Why is my exterior sump pump working itself out of its install pit?

January 3, 2008 11:58 AM

Plumbing enabled folks please read on. My exterior sump pump installed at bottom of the stairway landing outside my walk down basement is working itself out of its hole. I need a sanity check on the contractor's planned repair.

See this previous thread for background and pictures. I noticed last week that the sump pump had risen several inches above the surrounding concrete pad. I'm guessing that the ground under the pump froze, and ice being denser than water it pushed the pump up. The contractor that did the work came by today and will be back Saturday to repair the problem. His solution is to jack hammer out the hole a little wider to allow some room for ground expansion, and then to caulk it to try and keep excess water from getting under it.

Does this sound like a decent plan? I'm under warranty so this work is not costing me anything. The stairwell is about 8 feet deep at the bottom, so the bottom of the hole where the sump pump is installed is about 10 feet below the general grade of the yard. I live in VA and I'm a little surprised that the ground under the sump pump froze enough to cause this problem. It simply has not been that cold here.

And yes, I tried the obvious stand on the pump and try to work it back down solution. It's not going back down on its own.

posted by COD to home & garden (11 comments total)

Do you have pictures of the current situation? I guess from your previous thread's images I can't imagine where the sump pump went and where it is now.
posted by yellowbkpk at 12:28 PM on January 3, 2008

That sounds like a way for your contractor to put together a cheap fix that won't cost him much time -- he would have installed it that way in the first place, I'd bet, if it was appropriate for your situation. Pictures would definitely be helpful. Is it just a pump, stuck into a hole in the concrete? Does it have an open area surrounding it, with gravel and landscape fabric, or is the pump in direct contact with the soil?
posted by Pantengliopoli at 12:42 PM on January 3, 2008

I don't think that diagnosis is correct, yours certainly, or his if I understand it correctly. Neither is the planned repair.

There has not been enough freezing to lift your pump out of the hole unless there has been a hard sheet of ice on the concrete landing (and even then probably not).

But there is another force that can lift your pump: the back pressure of the ten foot water head that develops when your pump is pumping the water up to the level of your yard. A 33 ft. head of water is 1 atmosphere, 14 pounds per square inch, so the static back pressure you pump must work against is roughly a third of that, say 4.5 lbs./ sq in., and if you have a 1in diameter pipe out of your sump pump, that means ~4 lbs. static force. The dynamic force will be much greater than that, however, probably a significant multiple of that, depending on friction and total mass of water moved per second, and it is moreover a force exerted cyclically-- ideal for backing something out of a hole.

The problem is, then, that the pump was not anchored to the slab or the ground underneath well enough.
posted by jamjam at 12:43 PM on January 3, 2008

After posting it occurred to me that freezing at the outlet or in the outlet pipe would also produce really big back pressures, essentially the total pressure your pump can exert at maximum (at which point it should be turning itself off automatically). That's a more complicated repair.
posted by jamjam at 12:55 PM on January 3, 2008

Pictures of the sump pump...

Pic 1
Pic 2

The pump is not anchored at all, just sitting in the hole in the concrete. This is all good info, thanks.
posted by COD at 1:22 PM on January 3, 2008

Is that the sump pit itself that has lifted out of the ground?

It is possible that it is actually floating on ground water that exists under the pit. Some sump pits have holes or even no bottom to ensure that this doesn't happen.

I am thinking that the water has floated the pit, and the backfill slid underneath - this is why you can't push it back down by standing on it.

Perhaps the contractor should pop some holes in the bottom of the pit to allow the water to flow up and be pumped away.

I don't think that jamjam's answer is entirely correct in this case (particularly if it's not just the pump that has lifted)

Please let us know if it is the pump itself or the entire pit that is lifting out of the ground.
posted by davey_darling at 1:37 PM on January 3, 2008

I agree with davey_darling.
posted by jamjam at 1:41 PM on January 3, 2008

I'm not sure that ice is what is pushing the pump up. Ice is less dense than water (ice floats). Ice will crust over the top of water not under it, so I would have the contractor check to see if ice really is the problem before attempting any repairs
posted by estronaut at 1:59 PM on January 3, 2008

That is the entire plastic pit coming up. I just pulled the lid and took a look. There are few inches of standing water in the bottom so it's hard to tell for sure, but it looks like a solid plastic bottom on the pit. I didn't see any holes. Also, the fact that I've got standing water after the coldest 48 hours of this winter I think tells me that that there is no way that freezing last week when it was warmer was the issue. Water underneath floating the pit combined with backfill sounds like the problem, which is also an easier fix than jackhammering a bigger hole.

Thanks for all the help.
posted by COD at 2:05 PM on January 3, 2008

Frost heave can certainly move things around, but it doesn't sound likely. Is there gravel in the bottom of the pit?
posted by electroboy at 2:08 PM on January 3, 2008

From everything I see here, water is getting underneath the plastic pit, freezing, and frost heaving the pit up. The water isn't necessarily pooled under the pit: it's freezing with the dirt/gravel in it. If you want to see the power of frost heaves, drive in New England in the winter :)

You need to: pull the pump and pit, clean out the hole, and find out how water is getting under the pit, then put it all back together.

It's my guess that drilling some holes in the bottom of the pit will allow water that infiltrates underneath it to filter back into the pit. I would put some landscape fabric down underneath the pit first, though, to prevent dirt from migrating into your pit (and undermining it, creating another problem).

But I'm not an expert in these things.
posted by jdfan at 7:45 AM on January 4, 2008