Fixing a poorly executed sump pit....

[ADK_MechETech]

So when I moved into my house 7+ years ago, there was a sump pit in the basement that was clearly an afterthought. A year or so after moving in, I found out that the year after the house was built (built in 2004), the owners had a serious water problem. The hill behind the house literally gushes water almost year round. The contractor came back and did a ton of drainage work on the exterior of the house, and most likely installed this "Sump pit".

Flash forward to today, I have never seen water in my basement and the pump has not even been plugged in for the 8+ years I have been here. I am now semi-finishing the space to be used as a laundry room and my home office. I am trying to figure out the best way to:

1. Stabilize the gravel and fill in the hole.
2. Make it clean so a pump doesn't suck up gravel and dirt if it does have to run.
3. Eliminate any standing water.

Here are a couple pictures of what I am working with. The opening is roughly 23"x18", and it is 12" from the slab surface at the deepest spot. I'm open to any/all suggestions at this point. I don't think installing a plastic sump pit is realistic at this point as I would not be able to backfill under the slab around it.

 

[Tuttles Revenge]

Plastic sump pit is the way to go. You might have to stabilize the gravel. But otherwise you're just going to have a pit churning up rocks.

 

[Sponsor]

 

[ADK_MechETech]

Plastic sump pit is the way to go. You might have to stabilize the gravel. But otherwise you're just going to have a pit churning up rocks.

I agree, but how would I go about backfilling and patching the concrete well enough so that there is not a huge void under the slab when I am done? I don't really want to make the hole any larger than necessary at this point.

 

[Reach4]

You will want to fill the space around the pit with gravel. I would lean toward rounded pea gravel. Others might tend toward crushed stone.

Then use some kind of mortar around the rim to fillup that clearance.

Fill the liner with enough water or other weight to prevent the liner from floating up until the mortar is set strongly -- maybe a couple days.

You will want to drain holes, maybe 3/16 in the sides of the cemented-in liner. The holes should be small enough to not pass gravel. I like holes only high -- between the bottom of the slab and the bottom of the gravel that was already under the slab. Some believe in adding holes lower. I would only go high unless you need more holes to keep up.

 

[Tuttles Revenge]

I think you're worried about the gravel undermining as you dig. Valid. You might just have to dig a much larger area of concrete than you need for the sump, so that your excavation doesn't undermine the slab.

When you backfill, fill the sump with water and attach the lid and have it bolted down to the sump so that you know its not out of round and deformed.

 

[ADK_MechETech]

I think you're worried about the gravel undermining as you dig. Valid. You might just have to dig a much larger area of concrete than you need for the sump, so that your excavation doesn't undermine the slab.

When you backfill, fill the sump with water and attach the lid and have it bolted down to the sump so that you know its not out of round and deformed.

Exactly. The gravel is quite loose there. Unfortunately removing much more concrete isn't really an option at this point. I can go in one direction, but only about a foot or so. Poor planning on my part I suppose. I can probably get quite a bit backfilled by hand under there, but there isn't much of a way to prevent a void there. The slab isn't loaded heavily and the frostwall/footing is there also. This wall is backfilled to approximately 6' above the floor.

 

[Reach4]

Unfortunately removing much more concrete isn't really an option at this point. I can go in one direction, but only about a foot or so.

What size round or square opening would be an option?

 

[ADK_MechETech]

What size round or square opening would be an option?

From the picture posted, I can expand the opening to the right by appoximately 6-10" at most.

After giving this some thought, here is what I have come up with.

1. Obtain several 3-4' sections of 1/2" rebar, or t-posts (think steel fence posts).

2. Drive the rebar/posts around the perimeter of the hole as close together as possible, and as deep as possible. The stone under the slab is 3/4", but I am unsure how deep it goes. This may be impossible if the ground below the gravel is not clean fill. The area I am in tends to have a lot of rock from 6" up to the size of a car.

3. Excavate the hole to the depth required for the sump basin I have ordered (approximately 16").

4. Install and level the basin, fill with water, screw the lid on.

5. Remove the rebar/posts, or cut off if I can't get them out.

6. Backfill with appropriate stone, or use the material I removed. I will try to backfill under the slab where it has already fallen in as much as possible.

