High water table discovered after breaking up concrete for drain install

[Jason27]

I just broke up the concrete for a toilet, shower, and lav in my basement. I haven't began to dig out the trenches but did dig up in one small area to see what i was dealing with. Just under the plastic sheating is a pebbly dirt that was wet. Once scoop with my hand and I realized the water table was right there. Serves me right for doing this in the Chicago spring. I'm sure this is fairly normal though. I'd like to understand what I need to consider when digging out the trench and laying the pipe. I figure there will always be water there and it will be hard to dig out consistently, plus it will be a big mess. I figure I'll dry fit everything in the water and then remove from the water to glue it.

1)Any suggestions I should consider? Best practices or tips

2) Plastic sheating- Do i replace the plastic before laying concrete? If it's not sealed to the existing plastic, then what's the point? Is there a way to seal it to the existing plastic?

 

[Reach4]

I would add a sump pit and pump, since you are digging. This may or may not have a pickup pipe. On a prior house, got a sump liner and buried it to be flush with the floor, added some gravel around the outside, and filled the broken concrete with mortar. I put mine under a stairs.

I drilled 3/16 or 1/4 holes. In retrospect, I should have drilled holes only a little below the bottom of the slab, at the level of the under-slab gravel. One possibility is to not put any holes at all initially, but if you see seepage later, add the holes plus pump plus piping. I am not sure what to do with the sheeting. Maybe lay it across the gravel before you add the mortar/concrete.

I suggest a separate electrical circuit for just the pump. Even if rules call for a GFI on that circuit, I ....

 

[Sponsor]

 

[Jason27]

Already have a separate sump pit that runs every 20-60 mins or so.

 

[Reach4]

Good deal. The water level is probably where it is because your existing sump was dealing with it.

Did you have wire in your concrete? If so, will you do anything about wire?

 

[Jason27]

I'm not sure what you mean by, "The water level is probably where it is because your existing sump was dealing with it." The trench I dug is a good 20-25 feet from the existing sump pit. It's only 10 feet to the ejector pit but that pit won't be allowing the ground water in. There was no rebar in the concrete.

 

[houndzilla]

1) a deeper sump pit with help with your underslab water level. I'm assuming you have footing drains (aka french drains)?

2) the plastic sheeting isn't a water barrier, it's a vapor barrier. Moisture vapor transmission through slab on grade concrete is a serious issue for nearly all types of flooring and is tested via RH tests and calcium chloride MVER tests. While water moves I've read that vapor in these situations doesn't travel much more than ~3" horizontally. Ideally you would tape breaks/overlaps but in this case I don't think you would have an issue if you were to lay new plastic sheeting once you've backfilled and TAMPED your trenches (preferably crushed limestone, sand is okay, no dirt/clay!) and have as much of an overlap as you can. Taping would be worthless as the original is all dirty now.

If you are really up for belt and suspenders, rent a rotary hammer drill and use that to drill holes in the side of the slab openings and epoxy some rebar in there to ensure your trenches never settle.

Source: dumb architect

 

[houndzilla]

Also, regarding the ejector pit... if you don't concrete right away make sure you fill that up with water and weight it down... else the water level will pop it and all the attached piping up.

 

[Jadnashua]

Be aware that if you try to dry fit pvc pipe (or abs), you will not get the results you think you will! The fittings have an interference fit which means that you can't bottom the pipe in the fitting, as it wedges itself part way down. It's only when you add the cement that it melts the plastic and you can bottom the pipe. So, if you just dry fit, then later glue it, you'll be off a bit on each connection, messing up your layout. You need to carefully measure, not dry-fit.

 

[Cacher_Chick]

I can remember the first time I came to a job site after heavy rain and found the newly installed ejector pit and and the freshly layed piping, all floating in a foot of water.

The bottoms of the trenches need to be compacted at the required pitch and the piping bedded in place. Waiting a couple (or few) weeks might seem tough, but it will be a lot easier to do a good job after the water has receeded. I secure the piping in place by bending 4' sticks of rebar into a "U", and pound it down into the ground over the pipe. Sometimes we hit rock and move the rebar a foot or so one way or the other, but it needs to to secured every 4-6 feet if you have high water.
If you must work in the water, it is best to dig a pit and run a trash pump to keep pulling the water out from the area while you are working. If you pump water for a couple of hours and dont see the level go down, you need more pumps or more patience. June tends to be a good month for progeess.

 

[FullySprinklered]

Computer issues

 

[Atomic1]

I think the only way to install the pipe would be to dig a deep trench and use a temp pump to keep the water level low so you can glue the piping. Then backfill under the pipe.

However, i think you have bigger problems. This basement sounds like it needs a perimeter drain tile.