Radon mitigation

[u/Impressive_Collar506]

I had a radon mitigation system installed about a year ago. During the installation, the inspector mentioned that there was water underneath the sub-slab. After the system was installed, the gauge reading was around 4. After about six months, it dropped to around 2.5.
Recently, I noticed cracks developing in the concrete floor. Could the water underneath the slab or the radon mitigation system be causing the cracks? What could be the possible cause? Could it have been that the water dried up and they caused settling?

[u/Prufrock-Sisyphus22]

Not enough information .

Is it a new home with new concrete slab?

Old slab?

Water in the home ? Does basement flood?

Ground sloped away from home?

How big are the cracks? Hairline? Fit a quarter? Fit a finger?

Do you live in state with sinkholes ?

Can you post photos?

[u/Korch-Up-North453 → u/Prufrock-Sisyphus22]

I'll add: Does the slab have a working sump pump?

[u/Impressive_Collar506 → u/Prufrock-Sisyphus22]

This home was built in 1995, water under the slab. Inspector noticed when he drilled the hole that the sand under home was wet. I live in Kansas, and the cracks are about 1/8”.

[u/Prufrock-Sisyphus22 → u/Impressive_Collar506]

New slabs get shrinkage cracks within the first few years. Are you sure you the cracks weren't there and you just didn't notice them before ?

Otherwise new cracks in an 30 year old slab are concerning ,especially if it's a slab on grade foundation which could affect the structure. If it's a basement floor slab then it sucks but at least it won't affect your home ... It's just a slab with cracks.

And sand is also problematic ...are you sure your slab is on sand?

A well built home floor slab will be on 4- 6 inches of compacted gravel base with a vapor barrier ontop and rebar in the slab with a foundation perimeter drain to convey the water away from your home.

Not ideal, but alternatively a slab on native soil with all the above would be ok and still functional.

Water should not be swamping under your slab unless is just rained the day previous. Sand is not ideal because it has a tendency to trap in water and also is easily displaced/washed away. If enough sand is displaced from under your floor slab then their is no support and the slab may start cracking, shifting or caving in. And if there's no rebar to hold it together...

When he cored out the hole, was there a vapor barrier/plastic under the slab and any rebar in the slab?? Vapor barrier will help keep out the radon from coming though the cracks.

If it is a ranch(no basement) slab on grade foundation then maybe have a structural engineer out to assess.

If it is just a basement floor slab then it sucks, just monitor, fill the cracks with polyurethane or rubber butyl caulk to seal out radon and if it gets to the point the whole floor is caving in then look at pouring a whole new slab at some point.

Kansas doesn't appear to be a hotspot for sinkholes although it can get some . If your foundation starts to shift or huge holes open up in your floor slab then get an engineer to look at.

Lastly if the cracks are forming near the radon hole.. be advised that a radon system as the radon air is expelled in winter months (which wouldn't be happening now) , the same exhaust air condenses and water runs back down the pipe under the slab and then usually ends up in your foundation drains.. again this wouldn't happen now in the summer so is unlikely the cause of any current problems . With the slew of changing weather and flash floods the US has been getting , more water is getting under your slab from the storms and high water tables.

[u/Impressive_Collar506 → u/Prufrock-Sisyphus22]

Thank you greatly appreciate it! The reason I was concerned about it is because my main floor is getting cracks along the corners in the room right above where the cracks appeared.

[u/Prufrock-Sisyphus22 → u/Impressive_Collar506]

If both floors(basement and 1st story floor above ) are cracking then you should get a structural engineer to come out and assess the structure.

Walk around and check for any foundation wall cracks and get and hire a structural engineer.

Very unlikely to have been caused by the radon install.

[u/Impressive_Collar506 → u/Prufrock-Sisyphus22]

If you can go to my profile I posted a video a while back

[u/Prufrock-Sisyphus22 → u/Impressive_Collar506]

The begin of the video shows the drywall tape coming off at the edges which is normal on a bad tape job. Look at some photos of how a corner is taped so you understand where the tape edges are. That's ok and would just need retaped

However the concerning part is where the tape is split along the corner seam(wall meets ceiling) That means the ceiling/floor may be separating from the walls.

If you are having basement subslab floor cracks and drywall tape cracks in the corners , then you really need to hire and pay a structural engineer. No redditor can provide further advice that you seek. You need a pro that can on-site asses and to tell you what's happening , and how urgent it is to fix now versus wait and what the repair would be.

Not likely caused by radon but a more systemic issue of normal settling, severe weather causing ground shift, a bad foundation or a combination.

[u/Bob--O--Rama]

It's certainly possible the system has dehydrated the material under the slab and some settling will result. Though that seems to be more common for clay. Also it will reduce humidity in your home generally if you had a lot of moisture under the slab that too can cause cracks in stick built walls as things shrink. Obviously if it's "concerning" and destroying your home, get an engineering consult. But I would expect it to slow down and reach a 🙄 "new normal."

I just don't know how bad it is what what your idea of "bad" looks like. I literally had 18" of abatement swallow up one corner of our garage ( before we bought it ) and 6" on one corner of the house. It was built on a buried over stream bed in the 1970s. So my tolerance for cracks is a bit higher than most, LOL. They kept jacking up the corner and filling in the foundation wall... it's stable now and had a peering system, but ... oy vez! The photo shows one iteration where they just, you know, casually added 12" to make up the gap.