The blasted rock on site can’t be used to fill the space between the damp proofed walls and the rock. It’s sharp and would puncture the water-permeable landscaping fabric and possibly the 4” PVC pipe. It would also damage the damp proof coating. Therefor, a material that has only smooth, rounded gravel and sand is brought up.
Here you can see the water-permeable landscaping fabric in the shadow. It’s the black material, stuck on to the bottom of the walls and coming out a couple of feet from them. This fabric is very permeable, since it has to allow water to get to the perforated PVC pipe that it’s covering. It’s job is keep the dirt away from the pipe so it doesn’t get blocked.
One load of about 12 cubic yards of the backfill material being delivered. Notice that the gate is hydraulically actuated, not just gravity opened.