Water Table Under Slab

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Posted by: from Rouyn Noranda
6/8/2021 at 9:24:09 AM

Hi, I'm repairing a sewer line under a house that's been standing strong for over 90 years in Northern Quebec Region. House was built on expansive clay but the perimeter of the house was backfilled with sand and gravel except under the house. A french drain was installed (20 years or so ago) over the slab instead of under since the sewer line didn't allow it to be deeper and still have required slopes. When I plugged the concrete, the clay under it was pretty moist but what concerns me is that the plumbing crew filled the hole with gravel and now, there's sitting water in the gravel. I found out the water table was about 4 inch under the slab if clay isn't present. When all of it was clay, water didn't pool anywhere. I guess what I'm asking is, is it okay to have slight water pool pool in the gravel or can water vapor make it's way throught the concrete and ruin any subfloor? I'm pretty sure there's no risk of the water rising to a point where it would actually touch the slab.

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Date/Time6/8/2021 at 10:31:47 AM

I recently built a house close to Constance Bay in Ontario with a similar problem. Hydrostatic water intrusion is defiantly a real issue if its serious but it usually is not a huge concern. French drains and sump pumps typically solve this problem. Be sure to plug all opening through the slab around drains and pits and well heads.

I wouldn't worry too much about flooring in your basement becoming damaged as long as the following is followed.

1- vapour barrier/rigid insulation was or will be installed under the slab.

2- proper subfloor is installed on the slab such as dricore or amdry products

3- the flooring used on this subfloor is rated for humid areas i.e. Tile, engineered hardwood or LVT, SVT or VCT flooring

4- humidity is monitored, checked and managed properly as any typical basement should be

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Frédérick in Rouyn Noranda
Date/Time6/8/2021 at 11:05:44 AM

There's no water barrier under the slab. Do you suggest I add one over the gravel where I plugged before pouring replacement cement?

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Date/Time6/8/2021 at 12:24:36 PM

I think it should be alright. There might be a risk of the water rising past it's point.

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Date/Time6/9/2021 at 10:02:06 AM

Hey Frederick,

Installing a barrier under the slab will only work if the entire slab has the barrier, adding one in just one location wont do much. I honestly wouldn't worry about it too much. If you are concerned my best recommendation would be a sump pump. That will guarantee that the water will not rise high enough to damage anything in your basement.

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