7. Patch around the basin with fiber reinforced ready-mix.

8. Celebratory beers.

Yay, nay? Suggestions, objections? I'm going to go pick up some fence posts tomorrow and see how the first couple go in. That will determine whether the rest of the plan will work.

 

[Reach4]

I presume you are thinking ground water...

7.1 drill holes big enough to block gravel, but allow water. I am thinking 3/16. I think drilling the holes at about the level of the gravel under the concrete.

Some think that drilling holes lower too is good to accept more water. If the higher holes can do the job, you avoid pumping groundwater that is not rising to a level that is a problem.

7.2 Plan to put a sump pump in if you get groundwater into the basement. 1/3 HP should be plenty. Those don't go into sealed basins.

 

[ADK_MechETech]

Yeah, exactly. I left that step out. I purposely purchased the non-perforated basin so I can put holes where I need to. The ground water level seems to be pretty consistent at ~8" below the under side of the slab. I'll put holes in a little above that. In the time I have been here, I have never seen that level chaange more than a 1/2", even with heavy rain.

Yes, I will be getting a pump to go in, but my main concern was getting the basin set, and the floor repaired before I go putting up the rest of the walls. It would be impossible to get at this later. Thankfully though, the basement is pretty dry. The only time I get a slight amount of water weeping in the corners is when I fail to keep the downspouts clear.

 

[ADK_MechETech]

I got sidetracked today, but I was able to put in the first two fence posts and it seemed to work well. I got 3' posts and was able to get them over half way in. I can probably go more but I wanted to be sure I can get them out first. I also cut the hole about 4" wider as the basin was going to be a tight fit already. I'll take some pictures tomorrow after I work on it some more.

 

[Tuttles Revenge]

I think you have a solid plan. The one I had in my head and kept to myself was way more complicated, but one that I see on large construction sites. They spray shockcrete against retaining walls to hold the dirt in place.

Pea gravel will be good to backfill under the slab since it is self compacting and fit into those tiny spaces you cant reach.

 

[ADK_MechETech]

I think you have a solid plan. The one I had in my head and kept to myself was way more complicated, but one that I see on large construction sites. They spray shockcrete against retaining walls to hold the dirt in place.

Pea gravel will be good to backfill under the slab since it is self compacting and fit into those tiny spaces you cant reach.

Definitely an option if it was on a larger scale, but would defeat the purpose of putting the sump basin in. At a minimum, I would have to break up that concrete to allow water in if it was ever necessary.

 

[Reach4]

Definitely an option if it was on a larger scale, but would defeat the purpose of putting the sump basin in. At a minimum, I would have to break up that concrete to allow water in if it was ever necessary.

Pea gravel passes water effectively. Especially river pea gravel with more rounded pieces.

 

[ADK_MechETech]

Pea gravel passes water effectively. Especially river pea gravel with more rounded pieces.

That comment was referencing shokcrete. I will use whatever the installation instructions say for the basin which I believe is 1/2"-3/4" rounded gravel.

 

[ADK_MechETech]

Well, I didn't get so lucky this morning. All the posts went in fairly well, but once I started digging, I came to a screeching halt at about 15". There's rock, or ledge of some kind in there and the hole is not getting any deeper.

Unfortunately, for the Jackel sump basin I have, it needs to be at least 17". I have looked around quite a bit now and have not been able to find anything else slightly smaller that is not perforated already. Any suggestions?

Can I just leave the sump a few inches above the floor level, and add some holes at the slab surface if necessary?

 

[Reach4]

Can I just leave the sump a few inches above the floor level, and add some holes at the slab surface if necessary?

I think so.

 

[ADK_MechETech]

Well, it isn't exactly what I wanted it to end up, but it is definitely better than it was before. The basin is sitting in about 3" of standing water, and about 3" above the finished slab. Once it is all set up, I will drill some holes. There wasn't much more I could do with it, but I think it will be just fine. The concrete is just setting up now and I'll go back and smooth it out, sloping it slightly toward the basin.

I am not going to bother with a pump for right now, but part of the work I am doing includes replacing the piping for it anyway. I will be monitoring it to see if it accumulates any water and put a pump in at some point.