I couldn't find any writing about why do they hang laces at the excavation site, like what your photos suggest. But my best guess is that, it could be meant to "signal" something, that exact position could be a mark for an important artifact underneath.
My second guess, it could be a safety signal, if let say the cave is caving in gradually, it can be observed through the height of the laces from ground. If it touches the ground that means that the cave roof is going down.
Omg, I'm so imaginative :)
I did check photos of actual excavation sites and I never saw anything like that of your photos.
I maybe be wrong or correct on my guess, but I still do enjoy reading your blog and learned something new.
Cheers!