As the prisoner there should tell you, there is a hidden storage room in the cell diagonally opposite from his (left of the cell with the broken door). Check the map and it should be obvious which wall section inside is movable; there are a couple vertical wooden beams at the back of the cell, with a button on the left beam. After that there is another button (on the right IIRC) to remove another false wall.
There are two small kegs of explosives inside, one to remove the cave-in and the other can be used shortly after that.
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