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Adjustable Manhole Frames Avoid Costly Problems
“First,” he said, “the standard concrete grade ring is placed over the manhole opening. The support ring then is placed on top of the grade ring and the adjustable frame on top of the support ring. Using a 5/16-inch Allen or socket wrench, set screws built into the frame are used to adjust the frame’s elevation to the required elevation and grade.”
Next, ½-inch aggregate 20-30 MPA (megapascal) concrete is placed around grade rings, the adjustable frame and between the frame and support ring.
“The entire process takes 15-20 minutes,” Nadasde said. “Old, conventional methods would take close to an hour and in most cases structural integrity would be questionable with a very high failure rate, resulting in costly maintenance for years after. The new adjustable frame and support ring are far superior. They provide workers with a product that is easy to work with and provides long-term structural strength.”
Setting frames properly
Manhole frames need to be set accurately with a road surface on longitudinal grade and with cross fall. Yet, Nadasde said, conventional manhole frame design and adjustment procedures that are typically used to make adjustments are not adequate to prevent failure.
“Manhole frames are heavy, cumbersome, and frustrating to adjust,” he continued. “Workers are required to lift the frame on and off the manhole opening several times while trying to find the right thickness of wood using rocks, wood shims, old pieces of broken asphalt or concrete, pieces of scrap metal from fabricating shops and all kinds of other materials. Even if grout is used, such materials can deteriorate or leave point loads on grade rings which ultimately damages and weakens the manhole chimney structure. Historically, adjusted frames in this manner are very unstable and get knocked off adjustment by road building and paving equipment. It’s hard to believe, but without improvements, our industry has been setting frames like this for more than 100 years.”
In addition, he said, current specifications for adjustments are insufficient and do not consider the importance and diversity of the task.
Using the new adjustable frame and support ring provides the means to achieve perfect elevation and grade, minimizing inflow and reducing damage from traffic.