[RPG] How do underground tunnels stay up? Is it possible to dig under a castle wall without magic to support the tunnel

system-agnostic

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?

Best Answer

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.