proper fill beneath monolithic, insulated slab
Just getting ready for site preparation, and wanting to double check a couple of things. I contacted the engineer who drew up my plans to clarify, but I’m getting crickets at the moment.
The site has a big elevation difference from one end to the other, like at least 18″. We have a big property, and had been planning to move some sub-soil from a nearby high spot to upfill. I seem to remember that being done for my house, but I’m not 100% sure on that. (We have sandy soil below the organic topsoil). The excavator/concrete guy thinks it will be fine, but I noticed on the plan it says “Up-fill site as required for the building. Use good clean granular fill (crusher run for thicker fill depths, clear 3/4 stone or granular A for final topping of fill). ” Note that the engineer has never seen the site, nor the geotechnical survey, so maybe he’s just using boiler plate language).
Also, what is best for drainage under the insulation? My house plans specified 2-3″ of sand screenings, but this plan says clear 3/4 or granular A. Is one option better than the others?
Climate zone 6.
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Trevor,
Our code specifies you have to build on undisturbed soil of sufficient bearing capacity. You can't move it around. if you have to bring in material to level the excavation it needs to be engineered fill.
Directly below the slab you want a layer (6" or so) of granular fill clean and large enough so that it acts as a capillary break.
Thanks. I did hear back from the engineer and he said as long as the fill is free of stones and other debris that would hinder compaction, using local soil is ok. I have passed on the 6" clean granular fill requirement to the contractor.
You will obviously have to compact your fill that you move. In my jurisdiction this requires compaction testing even with site soil to ensure proper compaction, and is done in 18" lifts, with a very large hydraulic compactor.