Geotextile under slab?

[maverick302]

Is there a benefit to using geotextile beneath where the slab goes? It would be placed beneath the stone like so:

Slab
————————
2A modified stone, 6-10”
————————
Geotextile fabric
————————
Prepped dirt pad


Seems there are some conflicting opinions on this. I believe the purpose is so the dirt doesn’t migrate up and mix with the stone, essentially sinking the stone and losing your compacted base. I’ve read some people disagree that this mechanism is a valid concern and there’s a reason it isn’t common.

 

[matt_i]

Can't believe it would ever hurt you. Over a long time the soil fines could start to migrate. Also the criticality of maintaining your slab height is important.

Some of it depends on what kind of point loads you are putting on it.

Thick concrete acts like a washer distributing the load over a larger area thus lessening the propensity to smash little rocks into the subgrade.

 

[Superbowl]

Nobody does that so it can't be necessary.

 

[Bondo]

Is there a benefit to using geotextile beneath where the slab goes? It would be placed beneath the stone like so:

Slab
————————
2A modified stone, 6-10”
————————
Geotextile fabric
————————
Prepped dirt pad


Seems there are some conflicting opinions on this. I believe the purpose is so the dirt doesn’t migrate up and mix with the stone, essentially sinking the stone and losing your compacted base. I’ve read some people disagree that this mechanism is a valid concern and there’s a reason it isn’t common.

Ayuh,..... It depends on what the Prepped dirt pad is like,.....
If it's stable solid ground, probably a unnecessary expense,.....
If it's on a pool of clay or possibly wet area, yes, it'll help, alot,....

 

[Dodgeboy2020]

I'm not an expert on soils as such but I know I have been around a fair amount of slabs being installed and never have I seen the geotextile fabric used. Now that being said when I had my house built I did have them put fabric under the stone that made up the base for my driveway. Reason for that was the property has been a farm field for over 100 years and there was known areas of unstable soil in the path of the driveway. The fabric has done it's job, there was almost zero movement of the compacted stone during the course of construction to the point that the mason even asked if I had installed the fabric as he had experienced concrete trucks getting stuck in similar situations in the past and was surprised at how well my driveway held up. As far as the slabs in my garages was concerned there was no fabric placed below the stone base and there has been zero movement or concerns and it has been almost 7 years now.

 

[RivennHewn]

Geo over 6-10” rock will create air pockets that lead to settling. A course of compacted 5/8 -minus over the rock, then the geo will provide a stable base

As Bondo stated, it all depends on the native, undisturbed base you’re working on.

 

[KenC]

Geo over 6-10” rock will create air pockets that lead to settling. A course of compacted 5/8 -minus over the rock, then the geo will provide a stable base

As Bondo stated, it all depends on the native, undisturbed base you’re working on.

I think you are misinterpreting his terms. I see 6-10" thick crushed stone. 2A modified in my area would be 2" crusher run, a really solid packing base material.

The lowest layer is the most critical IMO, the type of 'soil' and layering/packing that is done. If that part is done well I don't think the textile layer is needed. I would however place a vapor barrier as it seems like everywhere I want to build has water too close to the surface at some time during the seasons.

 

[welder4956]

Is there a benefit to using geotextile beneath where the slab goes? It would be placed beneath the stone like so:

Slab
————————
2A modified stone, 6-10”
————————
Geotextile fabric
————————
Prepped dirt pad


Seems there are some conflicting opinions on this. I believe the purpose is so the dirt doesn’t migrate up and mix with the stone, essentially sinking the stone and losing your compacted base. I’ve read some people disagree that this mechanism is a valid concern and there’s a reason it isn’t common.

This is exactly the configuration used on the slab in my garage. There was a concern on the compaction for a slab on grade, so it was excavated to about 3 ft. on one end and 1 ft. on the opposite end. Then block wall foundation, geotextile fabric, fill with 3/4" stone, vapor barrier, 6 x 6 mesh, 4" of concrete. Not a single crack in 9 years so far.

 

[RivennHewn]

I think you are misinterpreting his terms. I see 6-10" thick crushed stone. 2A modified in my area would be 2" crusher run, a really solid packing base material.

The lowest layer is the most critical IMO, the type of 'soil' and layering/packing that is done. If that part is done well I don't think the textile layer is needed. I would however place a vapor barrier as it seems like everywhere I want to build has water too close to the surface at some time during the seasons.

I stand corrected.

I still like the 5/8 minus over the 2”, especially under a VB

 

[TomBexley]

The common thread in the replies seems to be that the native soil matters more than whether fabric is automatically good or bad. If it were mine, I would want to see what happens while the first lift of stone is being compacted. Does the subgrade stay firm, or does the stone start pumping down into soft soil and bringing fines up? That seems like a much better clue than whether the last five garages somebody saw had fabric under them. Also ask the concrete guy what he thinks the actual subgrade is there. Six to ten inches of well-compacted stone over solid undisturbed ground is a different situation from filling over wet clay.

 

[Hank11]

A site visit by a civil engineer would answer the questions raised here. No one here has any real knowledge of the OP’s soil conditions.

 

[welder4956]

A site visit by a civil engineer would answer the questions raised here. No one here has any real knowledge of the OP’s soil conditions.

I agree. My use of geotextile was recommended after a compaction check by a geotechnical engineer.

 

[phred]

I don’t see to purpose unless you have poor soil. Even then it would do little good. Properly compacted soil, adequate stone base and a reinforced slab should not compact the sub grade. We’ve poured slabs over very loose souls with the correct reinforcing and none have moved in over 30 years with traffic loads on them.