RE:RE:RE:RE:RE:RE:RE:RE:RE:RE:Lol…nice assultI wouldn't use "overburden" relating to a potential block cave. It suggests removal of material at substantial cost. I believe the term is "subsidence zone" and it will normally run all the way to surface from almost any depth as above the drawbells the rock fractures just keep running vertically. The resulting subsidence is why no engineered structure can be above the drawbell regions and have to be off to the side - like lifts, tunnels, ramps etc to access the drawbells, undercuts, extraction levels.
it IS expensive. Which is why scale is so important for feasability. The bigger the better. Deep adds some cost but is far from the main consideration. Grade, tonnage, access. Spend billions, make billions.
cg