Backfilling the Garage
Screened tailings fill most of the volume of the garage foundation and minimize the amount of compacting we have to do
Because of the sloping lot, the garage foundation walls had to be built 8 ft. tall. That meant the inside needed to be filled with a lot of material. 
Ordinarily we would have had to compact in lifts to ensure the garage slab didn’t crack or, worse, sink into a void. My friend Mat had an idea. Rather than backfill with gravel or other material that would require compacting, we could backfill with tailings—the small stones screened out of fill. Ranging in size from 2 in. to 8 in., the stones would only require tamping with the excavator bucket when spread in the garage. Around here, tailings are often used as driveway base material when the soil beneath is clay or silty and prone to moving under traffic.
One of Mat’s guys hauled a bunch of loads of stone and dumped them into the garage, filling it to within 2 ft. of the final grade. 
After Mat spread the tailings with the excavating machine, I placed a layer of old lumber tarps over the top. The tarps will separate the top course of gravel from stones beneath and prevent the fine sand in the gravel from filtering down into the stones.
Mat bailed gravel into the garage in layers and compacted each until we reached the final grade. 
We raked the final layer to match the slope of the future concrete slab, and then he ran the tracks of the excavator back and forth to pack the last few inches in an even plane.
View Comments
I would be interested in using the screened tailings to make a several foot high pad (on a 4% slope) for a shallow slab house foundation. Basically we have a slow drain soil with seasonal high water table and want to devise a well-draining base and then construct the frost-protected shallow foundation (FPSF) on top (no stem walls). Using tailings has been suggested and I am interested in hearing about pros and cons of their use as such a foundation base, such as cost, likely compliance with building code, settling over time, stability when stormwater runs through, common failure modes and so on. I am in MA. Thanks, Ken.