Long story short, I am currently running a campaign where there is a group in a dwarven mine that needs to do a time-sensitive mining operation. The group has about 6 dwarves with 14 strength each (and are equipped with miner's picks).
Are there any rules about how far one can mine in a day? If not, what is a good rule for this? How does strength affect this?
If you'll accept rules from older editions, the AD&D1e DMG has the numbers you need on p. 106.
To summarise, properly equipped dwarven miners can each remove 90 cubic feet per 8-hour shift in very soft stone, 70 cubic feet per shift in soft stone, and 35 cubic feet per shift in hard stone. It also points out that with appropriate scaffolding, digging can happen simultaneously at floor level, and a few feet above that. It has no scaling for strength, but I might give a 10% bonus for all the miners being strong. Humans can only manage 75, 50 or 25 cubic feet per shift.
Given that the digging is time-critical, the tunnel will probably be as small as practical. I've been in a tunnel dug through rock under extreme pressure of time, the counter-mine at St Andrews Castle in Fife, Scotland, which looks like this (picture from Wikipedia):
For scale, the trough in the floor is about a foot wide, and is where you're intended to walk; the larger width of the upper part of the tunnel allows for swinging a pick, or a weapon, and is 5-6 feet wide. The tunnel is only about 5 feet high from the bottom of the trough to the ceiling, and dwarves might well make it lower.
It was dug during the siege of 1546-47, when the besiegers started to dig quite a large tunnel under the walls. That tunnel is about 10 feet wide and 8 feet high, and was intended to allow planting a lot of gunpowder under the wall to collapse part of it.
When the defenders realised what was going on, they tunnelled desperately to reach the enemy tunnel before it got under the walls, which is why theirs is so small. They succeeded, just.
They didn't know exactly where to head, and were having to work by listening to the sound of the other side digging. That's why this tunnel has a bend in it; they made a mistake and had to change course.
For your case, if you're digging a small tunnel, I'd have two dwarves per shift, one digging as fast as he can and the other resting from his efforts. If you have other people around to help, they can clear away the rubble, saving the dwarves for the digging.
For a 5'x5' tunnel, 35 cubic feet per shift (dwarves through hard rock) is 1.4 feet of forward progress per shift, or 4.2 feet in 24 hours.
As a DM, I generally refer to the Stone Wall spell for stone toughness, which states that a 6 inch 10x10 wall of non-magical stone has 15AC for sheer hardness and 30hp. As such, when the players want to tunnel through, I make them take attacks on the wall, with the wall chipping away 1 inch per 5hp, 10 feet tall and 10 feet wide. I treat mining picks as war picks for damage rolls, which base off strength.
For extended mining like you're talking about, nobody wants to roll for each attack over the course of hours. I'd get an average damage score from 2 characters (because 10 feet is enough space for 2 normal sized creatures) probably three rolls each. Any misses would count as zero, as the swing glances off the rock without doing damage. Average the damage and combine the two character's scores. 1 inch per 5 hp. I'd say that's the average distance they cover per round.
You'd also want some way to move all that rubble out of the way, so keep in mind carrying limits and the average weight of stone (150-200lbs per cubic foot based on stone type) so your miners don't mine faster than the rest of the party can clear the path.
I also apply the forced march rules for extended strenuous activities, but like everything else in this answer, it's all homebrew.
I've consolidated the information in these posts in to tables for the convenience of all. A “shift” here is an 8 hour shift of work. - happy gaming everyone -';..;'
| Digging (Non-Dwarf) | Standard Mining Rates (cubed ft/shift) |
Standard tunneling rate (5'x5' tunnel ft/shift) |
|---|---|---|
| Very soft stone | 75 | 3 |
| Soft stone | 50 | 2 |
| Hard stone | 25 | 1 |
| Digging (Dwarf) | Standard Mining Rates (cubed ft/shift) |
Standard tunneling rate (5'x5' tunnel ft/shift) |
|---|---|---|
| Very soft stone | 90 | 3.6 |
| Soft stone | 70 | 2.8 |
| Hard stone | 35 | 1.4 |
Longer answer: 5th edition doesn't say, but we can use earlier versions to aid us.
According to the AD&D Dungeon Master's Guide p. 106, it depends on the type of rock. For 8 hours of labor, one properly equipped dwarf can mine 90, 70 or 35 cubic feet of rock depending on whether it is very soft (limestone), soft (other sedimentary rocks) or hard (igneous) respectively.
It is also assumed that they would only do 8 hours of labor per day, but you could use the forced march rules if they tried to carry on working for longer this. As this is a time-sensitive operation, if they have fewer than 8 hours, I'd consider letting them mine faster than this, but they would become exhausted much faster. For n between 1 and 2, perhaps n times faster, but they become tired after only 8 divided by n squared hours?
According to the AD&D Dungeoneer's Survival Guide p. 50, each 10 ft section of tunnel would also require shoring with either stone or wooden support pillars if you want to protect against the risk of collapsing.
The Dungeoneer's Survival Guide p. 7, also suggests that the most commonly encountered caves will be limestone, and therefore very soft stone.
The Dungeon Masters Guide p. 106 goes on to say that 16 dwarves could work simultaneously to create a 10 ft wide tunnel, so it would seem reasonable to allow your 6 dwarves to work together to create a 5x5 ft tunnel. (I assume they progress in a V formation to avoid congestion?)
Of course the displaced rocks will need moving out of the tunnel to avoid becoming trapped and potentially suffocating. However using the usual movement rules for carrying/dragging this is likely to be fairly negligible in comparison to the slow speed of mining for all but the longest tunnels.
For 6 dwarves mining a 5x5 ft tunnel through limestone they could displace 67.5 cubic feet per hour between them, which is 2.7 ft progress per hour, or 21.6 ft in 8 hours, after which they'd need to rest for the day, or use forced march rules to see if they become exhausted.
The rules don't give any bonuses to mining for strength, but it would seem reasonable to do so; your players are heroes after all! If we work backwards, take some liberties switching between AD&D and 5e rulesets and gloss over trying to mechanically explain why dwarves are faster, we can get a rough idea. Lets assume that the soft limestone has an AC of 13, and your typical NPC dwarf with a STR of 10 has a +2 to hit and is swinging once every 6 seconds, doing 1d8 points of piercing damage on a hit. In 8 hours, that's 4800 rounds; an average of 2400 successful hits; and about 10,800 damage to the stone. Given they would displace 90 cubic feet, it suggests each cubic foot has 120 hp. Your 14 strength dwarves have +2 STR modifier so will be hitting 60% of the time instead of 50%, and doing 2 more damage every time they hit. This works out at 156 cubic feet for 8 hours of labor.
Therefore your 6 14 strength dwarves mining a 5x5 tunnel through limestone could displace 117 cubic feet per hour between them, which is 4.68 ft progress per hour, or 37.44ft in 8 hours, after which they would need to rest for the day, or use forced march rules to see if they become exhausted.
