Fill and Geotextile Fabric

[BigTicket]

Hello everyone. I finally got my permit in hand today, so I am excited to get started on my detached garage. I have slightly under 3 feet of fall in grade from the front of the garage to the rear. My rear 8" CMU wall will be about 4' tall from the top of the footing. I won't know until I dig the footings, but I am estimating about 2' to 2'-5" of fill to build up the rear of the interior. I had initially planned to use crusher run for my sub base and screenings for my base course under a 5" slab, compacted in 8" lifts and using fabric between lifts. I talked with an engineer ( he didn't analyze but just shared ideas based on our conversation), and he suggested that I back-fill with #57/67 around the inside of the foundation to prevent the compaction of the fill from pushing out on the rear wall. I think if I use the woven geotextile fabric, that would mitigate the outward forces. Now I am reconsidering going with all #57 fill everywhere, although the costs would be higher. I can't see anything wrong with a full-stone sub-base/base under my slab. I know the #57 won't compact much, but I would still run a plate compactor over it and place it in 3 lifts. I would put the fabric on the ground to keep the stone from settling into the dirt, but would there be any benefit to laying fabric between the stone lifts?

 

[jack stand]

I've never seen fabric used in this situation.
You will be removing the "organics" from the entire slab/backfill area right?

 

[racecougar]

I haven't heard of fabric used in that way either. I had nearly 4' to fill on my low end. I used 1"-minus compacted in 4"-6" lifts to build a very solid base.

Are you not backfilling outside the foundation walls?

 

[larry4406]

On our new homes we do one of two things when we encounter this.

Option 1 - remove all loose fill and fill foundation with 57 stone. Then prep and pour slab. With unbalanced fill against the block wall on the back side (low grade) you don’t want to be compacting against the wall thereby bowing and cracking it.

Option 2 - install a structural slab with grade beams and leave the loose fill in place as it’s your mold/form material for the slab and grade beams.

Weigh the costs between 1 & 2. We usually do Option 2 as we don’t then need to haul off the spoils which you need to figure in your cost analysis.

What will be your framed walls? 2x4, 2x6, ?

For Option 2 you need a slab ledge and pockets for the grade beams. These are typically 4”. Thus you can use 8” block, change to 4” block for your stem walls and locally at the grade beam pockets, then frame up.

We don’t do the fabric and compaction under slabs as you proposed.

 

[Firebrick43]

Geotextile will do nothing to prevent push out on the wall.

I would never place crusher run under a slab, completely unsuitable.

I don’t understand why you think 57’s will compact less than crusher run, it should compact tighter being smaller rock. I would say even 57’s are to big and would suggest #8 stone.

Don’t confuse compaction with stone being locked in place laterally with fines like in crusher run. Two different things.

Are you going to place rebar and peacrete cells in the cmu wall every 3 or 4 feet?

If you evenly fill and compact the out side of the wall as you do the inside there is no issue as well.