Not scientific, but I used to use an 8# sledge hammer and dropped it from a couple of feet as a rough compaction test... This was for stiffened slab foundations in rural Lindsay, Ontario area... I'm not familiar with rebar and gravity hammer.
Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
Penetrating soil with any rod will give you a feel for the density, strength from one location relative to another, i.e. the bar is harder to push in at this location than it was at that location so this area seems to be denser.
However, in granular material, catching the edge of a large gravel or stone can throw you "feel" way off so it becomes useless.
I worked in an are where the soils were essentially silty CLAY till depth. Large subdivisions would be filled with CLAY. We use to use a gum spear (google it) to get a feel for areas where the material was less stiff. We would then do a hand auger borehole with shear vanes every 300-500mm for acceptance testing to see if the fill met the required shear strength.
But trying to relate any of that too a bearing capacity is worthless as Ron has said. Dynamic cone penetrometers (DCP) have been invented for a reason.
