Drilling holes in ground and filling with gravel - to what ends?

Question

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.

Some background:

• 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):

The funnel can be hoisted up near the end of the pipe, or lowered to be filled by front loader.

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?

Answer 1

You might be right

Your first thought about using the holes for drainage could be right. This would be similar to a French Drain or a Dry Well.

French drains are usually horizontal and dry wells aren't usually installed on hills.

The likely option

What is more likely is that the contractor is installing a form of compacted aggregate pier. See this for a description from a company that does them (vibro aggregate pier).

The main idea is that a hole is drilled and then filled with compacted aggregate. This provides a firmer foundation without treating the entire area or installing steel or concrete piles. The aggregate piers are sometimes used instead of micropiles, but they aren't as good of replacement for large concrete piers or driven steel piles.