary to ensure the safety, serviceability and structural integrity of the manhole and pipeline system.
Precast concrete manholes must be large enough to accept the maximum pipe size as well as provide a minimum structural leg of 6 inches or more between pipe openings. Anything less than this width may allow cracking, which can lead to leakage, structural distress or durability concerns.
In addition, openings in the wall of the manhole for influent and effluent piping should be 6 inches larger than the outside diameter of the pipe to allow for sufficient annular space between the exterior of the pipe and the interior of the manhole opening to either install a flexible connector or otherwise apply sealing material in the annular space.