Near where I live there's a construction site for a new supermarket. The last week or so they did things I don't quite understand.
- The ground in our area (Ruhrgebiet) is rather clayey
- The site is near a small hilltop, the area generally drains into the Ruhr
- The site was excavated maybe 2m deep (below ground level) and filled in with 1.5m of sand gravel mix.
Now the part that I don't understand:
- An excavator with a drill drilled through the sand-gravel and the soil beneath, maybe 3m total depth (below ground level), I guess 0.5 m diameter
- The holes are set in rows, maybe 2m apart
- A huge machine that on first glance looks a bit like the ones they set sheet pile walls with drives up to each hole, inserts a pipe and (appears to) fill the hole with gravel - not the sand gravel mix, but fairly large, roundish individual stones with no small infill material
- I see no way that a binder or water could be added to the gravel - there's just a funnel for the gravel and a loooong pipe that is entered into the hole. I don't think it's for a pile foundation.
Picture of the machine (not from "my" site, but from here - sadly no explanation what the ultimate purpose is):
My theory is that this is for surface water drainage - water will percolate through the sand-gravel mix and then into lower ground layers though the highly porous gravel. There are a few other sites close by dedicated to infiltrate storm water into the ground, so maybe this is a thing in my locale. The image source has someone describing the machine purpose built for "Kieseleinbausäulen" (Gravel infill piles) but I don't find that term anywhere else.
Ultimately I don't know (and the only way to really find out is to ask one of the guys on site, but so far they've been to far away from the fence when I came by), my question is: To what end does one drill holes and fill them with gravel? Is my hunch correct, are there other explanations?