Charge money for excavation work

Please keep in mind, real cameras have become so inexpensive nowadays that the effort for a dummy might not be much less. Deterrence is necessary, the "Attention Video Surveillance" sign must be large. And possibly motion detector floodlights. Ask local security services if they rent anything cheaply. Three villages away, various construction sites in the new development area were cleared out by thieves. Since then, there is the most colorful mix of barrier fences, floodlights, surveillance cameras, and dummies you can imagine...

And possibly motion detector floodlights

That's what you call using a sledgehammer to crack a nut. Where a fence and a sign are enough, maybe go and observe sometimes, here an upgrade is being listed.., no words. From me now also the NATO fence, a ditch around the fence and an automatic weapon system - fitting to the floodlights

I would just install a trail camera hidden somewhere, and if you have captured the culprit, you can still put up a warning sign "Achtung Videoüberwachung". It's not really allowed, but sometimes you just have to defend yourself against others.

That's what you call using a sledgehammer to crack a nut.

The OP should dispose of his pile, and depending on the pollutant class and costs, he will decide what the cannons and sparrows are. In the worst case, such a floodlight is the sparrow and the illegal dirt pile the cost cannon...

Pragmatically speaking, construction fence, dummy camera, sign.

However, maybe you have a tip on how to proceed in this case

Hello

set up a "camouflaged" trail camera. I have several of them in nesting boxes. Homemade.
There are inexpensive ones for 80 euros. I just bought a new one with a black light flash for 200 euros. It also takes good pictures at night and you don't see the flash.
From martens to two men masked with balaclavas, I have captured everything in my garden.

Steven

Please keep in mind, real cameras have become so cheap nowadays that the effort for a dummy camera might not be that much less […] And possibly motion detector floodlights.

Cute that you mention that in the same breath: yes, cameras are no longer much more expensive than their dummy counterparts, but the installation of a functional system is still noticeably more costly than just hanging cameras as "dummies," even if they are real; however, good night vision cameras have also become cheaper, which probably take somewhat better pictures without floodlights.

I would simply install a hidden wildlife camera somewhere; if you catch the culprit on camera, you can always put up a sign "Warning: Video surveillance." Although none of this is actually allowed,

Oh, is it? - Martens are wild animals, they can neither read nor would they have an owner defending their image rights (if they had such as civil law “things”). And there are plenty of plastics on construction sites that need protecting.