Two suggestions to make the cave experience richer.
I) Buildable one-tile and two-tile doors - costs about the same as a strongbox (2x cost for cart-passable two-tile door) and has comparable soak to strongbox.
II) Reinforce tiles - possibly an adventure action along with repair and destroy - expend nuggets to raise the Str requirements to mine a tile.
Example using Str as the limiting stat to overcome tile hardness (maybe in reality axe q matters as well?):
Two nuggets of any hard metal raises the tile hardness 10 points, hardness can not exceed character Str + 10.
(meaning that if you spend alot of nuggets you can make a vault impenetrable for charaters of your Str level.