Plastic Sump Repair, Glue/Goo to Use?

[PapaDisco]

I have an Everbilt plastic sump pit with extension ring installed in a concrete basement floor and serves an under slab de-watering system. The pit is about 4' deep with the extension, more depth than needed. Its bottom was drilled with holes and up the sides about 2-3" from the bottom. I want to eliminate those holes as more ground water enters from them than from the drainage pipes, and by a large margin causing the pump to run nonstop in the rainy season. In essence, the deep pit installation manages to just hit the water table in the rainy season, whereas the de-watering pipes are another 3' up and collect no water at all.

My plan is to cut the bottom off another Everbilt sump and slide that as far down as I can into the installed sump; likely 4" from the bottom based on how tightly these things next together.

Here's the question: what's the right sealant/glue/goo to fill the space between the old sump and the new? It has to support the weight of the dual pumps (40lbs) and also be a good sealer against the rainy season water table pressure. Concrete would obviously be supportive but not a great sealant. Is there some elastomeric compound that won't shrink and will also adhere well to the plastic?

 

[wwhitney]

Without comment on the wisdom of letting the water table be higher under your slab/crawlspace/basement, why not just raise your pump up within the existing pit? E.g. setting it on a concrete block. Or if your pump float controls are separate from your pump, just raise the float controls.

Cheers, Wayne

 

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[Reach4]

I would be considering screws to fill the holes. No specific suggestion, but something that does not rust.

For an adhesive I would consider https://www.3m.com/3M/en_US/p/d/b40066983/ or https://www.3m.com/3M/en_US/p/d/b40066971/ during the dry season maybe. I would want to inject whatever sealant I used so that it mushrooms on the other side.

 

[PapaDisco]

The drilled holes are 1/2" diameter. Agree with the strategy of getting sealant to 'mushroom' on the other side but not sure how to execute on that. The soil here is sand and silt; extremely hard when dry and compacted, but water flows right through it during the rainy season.

 

[Reach4]

That is much larger than I was expecting. I drilled 1/4 inch holes, and might choose 3/16 if doing it again, and all holes up high if doing it again for the reasons you cite.

 

[PapaDisco]

There's one school of thought that says to drill holes in your basin to avoid it floating. Another school says the slab will hold it in place. There's another sump of exactly the same size and depth in this floor that wasn't drilled (it's for a future bathroom septic) and in the rainy season it stays perfectly dry inside and of course does not float.

The de-watering system has 6 mil plastic under the concrete, and then 6" of 3/4" gravel and 18-24" deep drainage ditches under the slab perimeter with 3" pipe that drain to the sump we're discussing. With the highly permeable soil here (sand and silt) the water table likes to flow at a very even, flat level and so the extra two feet of depth in the sump, and the holes, end up acting like an unintentional 'well' in the rainy season. Without the holes the water would continue to just flow 4' under the slab all winter long, nothing is reaching up to the drainage rock under the slab.

 

[wwhitney]

Say you want your water level under the slab to never exceed 24" below TOS (top of slab), and the bottom of your sump pit is 48" below TOS. Three possible approaches:

A) No holes drilled in the sump pit below 24" below TOS. Pump with attached float switch sits in the bottom of the sump. When enough water enters through the pipes or the holes, the pump run and leaves little water in the pit.

B) Holes drilled all the way down the sides of the sump pit. Pump with attached float switch sits well above the bottom of the sump, so that the pump will activate when the water hits 24" below TOS. Pump runs to pull the water down to its shutoff level, leaving ~20" (depending on the height of the dead band on the built-in float switch) of water in the bottom of the sump pit after running. If the water table falls below this level, excess water will flow out of the pit.

C) Holes drilled all the way down the sides of the sump pit. Pump with external float switch sits at the bottom of the sump. Float switch is set so the pump activates when the water is 24" below TOS. Off trigger on float switch might be set to anywhere from 28" to 46" below TOS. Some amount of water never gets pumped out of the sump, depending on off trigger setting. If the water table falls below the off trigger level, excess water will flow out of the pit.

So what is the advantage of option (A) vs option (B) or (C)? Both (B) or (C) seem easier to implement given your existing sump pit.

Cheers, Wayne

 

[Reach4]

Good point. Adjusting the tether on the float switch to not turn on until the higher level is reached, you could have much of the advantage of plugging the holes.

