22.02 What is important about underground tank manhole frames and covers?

Manhole frames and covers are designed to protect tank sumps and risers from traffic loads above the tank. The important functional characteristics are (a) resist traffic loads by design to minimum H-20 traffic load, (b) transfer load to concrete pavement and isolate load from tank or sump, (c) direct surface water away from tank sumps, and (d) allow access to the tank by lifting and removing the cover.

Proper installation of the manhole frame and cover is critical. The top of the frame should be set about 24-50 mm above the surrounding pavement elevation with transition to the pavement elevation over an approximate 1 meter radius. Since many drainage manhole frames are set low to collect water at a low point, it is important to indicate for the concrete contractors the importance of this requirement. Another aspect of the installation is to note the isolation of the frame from the tank sump by a minimum of 75 mm, so that if any settlement occurs, the frame will not bear on the tank sump.

It is highly recommended that manhole covers be constructed of lightweight composite materials so that personnel can remove the covers without injury. An alternative is a cover designed that is hinged with a spring assist.