What is manholein construction?

Subtract the lowest invert elevation from the rim elevation, both printed on the utility plan or the profile sheet. That vertical dimension sets the number of precast barrel sections, the excavation volume, and whether the dig needs shoring, since OSHA requires a protective system in trenches 5 ft and deeper. Estimators group manholes into depth classes or carry a base price plus an adder per vertical foot beyond a standard depth.

From the bottom up: a base section with the flow channel formed into it, barrel sections stacked to reach height, an eccentric cone or flat slab top, grade rings to fine tune the rim elevation, and a cast iron frame and cover. Sanitary structures add gasketed joints and flexible pipe boots at every penetration. Each component is a line on the precaster's quote, which is why depth drives the structure price.

The structures look similar, but sanitary manholes must be watertight: gasketed joints between sections, boots where pipes enter, and in many jurisdictions a vacuum or exfiltration test before the line is accepted. Storm manholes typically carry looser joint and testing requirements. The cost difference is real, so estimators keep storm and sanitary structures on separate lines even when both appear on the same utility plan.