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I'm considering running a quest where the players must deal with a large, powerful, and well organized underground Kobold tribe. They initially find the opening of a tunnel in the middle of a village the kobolds invaded during the night. They dug the tunnel from their home base about a half a mile away so it opened up in the middle of the hamlet so they could attack with minimal detection.

How hard is it to dig a tunnel that is >30 feet underground without having it collapse? Is it even possible if the miners have to dig under a small stream, or under an extremely heavy structure like a thick castle wall? Will the pressure of the earth above the tunnel be diverted to the tunnels' walls, or will special supports be needed for the tunnel to work? In addition, how difficult would it be for the villagers to detect that a tunnel was being dug deep under their homes?

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Routine tunnel mining uses wooden, concrete or steel reinforcing to prevent collapses.

In rocky compact soil, tunnels and caves can last for years or longer. In loose soils, one may have to advance the supports for even a foot of progress.

Narrow tunnels last longer than wide ones. For a given width, taller ones are more likely to fail by side collapse than short ones. Squared tops are more likely to fail than arch-shaped ones.

Tunneling through solid rock is even doable... the Romans did so, and many of the tunnels survive today. Some were several hundred meters long, a few were in the double digit kilometers (up to 19), and the longest was a little over 106km

The better the material, the harder the digging. That said, tunnels through root systems of trees are surprisingly durable, and yet much easier than would be the case for similar durability in non-root-bearing soils.

In good chalk, a skilled miner with hand tools can dig up to a cubic foot an hour or more, provided he doesn't need to police his own tailings. In particularly hard rock with poor tools, it can be a cubic foot a week or less.

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  • \$\begingroup\$ Thanks for that line about the root systems of trees. It gave me an idea for my campaign. \$\endgroup\$ – called2voyage Mar 4 '14 at 14:32
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Good question! You're referring to "mining," and it was a common tactic in medieval Europe castle battle. The attackers would either try to undermine the castle's foundations, or dig all the way in and burst through into the castle. See http://en.wikipedia.org/wiki/Mining_(military)#Middle_Ages

Now, going 30+ feet underground sounds like a bit lower than normal. But yes, this did happen, so it's quite reasonable.

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  • \$\begingroup\$ Specifically regarding the issue of supports, I'll add that braces were sometimes used sufficiently, sometimes not (leading to catastrophic collapses) - a function of how skilled the sappers were and how much resource/money the army was willing to spend on it. The opposition, if they detect it, may try to blow it up/flood it with water/etc. \$\endgroup\$ – mxyzplk says reinstate Monica Dec 18 '11 at 2:06
  • \$\begingroup\$ Look for an old musical called "Paint Your Wagon", that's the theme of the whole movie! :) \$\endgroup\$ – Bill K Dec 19 '11 at 17:19
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    \$\begingroup\$ Not all the collapses were unintended, either; often, the point was to trigger a tunnel collapse so as to bring down a section of wall. \$\endgroup\$ – aramis Feb 1 '12 at 18:36
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Historically, this sort of tunnel would require skilled tunnellers (right up to WWI miners found themselves drafted when plans like this arose) and/or a very long time. The mining/countermining battle could become a miniature war; any good history of sieges or sappers will give you details.

But this all becomes moot in a FRP setting; how much can a kobold or dwarf excavate per hour compared to a human miner? Are they better or worse at direction-finding underground? Does a rock-destruction spell cause any noise? For humans, digging that length of tunnel to destroy a village would be a disproportionate effort: do kobolds feel the same? Most systems don't have answers to these questions, (how odd), so it's a GM call. It's certainly not impossible, and if it advances your campaign, go for it: just prepare for the inevitable moment when a player asks how long it will take to dig a tunnel into the kobolds' lair.

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  • \$\begingroup\$ What does FRP mean? \$\endgroup\$ – Joe Apr 15 '14 at 23:49
  • \$\begingroup\$ @Joe Fantasy Role Playing \$\endgroup\$ – Matthew Najmon Apr 16 '14 at 0:41
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In answer to the bit that's not covered by Brent's answer (or at least I didn't see any reference to it in the Wikipedia article), it is possible to detect that a tunnel is being dug, although they would probably require someone dedicated to this task. On a school field trip many years ago (because we have real history in England ;) ) I remember being shown the big wooden barrels of water that would be used to detect miners by watching for ripples in the surface. Unfortunately I didn't think until afterwards to wonder what they actually did about it in response - I would guess counter tunnels, as mentioned in the Wikipedia article, although I don't know how they would know in which direction to dig...

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How hard is it to dig a tunnel that is >30 feet underground without having it collapse? Is it even possible if the miners have to dig under a small stream, or under an extremely heavy structure like a thick castle wall?

Excavating underneath a small stream means the miners are digging through rock or earth submerged under the water table. The tunnel will flood with water unless the kobolds have mechanical or magical means of pumping water.

Likewise, if the village is located next to a river or lake, then likely the kobolds will not be able to dig >30 feet down without encountering flooding issues.

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    \$\begingroup\$ That entirely depends on the local geology. If the stream, river, or lake exists because it's over non-permeable soil/rock layers, then it indicates nothing about the depth of the water table. \$\endgroup\$ – SevenSidedDie Apr 15 '14 at 18:05
  • \$\begingroup\$ @SevenSidedDie - You're right there are special cases, such as the salt mines that extend out under Lake Erie in the United States. Mines and dungeons in mountains and deserts could be fairly dry. However for really deep dungeons located in temperate locations, groundwater will be an issue. You dig a hole in the ground, it will fill with water. Maybe if dungeons are built with impermeable walls that can resist the water pressure. I'm curious if any D&D dungeon designs address the groundwater issue. Like the vast Undermountain complex in the Forgotten Realms setting? \$\endgroup\$ – RobertF Apr 15 '14 at 23:01

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