Sealing a sump pit


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Old 08-27-17, 12:10 PM
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Sealing a sump pit

I have a 15" diameter sump pit that's been in the house for at least 60 years. For most of its existence it was just that: a pit. For quite a number of years now, we've had a sump pump.
The existing one just died and I bought a replacement, this time an upgrade with a combination primary & battery backup, so I decided to try to do things right. There was a LOT of gunk down there, some sort of pitch-black sludge.

Between digging and pumping it out, I found that there is no real bottom to the pit: it was dug through the foundation and a layer of bricks were put in as a bottom--presumably as a place to rest the pump, though for all I know it predates that. The bricks weren't cut to fit, so rectangular bricks in a round hole means that there are big gaps around the edges. Presumably, the sludge is the bog-like material that sits down deep all around this area and it keeps infiltrating the basin (not great for any pumps).

I'm guessing that the bottom originally wasn't sealed so that water could drain through. Not just (or even mainly) rain infiltration--I leave near a salt bay, and with coastal flooding basements WILL get water in them. But the sump pump is what i want to get rid of that water, and so that open bottom is, AFAIK, only a liability.

So. First, is there ANY reason I shouldn't try to seal that bottom? I.e, does it help relieve hydrostatic pressure? The water table around here is REALLY high, maybe so high that it actually is pretty close to the pit's bottom. During a major rain or coastal flooding it could be pushing upwards through the bottom. I can't imagine that a 15" pit relieves the pressure for the whole foundation slab, but stranger things have been true.

Second, if I were to do so, how would I? Just slather on as much hydraulic cement as it takes to seal it all up (tricky with all the gunk there), or do I need to somehow manufacture a physical object to fit and cement THAT in place? Or something else?
 
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Old 08-27-17, 12:16 PM
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I can't imagine that a 15" pit relieves the pressure for the whole foundation slab,
Oh.... but it can.

A sump pic bottom must remain open. If you were to seal it and the water level outside the foundation rose..... it would be forced out of another area. Usually higher than the pit and in a place where it's not wanted.

Some water may drain thru the bottom of the pit but that wouldn't be a primary function.
 
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Old 08-27-17, 12:24 PM
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So should I dump a few inches of gravel into the pit? That might keep the sludge out but let the water in (getting a plastic basin in might be tricky, due to sizing). Or raise the pump up on some sort of mesh? A long time ago I'd put in some sort of plastic bucket in, cut down to about 3" high but that that was all filled up, too. Here's another look at what the pump was dealing with--at least 4-5" inches worth:
 
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Old 08-27-17, 01:30 PM
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The sludge is mud and grease that leaches into the pit. Yes.... you don't want the pump sitting directly in it. I'd recommend using larger sized crushed rock to line the bottom.

I will usually put the pump on two bricks or a paver.
 
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Old 08-27-17, 01:36 PM
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The pump is on a bunch of bricks; there's so much sludge that it overflowed the bricks. I think the gunk is mud mixed with a load of decaying plant matter. A couple of years ago the town dug up the streets for a drainage project and you could see (and smell) the gunk underneath, especially where they dug up the stormwater catch basins.
 

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Old 08-28-17, 07:27 AM
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Operationally it makes no difference whether a sump pit liner has holes in the bottom.

But ...

Holes in the bottom may let unwanted dirt come in. In your case it did.

No holes in the bottom might, given a high water table, cause the pit liner to float up and out (a rising tide floats all boats) possibly damaging other components and/or allowing the raw sides of the pit to collapse and prevent depressing the liner back into place.

A properly installed and operating (unclogged) perimeter drain system emptying into the 15" pit allows the latter to reduce the pressure for the entire foundation slab and protect the entire basement from flooding. (Sometimes an extremely large slab may require drain pipes going down the middle.)

An ideal pit has a volume of at least 20 gallons below the level the perimeter drain pipes dump in. To achieve this, a diameter greater than 15 inches might be used.
 
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Old 08-28-17, 09:09 AM
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There is no liner, nor are there any drain pipes of any sort. It's simply a 15" hole in the concrete slab in a (relatively) low point in the basement. The pit's walls are solid concrete (not separate at all from the slab). It's been that way since before the Eisenhower administration. Things only drain in from the top (anything flowing over the basement floor into the pit) or up, through the bottom. Nothing from the sides.

Edit: I don't know if this is relevant or just tangential, but, as I said in the first post I live near a bay. I mean RIGHT NEAR, as in I could throw a rock into it from the back deck. So the couldn't actually build a basement by digging down because the water table is maybe 18" down, or not much further.

The foundation slab is built on the ground (though it maybe sank a little below ground level). The cinderblock foundation walls were built on that, and a dirt hill surrounds that, making the basement "in ground" but not "below ground."
 
 

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