If these plugs had a system for removal, I would presume that every time they were replaced, there would need to be some patch work and some plaster type material spread on the surfaces at the bottom so that when the plugs slid into place, the gaps would seal well for pressure. In this case, the plaster wouldn't need to be that of a strong hardening bond as it would only need to remain solid enough to fill gaps under great pressure from the granite plugs leaning down on the bottom surface.
If that were true, (multiple assumptions here but...), then the force you'd be facing to remove the plugs would be the coefficient of friction of these blocks initial wedged position. After they were pushed free from that stuck position, the pressure necessary to simply push them up could be closer to a one to one in force from the water weight to the stone weight as they're both loading pressure on the same slope.
Any discrepancy of this system and force measurement can of course also be affected by a mechanical linkage in the system that affords 'advantage' if necessary. Meaning you adjust it to a lower gear, say a 2 to 1 for the weight of the water pushing on the rock, which give the equivalent of 100 tones of water vs 30 tones of granite. Its presumable that might be sufficient or some mechanical advantage could solve the force requirement necessary for that first most difficult push to break the static coefficient of friction. Again, after that, it might be closer to a 1 to 1 push for distance for every fill and jack, (hence my proposal that a few days would be all that's necessary for this system to complete its plug push up into the GG.