Something is excavating sand/mortar from between paving bricks, as seen here. I thought it was ants, but I've placed ant traps and bait and there seems to be no effect. Also, when the mounds are disturbed, no insects are visible.
Here's a close-up view of the large pile at the left.
That's a significant amount of sand/mortar being removed.
If it's not ants, what insect is causing this?