EXAMINE WHY A MANHOLE COVER JUMPED OUT OF ITS CASTING

THE TIDDLY WINK EFFECT

REVIEW OF THE EVENT

A vehicle was driving along a main road when it struck an open sewer manhole, lost control and crashed into a tree. The road had been re-surfaced a few months prior. The sewer manhole cover had been raised to match the roadway surface with a casting riser. After the re-surfacing the manhole cover began banging when a vehicle drove over it. A few weeks prior to the crash a neighbor had seen the manhole cover pop out of the casting and roll away. The neighbor found the cover and replaced it . As cars drove over the manhole cover it banged until one night the cover either broke and fell into the manhole, rolled away or someone took it.

Photographs & videos of a tire rolling over an improperly seated manhole cover.

      
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View video of tire rolling over properly seated manhole cover at 30 mph
tire_over_good_mh.wmv
---------------------------------------------------------------View video of tire rolling over improperly seated manhole cover at 30 mph
tire_over_mh.wmv
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Graph of Kinetic Energy v Potential Energy
When the Kinetic Energy exceeds the Potential Energy of the manhole cover, the cover will jump/slide out of its casting.
Manhole_Energy_Motion2.avi
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OBSERVATION AND ANALYSIS

Following were the writer's observations, analysis and engineering opinion as they apply to this case:

A few days after the accident a video was taken of vehicles driving over the manhole cover and it could be seen moving up and down. A pedestrian stood on the cover and rocked it back and forth. Below are the engineering calculations that cause the manhole cover to jump out of the casting.

The graphs below show that as the gap between the bottom of the manhole cover gets larger, the rocking becomes greater and the angular rotation increases resulting in a higher probability of the manhole cover jumping out of the casting. When the red line is above the blue line the cover is jumping out of the casting.

SUMMARY & OPINION

In conclusion, it was my opinion, as a Licensed Professional Engineer, within a reasonable degree of engineering certainty, that the municipality created the condition of the rocking manhole cover by inserting an improper riser ring to raise the manhole cover when the roadway was resurfaced.

 Last update: 12/22/2007 Top of Page