A mining shaft would be an option for a base in a Zombie Apocalypse. It is unlikely that they will be any zombies inside, they also won't be a target for Bandits because they have no valuable loot in them. However, despite being secure they have no source of food or water meaning it would be necessary to go out in the day to hunt or scavenge for food. While doing this you could get killed by zombies or bandits could follow you back to your base. Another flaw would be that always being in the dark could cause physiological problems for people that live in them. In conclusion, the cons outweigh the pros so they are not recommended. If you do decide to got to a mining shaft know that they only work for small groups (less than 20 members), because of lack of food.