Pouring a concrete slab around the “wet well” isn’t as straightforward as you might think. Over time, the wet well may settle into the soil, especially when it’s full of sewage, and if the slab were rigidly attached to it, would crack. So the slab is poured to as to only attach at the rim of the wet well, with sacrificial cardboard raising it above the excavated terrain.
The radioactive instrument used at the 5:00 mark to measure compaction of soil is a nuclear densometer, which measures density of soil by transmission or backscatter of gamma radiation (usually from a Cesium-137 source).