Foundation Preparation / Dirt work.
This is from my observations on our property.
- Reviewed Geotechnical reports.
I found a Geotechnical report published by LSU AgCenter dated in the 70's. We ordered a Geotechnical review of our property to be sure we understood just what type of foundation would work best. We had the old slab on grade and neighbors pier and beam to measure success. We never had a crack and didn't need to elevate from the sinking our neighbors had. So a simple footer with columns was not the correct method of supporting a home in our area according to observations made of homes built between the 30's and 60's.
- Soil type and depth of each layer change.
We have light sandy clay from 10" to 32" and on top it was pure mixed dirt, pine needles and oak debris. Nothing really good to support a home. The Geotechnical report told us to scrape down 12 inches. We did, found wet areas so we when another foot below average grade and it started to dry out in more than 80% of the total foundation.
In the wet areas at about 24 inches we dug another 6 to 10" but not more than 30" because of our testing showed water and pea size gravel at that point front and back of our foundation.
We added Lime to the base where it was wet and mixed the clay fill in with it. This is the actual product we used from Tractor Supply. Pro Aglime.
What I noticed as I spread the 50lbs of lime over the wet soil/clay on our property is it started to make the sandy clay stick better as if it was a heavier clay. The lime mix made a muddy type soil into a harder and easier to walk on soil in just seconds. It joined the sandy clay tighter together for a better base.
Clay Fill was hauled in and compacted just about every 12 inches.
I estimate we dug 24 inches below average grade and with an skid steer and excavator the new fill clay was spread and compacted as it was added. We spread about 2 feet wider than our actual footers. We had our Surveyors mark the edge of our foundation then placed a metal fence pole about 2 feet out from that marker.
Once we added enough clay fill to reach our average grade we measured the recommended mound build up from within the same Geotechnical report. We were told to add 12 to 14" of fill above grade and compact it. We went to 14" above grade. This will give us a total of no less than 16" above grade from the top of our foundation when the foundation is poured.
To recap the process, we dug down to get to dry and as close to solid soil as possible. We added lime to dry the soft sandy clay. We compacted hard clay to 14" above grade.
Our foundation will be dug a 12" below grade leaving us with 12" of compacted clay fill. We are actually going to dig to at least 14" and add a 2" layer of pea gravel because of our oaks. We found that were the old slab had gravel / shell mix all major roots were below the gravel mix. It was a trap for moisture and the roots didn't have to travel the surface. We compared this to a section of patio that was 13' by 65' long that had major roots to the concrete.
With 14" below grade our remaining footer uses the compressed clay as it's earth mound. Our Foundation Engineer asked for 22" of footer depth with 4" of slab pour. This gives us 2" 2" from below grade to top of slab with an estimated 38" of newly compacted clay below our poured concrete foundation.
We had just about 3,700/sf of foundation to prepare for. We used 47 truck loads with each being about a 20 yard load of clay.
We used 20 50lbs of Lime at 24" below grade.
Compaction testing showed 8 test points to be 103% or higher.
Our dirt work was completed by Lee's Trucking and Land Services LLC. Lee and Lisa Jacobs.