Manhole covers lifting with excess rainfall

[N_Cook]

Would anyone have any input on what "pressure" of rainfall in a combined sewer system is required to lift a steel manhole cover in road surface. Yesterday about 3 inches of water jetting thru a vent hole , no lifting of steel cover. Previously I've seen about 6 inches and cover not lifting. I've seen an impressive full manhole bore width of geyser shooting 30 foot into the air , having discardeded the cover some distance. So about what height of jetting , ie inferred underlying pressure of water, would indcate imminent lifting of a cover, perhaps wobbling? So somewhere between 6 inches and 30 foot , if anyone could narrow it down a bit. I appreciate that because of the way air is expelled from sewer pipes in extreme circumstances and the change in boundary friction with the pipe surface at the point of going to 100 percent capacity with water, the pressure can rise very rapidly, but prior to that or not getting to that , ie water still flowing with air gap at the top of the pipe. All from heavy rainfall on high ground getting into the sewer system passing through lower ground. Substantial heavy road trafficed grade of steel cover , like these

[Martin Brown]

I doubt there is any warning since one moment it isn't moving, held in place by accumulated grit and grot and once it starts to move at all dynamic friction is much lower and it accelerates accordingly.

I have seen fountains about 6' high without the things lifting and the one at the bottom of my drive would sometimes lift with only about 3' head of water behind it since the field drains carry a hell of a water volume after a serious rainstorm and hits it with a big thump. I think it might have a latch on now or something since the last repair when it was broken after jumping onto the road it hasn't done it for ages...

I'm not sure if it is the water hammer effect or stored energy in compressed air trapped in the system that makes the difference. One thing is certain - once there is enough water on the ground not to be able to see where you are putting your feet you have to assume that manhole covers may have lifted. Falling into one can be lethal.

Guestimate the weight and area double it for stiction and I would think that won't be too far off the mark.

Speed the water hits it may well be more important than static pressure.

[newshound]

Martin is right, it's all about stiction in the housing

"Light, domestic" cover, 25kg, 500 mm diameter, 0.18 psi "Heavy, road type", 100 kg, 900mm diameter, 0.22 psi.

1 bar or 15 psi is about 30 feet of water, so this corresponds to only a few inches water gauge.

[Andrew]

High pressure water operated all the LOndon lock gates, the bed lifts at Barts hospital, and I believe lifts at many establishments

[N_Cook]

Assuming the utility company lifts these covers once a year for "backwards" jetwashing and gravel suctioning where required, that seems about how long they go round. But road grit and road traffic can soon wedge all that grit in really tight. For a heavily trafficed road, but not a worked loose rattling cover, then looks like it could be only 1 foot fountains as a minimum height thru the vents , before shifting.

[Martin Brown]

There must be quite a bit more stiction than I had allowed for then because I have definitely seen a 2' fountain come out of mine without it lifting. I expect because it is jammed in place by bits of gravel from the road, twigs and/or rust expansion.

I think the highest fountain I have ever seen one without one lifting was ~3' in Northallerton at the roundabout near the hospital. It happens there most years after a good downpour or thunderstorm.

[newshound]

Stiction is a funny thing. You would normally expect water pressure to give a pretty good "square" push on the cover. Assuming the cover is properly engaged in the housing, you don't need much misalignment to start adding friction there as well. But rising flood water won't apply a "shock load" and that is sometimes what is needed to get things moving. No simple answer!

[Martin Brown]

I think that may well be it.

When mine does lift it is probably because it has been hit by a slab of fast moving water that entered the field drain very quickly and then ran down a decent longish slope so it has real momentum. It takes a particular sort of quick thunderstorm torrential rain for it to happen.

Ordinary rain it is just a steady stream that you can hear but no more.

[N_Cook]

Going by my water meter lid ,set in the pavement, but Tesco vans etc regularly drive over it. Seems to be a wedging effect of thousands of bits of road grit, each vehicle passage and flexing of cover, some drop a bit further into the gap , repeated each vehicle passage .

[newshound]

Agreed. Also, the grit will fracture and the "dust" will tend to build up in the spaces.