My only problem with my design for your base bolt hole and what started my re-thinking of the emergency exits in general. They are for emergencies, so I don't think it is a good idea that it takes an hour or 2 to clear the shaft and all of that material needs to be moved into the bolt hole in general.
I was watching a documentary recently on the Diefenbunker
https://diefenbunker.ca/en/ and they explained the emergency escape shafts and why they where put in.
There are 2 emergency escape shafts built into the bunker. These shafts are all filled with p-gravel as well.
Here is how they work and how I think it may work for a bolthole as well.
1. Access to the shaft is gained thru a heavy steel door that swings into the escape shaft. The shaft is outside of the bunker.
2. This area has an open pit just inside of the door where a floor would be and is as big as the shaft above and extends to either side under this anti-room. Opposite to the opening there are rungs embedded into the shaft and forms a ladder up the shaft. Maybe 5 or 6 visible at this point.
3. Above the door is a steel hatch with a latch that closes off the bottom of the shaft. On the wall directly above the escape hatch is a lever. The lever is mechanically assisted and connects to a latch that holds the hatch shut. When you activate the lever the hatch opens under the weight of p-gravel and or sand that fills the shaft above.
4. The hatch above swings open, downward into the anti-room and all of the material in the shaft falls down into the open pit below. The suction from the material falling out of the shaft also clears the material that closed the shaft at the surface. The pit is big enough to hold all of the material and once fully filled, now becomes the floor that can be used to stand on to move to the now exposed shaft above and ladder in the shaft walls. A shovel or 2 could be placed to either side of the escape exit and would be used to only open the shaft at the surface if required.
The escape shaft would be usable in just a few minutes, not hours and described in Riverton and the 3rd/4th edition rule book.