True, local rock is cheapest - but the structure that you lift has to with stand tens of thousands of haul-up/lower-down cycles = standard blocks with enduring

True, local rock is cheapest - but the structure that you lift has to with stand tens of thousands of haul-up/lower-down cycles = standard blocks with enduring quality. Scrap iron does not pack well, unless melted into standard blocks = $200/tonne or more. Crushed granite and various sizes of graded rock (the MacAdam principal) with close to 5% of voids to which you add 5-7% portland cement, as well as a few rebars and cemented in attachment lugs is what these block are made of. They will have a rebar pattern that prevents fragmentation in there are any cracks over time. AS the cement cures, the cement re=hydrates and creates a void free very strong in compression block. With machine pick-up and place-down that carefully avoids crack-making impacts they can survive tens of thousands of trips - which will probably be 1-2 times/day - as power is needed. A water filled mineshaft would work, but most vertical shafts are 8-10 feet wide and perhaps 16-20 long, as they were usually made with 2 elevators - 1 goes up as 1 goes down. For this application they could fit the shaft. Water might erode and places with bad mix. These shafts can be several thousand feet deep - varies. These built on land pyramids are far wider/deeper = lower height needed for same storage as a deep mine limited in width. Most large mines now use 'trackless mining' = a long spiral highway 30 feet wide and 20 feet high going down at an angle, with rock hauled up by electric carts(huge ones) https://www.google.com/search?q=trackless+mining+wiki&rlz=1C1CHBF_enCA924CA924&sxsrf=ALiCzsZ2rVDPFQWR_uiq4KP1ETMzMLLRSQ%3A1655324254182&ei=Xj6qYujgCoH0tAbg2pKgBA&ved=0ahUKEwjoh6Kxo7D4AhUBOs0KHWCtBEQQ4dUDCA4&uact=5&oq=trackless+mining+wiki&gs_lcp=Cgdnd3Mtd2l6EAMyBQghEKABOgcIABBHELADOg0ILhDHARCvARCwAxBDOgUIABCABDoGCAAQHhAWOggIABAeEA8QFjoHCCEQChCgAUoECEEYAEoECEYYAFDbBlixGWChHmgBcAF4AIABwQGIAYkGkgEDMS40mAEAoAEByAEGwAEB&sclient=gws-wiz https://www.google.com/search?q=trackless+mining+wiki&rlz=1C... I have not seen how one of these surface ones operate, especially with wind/winter and how many of these cement blocks they use get cracked/dropped. Can you imagine an attachment failure at the worst time knocking down multiple blocks? Recovering from that would take days/weeks and the actual occurrence might halt things for a while while regulators sniffed into everything, esp if there were any lives lost?