Excavation and compaction.

Excavation started in early June. For the walk-out basement the building site basically just needed a notch cut into the sloping terrain. When staking the site beforehand I was sure to include extra space for the horizontal insulation that surrounds the footing as part of the FPSF design. The horizontal insulation required an extra 4' all the way around and 6' at the corners (minimum). I added an extra 1' to those numbers so I'd have some "wiggle room". I had the machine operator slope the bottom of the excavation slightly downhill as well for better drainage. This proved well for me as it began raining as soon as we finished...

A notch in the ground.

After the excavation, it was time to build the gravel pad. Another requirement for good FPSF design is to build on top of "non frost-suseptible material" - basically a well drained layer of compacted gravel. I used screened "A" gravel that packed really well. The gravel was compacted in 6" lifts with a 1000lb diesel plate tamper to 100% SPDD. You can tell when you reach this point because the plate tamper literally starts bouncing off the gravel. Because of the way the excavation was sloped, final pad thickness was 12" at the high end and about 18" at the low end. For best possible drainage, I installed 4" weeping tile in the gravel which was run downhill and drained to daylight.

Pad construction wasn't without it's problems. After compacting the first lift of gravel, several spongy spots were revealed. In one case a blister of clay oozed up through the gravel like toothpaste coming out of a tube. Standing on the spongy spots was like standing on a waterbed filled with molasses. It turned out that the spongy spots were in most cases caused by rock inclusions within the clay soil that were trapping liquid water from the rain we had had. I think running the heavy compactor over those areas had the effect of driving the water into the surrounding soil making it squishy where it hadn't been before. The solution was to peel back the gravel, dig out the gooey clay, trench downhill and backfill with rock and weeping tile - essentially creating a french drain under the pad. We used the excavator to track down the rock before covering and compacting again with gravel.

The deepest part of my french drain. After digging out the
squishy clay, it would have been a bad idea to just fill in the
hole with gravel since the hole would then act as a sump
for ground water. To drain this pit a trench was dug out to
an existing ditch further down hill and weeping tile run the full
length before back-filling with coarse rock.

The shallow end of the french drain. This whole area had to
be dug up. Lots of spongy spots.