 

[PapaDisco]

Last winter I would turn the pump off and watch the flow. Over a matter of two minutes water would bubble up from the holes in the bottom (floor only not the sides) of the sump until it was so full that water started back flowing into the drain pipes at the top of the sump. It was as if the sump was a pressure release spot for the ground water.

My other concern would be a single drainage point encouraging a focused ground water flow that might undermine a footing somewhere as opposed to the broad whole slab drainage system. It's good to keep water as far under the slab as possible, but it's also good to de-water evenly.

Wayne's analysis is a good one, but given my hydrology the main advantage of leaving holes in the bottom is that the pit fully drains at the end of the rainy season, but for 3 months it's pumping like mad. With no holes the pump won't run except in exceptionally high water table years. There's basically a river running 4 feet under this house for 3 months of the year.

 

[Reach4]

A 3/4 inch NPT PVC plug has an OD of 1.05 inches on its widest thread. I don't know if you could cut one down and screw into a 1 inch hole to plug it.

Or maybe you could dig out clearance behind the hole to use the whole plug. I have my doubts this would work for you, but that would be nice if it did.

Maybe a kneedable epoxy could be pushed in after you vacated some area behind the hole, and you could get that to mushroom.

 

[PapaDisco]

I have found online a wealth of rubber plugs of all shapes and have ordered several. The ideal shape would be mushroomed on both sides, but all of those are for panel thicknesses of 1/16-1/8" and the sump walls are a solid 1/4" thick. That said, there are soft single cap plugs that look like I'll be able to fold up the top and get it jammed through, then caulk it and it should be good to go. I'll still insert the cut off bottom half of a new sump basin after the fact and glue that in with polyethylene glue. I'll post up which solution works after I try the stuff on order.

 

[wwhitney]

n essence, the deep pit installation manages to just hit the water table in the rainy season, whereas the de-watering pipes are another 3' up and collect no water at all.

So you want to seal up the holes in the bottom of the pit, and just pump out water that comes in from the de-watering pipes? In other words, let the water table rise another 3' higher than the pump currently allows?

How is that any different from just raising the pump (or its float controls) 3' within the existing unmodified pit? (Semi-rhetorical)

OK, the pump "on" elevation is above the pump "off" elevation. So if the difference is 6", and the pump "off" elevation is currently the bottom of the pit, you'd only move the pump/controls up 2' 6", so the pump "on" elevation would be at the bottom of your entering de-watering pipes.

So it is a bit different, in your proposed configuration the water table can rise to the bottom of the dewatering pipes before anything gets pumped (well, the water table will be a bit higher, as it needs to be to generate flow into the pit). Pumping never lowers the water table below that point, it just pumps out the accumulated water periodically. While just raising the pump as I propose would let the water fluctuate between 6" below the bottom of the pipes and the bottom of the pipes (if 6" is the difference between the pump on and off elevations).

Still, now I think I would caution you against making the change to let the water table be 3' higher all at once. It could affect your basement conditions more than you expect. I'd suggest taking a phased approach: raise the pump 1' for one year, see how much it runs over the rainy season and if any water is entering from the pipes, or just from the bottom of the pit. If there are no problems, and the pump still runs too much, try raising it another 1' for the next year.

Cheers, Wayne

 

[PapaDisco]

Here's an update so y'all know what I did and how it turned out.

I didn't use the rubber plugs; it was just too difficult and too much upside down work to get those into the bottom of the sump (4' down). I vacuumed the sump and the holes and then squirted them full with a rubberized caulk and caulking gun, so it would fill the hole completely. Then I glued squares of thin polyethylene over the rubber before it cured using MMA500 adhesive. This stuff is expensive but is specially designed for low energy surfaces like polyethylene.

After the first heavy rains the ground was fully saturated (neighbor pumps are working overtime) but the water table hasn't risen high enough for my under slab system to be touched yet. The sump, which is 4' lower is also dry with no leaks. So the MMA500 is working.

It could be that the water table hasn't gotten up to the sump bottom yet, or it could be that the bottom of the sump is floating in water like last year. I thought it best to dewater from under the floor evenly, by only pumping when the gravel under the slab is reached. When the water was coming in from the floor of the sump only, it was like I was dewatering only a single corner of the slab, which seemed pretty asymmetric. Most rainy seasons, the water table will likely flow completely under the house by 3-5' and never rise to the level of the slab.