Around the retention pipe, the crews are layering pea gravel and native soil. The pea gravel is to ensure water can flow down. If the hole was just all soil, if water came through, the soil would just turn muddy and prevent water from flowing down into the basin as it should.
A compacter runs across each built layer over and over again to pack down the soil and gravel.
A worker from Speedie and Associates comes every few hours to test each layer to make sure the soil and the gravel are compacted as hard as the normal ground to ensure safety.
It's amazing how much they can get done in one morning/afternoon!