WW1 tunneling - bypassing the front line?

There are 3 problems to the idea.

1. The defense systems of WWI (particularly those on the Western Front) were often multiple miles deep. To tunnel behind the enemy line would easily require a tunnel a few miles long.

2. Given the limited flow of manpower over such a small pipe, it would be a fairly simple exercise for the defenders to contain the troops coming out of the tunnel.

3. the combatants had fairly sophisticated anti-tunneling capabilities, as tunneling was not a new idea.

While the above answers are quite valid, here's something else to think about: It's highly likely that the enemy would pick up on a tunnel dug for such a purpose, and move to defeat it, as counter tunneling efforts included listening systems to detect the construction and use of tunneling. Having many men stationed in the tunnel prior to the attack would be relatively easy to detect by that method, and so betray the presence of the tunnel.

While I like your thinking there are a few issues with such a plan:

• Emerging behind enemy lines means there may well have been other enemy troops (just as fresh) in the general area.

• With WWI technology it would be extremely difficult to reliably pick (and hit) a suitable exit point. Tunneling to the lines was comparatively much easier in terms of judging the distances etc. Also if you were slightly off the explosions still stand a decent chance of doing at least some damage to the enemy forces, and the distraction element (thus allowing your forces to close the distance overland) would still be largely intact.

• Digging a tunnel large enough for moving substantial numbers of men in a reasonable timeframe would be difficult - especially with WWI technology and battlefield conditions (digging a tunnel to place a single mine could take nearly a year and they would be much, much smaller than one capable of moving troops)

• If you did manage to dig a wide enough tunnel to move enough men to make it worthwhile the tunnel exit point would need to be wide as well - otherwise they are just coming out single file and one enemy soldier with a decent firing-rate rifle/gun and suitable piles of ammo could hold such an exit for as long as they want to really.

• In order to dig a tunnel exit sufficiently far back that it wouldn't be noticed (and either wide enough to allow troops out more than one at a time or in a quiet enough location to give you room to form up before attacking from behind) the accuracy in digging and positioning simply wasn't there in that era - not into territory controlled by the enemy certainly.

• Also if you built a wide enough tunnel (as above) you'd have the problem of keeping it stable - wide tunnels need matching amounts of structural re-enforcement to keep them from caving in under their own weight (technology would have been one of the limiting factors again as would time). And that's even before you take into account that the ground above will likely be receiving hits from mortar shells, tanks and other vehicles rolling across them and so on. Losing a substantial number of troops to cave ins would have been very probable, especially when you consider that digging such a large tunnel would have been difficult to keep covert, so if the enemy knew one was being dug it would be trivial to collapse it.

Conventional "over the top" charges weren't a good solution either (as you correctly point out) but considering the above it's difficult to see tunneling being any better - and quite possibly worse. Much of the "advantage" of such a plan relies on the enemy not knowing you were coming - you may be able to pull it off once or twice but after that it would be easy enough to prepare for it and there goes the element of surprise, and once the Germans had used tunneling to place a mine against allied forces (to great success) both sides were not only tunneling themselves but were actively watching and listening for enemy tunneling operations.

Update following the OP's own

To more directly answer the clarified question I'm not aware of any recorded instances where tunnels were used to directly place troops behind enemy lines in WWI where the troops themselves were used for the offensive action themselves, presumably because (when you consider the issues described above) it was nearly logistically impossible and in any case would struggle to be more effective than a mine/mines and far more dangerous.

The sort of "advantage" you describe (which I still think is highly unrealistic in the scenario; moving "a few hundred men" and their associated equipment through a WWI-era tunnel, undetected and in fighting shape when they arrive is pure fantasy IMO) would be negligible, when you look at the sheer scale of the troop numbers involved you can see why - on the 1st day of the Somme the British alone brought 13 divisions and a division was approximately 16,000-18,00 men so that's 208,000-234,000 men - a "few hundred men" wouldn't even move the needle on a battle of that scale.

And when you compare the offensive potential of a few hundred (or even a few thousand) infantry troops compared to the destruction even one of the mines could cause, not only in terms of enemy troops killed or incapacitated but the destruction to infrastructure, artillery and machine gun placements, and equipment it's not hard to see why no-one tried the idea of using them to deploy troops directly.

It really is no-contest: The Lochnagar mine alone obliterated approx 300 to 400 feet of German fortifications including nine dugouts!

Offensive mining was by no means a perfect strategy (trying to traverse a 300ft crater with infantry troops created something of a killing ground and combined with some sub-par infantry tactics didn't exactly help matters) but tunnel-deployment would likely have been a 99% certain death sentence for the men in question if the enemy soldiers didn't kill you your own side might - this was the era of large scale shelling, and wouldn't have achieved anything for it.

All the troop-on-troop conflict in WWI tunnels that I'm aware of took place in the tunnels themselves either by accident (tunneling into an opposing tunnel by mistake was quite common) or deliberately to counter a detected tunnel (at least in the early days of tunnel warfare the German tunneling techniques were much louder than the virtually silent "claykicking" employed by the British forces)

The closest I can think of to the sort of tactic you propose was the Chi-Chi used by the VietCong during the Vietnam war and it was reasonably effective - but that's a whole different tactical situation from the relatively static trench warfare scenario seen in WWI.