there's many reasons why this wouldn't be practical, but for a start once your trench is within 50 yards of mine, how much further are you planning to dig?

Because anything within 40 yards, I can reliably toss hand grenades into your trench the whole time you're digging.

Tunnels were very much used, generally to put explosives under defenses so the territory could be easily taken after the defenders were killed.

They weren't reliable though, because tunnel collapse was always a major risk, and defense in depth meant the enemy could pull back and make a new line of defense quicker than a massive tunnel could be made.

This was actually just what the Russians did in the Brusilov offensive in 1916, where they dug hidden trenches towards the Austro-Hungarian trenches to insure that the soldiers did not have to cross a wide no mans land.

I'm not sure, but maybe one of the reasons why this succeeded was that there was no vigorous patrolling of no mans land by the Austro-Hungarian army. If you detect such trenches, you are warned that an attack is imminent and you can direct artillery towards it.

A significant issue is that trenches weren't singular lines of defence, but rather multiple ones designed with defence in depth as a guiding principle which had massive ramifications.

Gaining the outermost trench wasn't all that difficult, holding it however was a nightmare. The outermost lines were lightly defended with the bulk of the troops stationed on the ones behind that, away from enemy artillery while the front line was still well protected by theirs. This meant if you gain that outer trench line, you wound be facing immediate counterassaults from large units of fresh infantry as well as being under fire by enemy artillery whilst you were still trying to reorganise your units and move your artillery up to defend your line. The enemy also held other high cards such as having direct communication trenches leading to frontline trenches, defences between the first and second lines being designed with counter offense in mind and having clear, stable lines of supply behind their remaining trenches whilst you were stuck with the question of how to lug HMGs, ammunition, wounded and a hundred and one various things through the quicksand like quagmire that was no mans land and get your artillery forward to support you (which given said forward positions were square in that quagmire was a difficult task in the least). Logistically and tactically, you were quite utterly screwed despite your success.

So even if you managed to dig a trench into the enemies outer line (and not have the daylights shelled out of you in the process), you were still massively exposed to counterattack, especially by the Germans who were notorious for quickly and aggressively pushing back, whilst lacking artillery and logistic support.

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