Manholes are the most important infrastructure objects for both sanitary and storm sewer systems in any new development or subdivision. To maintain unobstructed flow, they must be constructed properly. Proper inspection ensures long life and protects the city or county from future problems.
“Having an inspector on the construction site while infrastructure is being built is the best way to ensure everything meets current state standards,” said Dale Vandersommen, civil engineer with the city of Lorain, Ohio. “Our inspectors stay on the job when pipe is laid and manholes are built.”
The inspector should meet with the contractor before work begins and find a safe spot to observe construction. The materials used to construct the manhole should already be onsite. This is so the inspector can examine these materials and compare them to the construction plans for proper type and size, and to check for damage that may have occurred during shipping or unloading.
An excavator is typically the contractor's choice for manhole construction. Working off the stakes set by professional surveyors, the operator will dig to the proper depth or elevation for the invert of the manhole. The contractor must excavate a safe trench for the pipe fitters to work in. The contractor should either use substantial bracing, such as a trench box, or cut the trench to the angle of repose in accordance with Occupational Safety and Health Administration standards.
Once the desired depth has been reached, the operator will lay a bed of No. 57 limestone on the floor of the trench. A worker makes sure the bed is level, then signals for the operator to lower the manhole into the trench. After helping to set and level the manhole, the worker will then connect the sewer pipe to the invert opening. Once connected to the sewer pipe and centered, the grade of the invert will be checked again to be sure it is correct.
Each barrel of the manhole will then be lowered—one at a time—onto the invert or base level of the manhole. A ring seal is installed between each barrel level to keep infiltration of groundwater to a minimum. As each barrel is lowered into place, the pipe fitter will ensure that the ladder steps, which are either embedded in the walls of the manhole at the factory or grouted into predrilled holes onsite, are placed above each other from barrel to barrel. This guarantees that the ladder will be contiguous from the manhole cover entrance to the invert or flow level for cleaning or inspection in the future. Each level is installed the same way until, finally, the cone of the manhole is set in place.
The entire structure will then be surrounded by No. 57 limestone as it is backfilled, and the contractor will grout all the seams from inside of the manhole effectively eliminating infiltration of groundwater into the sewer system.
With a digital camera, the inspector can record the construction process as it is taking place and attach the pictures to his or her reports and field notes for future reference.
—Schrull is superintendent of water distribution with the city of Lorain, Ohio.