Tallulah_B said:
What's the reason for the ground to sink like this?
New answer, still not sure.
I fixed it today, not perfect but as I had to redo parts several times when I found other parts also sinking, I finally pulled up everywhere I thought might be bad.
I found tunnels, or channels, I do not know what formed them running in three directions.
My hopefull guess is they were made by the late Mr. Mole.
I poked a rod down four holes, and it went in free over a foot in two of them.
I was going to just pour dry sand in the holes but I though if water washed down there it will wash the sand down so I put the type of cement you use to build a brick wall down the holes first, tamping it in and then poured and tamped in dry sand.
I used one half bag of cement and full tube of sand, the type you use for weight in the winter.
I tamped till the sand did not run.
I had to scrape polymeric sand off of the bricks to reset them and found out that the safer salt you use in winter still corrodes paver stones where the salt water can seep between the stones because the polymeric sand has washed away . (due to tunnels barring the edge of the brick.)
Not real badly but bad enough I replaced some anyway with others I had left over from years back.
It is now supposed to rain, so I won't sweep polymeric sand in the crack till a good dry day and then I will put glossy paver coating over the top to seal them.
Now all I have to do is do those in front of the shed but there I know I am probably dealing with old tree roots.