How to assess and repair manholes with misaligned rungs, leaks and section damage from improper backfill operations.
By Gary K. Munkelt, P.E.
Drawings by Sue McCraven, NPCA
Rungs are installed directly below the manhole cover or hatch to provide access to the inside, and so it is essential to ensure that they line up from section to section during manhole installation. When the rungs are not in line, new holes must be drilled, new rungs installed and old rungs removed.
PROBLEM A: During installation, rungs are not properly aligned from section to section.
Most precast concrete structures on sanitary sewer projects must be watertight. Occasionally, leaks occur at the section joint or at the pipe-to-manhole connection. These leaks are easily repaired with chemical gel that expands when exposed to water. To repair, drill a small hole in the area of the leak and pump the chemical into the hole. When the chemical is in contact with water, it expands and provides a permanent, resilient seal. Use chemical gel in accordance with the manufacturer’s instructions.
PROBLEM B: Water leaks can occur at joints or at pipe-to-manhole connections.
The correct way to place backfill is to alternate compacted 12-in. to 18-in. lifts on both sides of the manhole. This allows the soil load from one side of the manhole to be resisted by the soil load from the opposite side (See graphic at right).
There are situations, however, when contractors will place backfill material on one side of the structure, all the way up to grade. This places loads on the concrete that are different from the design.
PROBLEM C: First, a tall thin structure such as a 48-in. diameter manhole could be tilted, leaving the chamber exposed to leaks from groundwater (See graphic at lower left).
Second, larger-diameter manholes may not tilt but the uneven load places a large force on the shiplap or tongue-and-groove joint. This could cause the upper section to move laterally so that the walls do not form a straight line (See graphic above right).
Third, sometimes the movement is so large that the base unit needs to be replaced. When the movement is small, however, repairs can be made in the field (See graphic below).
Gary K. Munkelt, P.E., is a consulting engineer with Gary K. Munkelt & Associates in North Wales, Pa.
Contact him at [email protected]