Geotextile Fabric Between Soil and Gravel Beneath Slab
Hello all, I am building a slab on grade structure using the techniques outlined on this and other reputable sites to prevent moisture and soil gas infiltration through the slab – i.e. 4″ of washed, crushed stone on compacted soil, with poly vapor barrier on top of stone and immediately under concrete slab.
One thing that I almost never see mentioned, is what, if anything, to put under the washed stone – i.e. a geotextile or filter fabric layer? Obviously we wrap foundation drains with geotextile to keep the soil fines out of the coarse drainage gravel, and we often use geotextiles under road and driveway bases to keep the structural stone base from sinking into/mixing with the native soil.
With foundation drains we are presumably actively filtering subgrade water, and on driveways we see “pumping” action when vehicles pass over. But what about under slabs? I *hope* to not have to filter much groundwater, but we obviously design for some or there would be no need for a vapor barrier. Likewise, there shouldn’t be much dynamic loading of the sub-slab gravel, but there is some static load – distributed and pretty low but still some.
I’m thinking that over time, clean crushed stone, with its open pores, could compress into/mix with the native soil, causing slab settlement issues and reducing the effectiveness of the porous stone base, which will distribute and vent soil gas and moisture.
Climate Zone 4A – hard red clay soil – reinforced monolithic (turned down edge) slab on grade.
Am I overthinking this? overlooking something? Thanks!!
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I am not a professional builder, but my assumption is that your compacted gravel with a slab on it is stable. Weeds aren’t coming up through. Hopefully bulk water isn’t going to be an issue.
I'd worry more about differential movement caused by changes in moisture from the middle of the slab to the edges.
Chris,
As you say, the crushed rock is there to stop moisture from moving upwards by capillary action, not any structural purpose, so the fill underneath would still have to be compacted to the design limit (+- 2500 lbs/spf), and there shouldn't be any settlement. If there is enough hydrostatic pressure to cause differential settlement, there must be something forcing water upward under the slab, and if a site has that problem it needs to be addressed directly. One solution is is a grid of perforated pipes under the slab. Separating the layers with fabric isn't necessary.
Chris,
There is no need for fabric under the crushed stone. The stone just sits there for decades.
Driveways are different -- constant traffic drives the stones or gravel downward into the mud, so fabric is useful under a driveway.
I firmly believe that you are not overthinking when it comes to the foundation. No second chances.
In my world (Zone 5, MA), there is a shallow water table (a few feet below surface in the summer, right up to surface in spring and wet weather, no possibility of a perc test passing). The soil is silty and stones seem to sink into it over time (and are pushed up during frost heaves). Too exciting for a foundation.
What I did: I had 3 ft excavated and then laid down 6 foot wide geotextile fabric in parallel lines (overlapping more than 6 inches) held in place with 6 inch landscape staples. Onto this was dropped approximately 3-5 feet of 'cobble' which around here means clean rocks ranging in size from 4 to 16 inches. This is both a capillary break and the structure support for a frost-protected shallow foundation. I felt sure that these stones would sink into the muck over time and I am very happy I used the geotextile to stabilize. Just dropping from the truck and rolling over it a few times and it was pretty much fully compacted.
The next layers included 1.5-3 inch crushed stones (6 inches of this, plate compacted) and then 4-6 inches of peastone. THen poly and the rest of the foundation on top. No 'gravel' (rocks and soil/sand) at all. Any water the hits the surface around the house quickly falls by gravity down to a foundation drain that runs atop the geotextile and thence downhill away from the foundation. Amazon has great deals on such geotextile fabric in rolls 6ft wide and 300 ft long. I chose the WOVEN WF200 because I preferred the water to shunt quickly to the foundation drain but both Woven and NonWoven will keep the soil and rocks separate.
I am very happy about the solution. We used more than 1 million pounds of stone to support 120,000 pounds of concrete foundation (+ the weight of the actual house). Torrential rain and storms all during the build cycle from November though summer but the foundation has never had unexpected wetness from below and has no settlement cracks in it. I have no gutters yet and still had no water issues at the foundation (other tham splash up to the exposed wood siding). Very likely the loose rocks should make soil gas (ie, radon) accumulation an unlikely scenario.
I will note that this scenario leaves lots of air gaps between boulders underneath the house. Voles, snakes and other critters are likely to take up residence therein. However, I (want to) believe that this soilless environment is not suitable for carpenter ant or termites.
So, I cast a YES vote for using geotextile if you believe there is the remotest chance that your support stones will sink into the subsoil over the years. Cheap insurance that can be DIY.
Kenneth - I'm wondering when you put your crushed stone on top of the cobble, did it not fall into the gaps between the cobble? Did you completely compact/vibrate it to fill in the voids?
-Walter
I think it's a fantastic idea to put filter fabric under crushed stone. Many of us that have excavated in dirt under a house have found dirt you can dig with your hands, often full of minerals from water intrusion. Over the decades, water can loosen the dirt. Loose dirt can fit between the large spaces between 3/4" stone. The load above 3/4" stone will push it down into the dirt. Your slab will shift drop and crack. Even if it does not, you are affecting your capillary break.
Here's my layers for a recent project:
1. Well-compacted existing dirt
2. Geotextile layer, overlapping a foot
3. 3/4" washed crushed stone, compacted
4. 3/8" pea gravel, screeded well
5. Rigid foam
6. Vapor barrier, taped to prevent concrete intrusion between
Etc
Anukeen,
I see where the concern comes from, but if the rock is capable of compressing your sub-slab fill, so is the concrete above. The substrate is either properly compacted or not, and composed of the right fill or not. If it is built properly, I don't understand what mechanism would allow the rock to drop and the slab to settle.
I can see how you would have the problem with the stack-up you have listed. it's very rare that the existing "dirt" is suitable fill that can be used under a slab.
I should have clarified. Layer #1, well-compacted existing dirt, is existing UNDISTURBED dirt. The top inevitably gets a bit of loose dirt, so it's nice to whack it down before covering.
Yes, that dirt below is well-compacted - mostly by being undisturbed, partly by mechanical compaction - but the fabric will keep it from ever getting between the 3/4" stone.