Reconstructing a manhole refers to the task of refurbishing an existing manhole structure by raising or lowering it’s casting and structure top from its initial roughly placed elevation to line up in harmony with the adjacent grade, pavement, curb, etc.
Reconstructing a manhole is different from adjusting a manhole in that is done to an existing structure where the adjacent proposed grade will be quite a bit higher or lower than the existing grade, or other major structural work will be done to the structure. The standard definition for raising or lowering a manhole more than 12″ is a ‘reconstruction’.
Reconstructions are also done if the existing structure is deteriorating or for some reason considered unstable. In this case the engineer may require that the structure will be removed down to the nearest sound area, but usually not past the lowest springline, and built back up with either a precast riser or brick or solid block and mortar.
Lastly, anytime an existing manhole casting/top elevation requires considerable lowering, it is typically considered a reconstruction. Lowering a precast concrete structure is commonly approached by:
-removing a precast concrete riser or top
-sawcutting the structure top horizontally
-or making vertical cuts (about 6-10″ apart) along the perimeter of the top of the structure, then jack hammering them off piece by piece.
Lastly, depending on whether the manhole is within an area with vehicle traffic, a ‘concrete collar’ may be required within the reconstruction scope. This is also typically the last task done when installing a new manhole and is often lumped into the overall task of ‘reconstruction’.
A concrete collar is a cast in place concrete section formed and poured around the casting giving additional strength and reinforcement to the manhole from vehicle traffic. The concrete collar also helps resist the entire assembly from shifting or moving with vehicle traffic or freeze thaw conditions.