Trying to just envision trench warfare here. If soldiers successfully forced enemies to fall back past the reserves, what was beyond that? Did the enemies all just die? Did they have an HQ of some kind?
I know there is artillery at the furthest line of defense, but was there an HQ that had to be raided? Were the artillery lines enforced, like cement turrets where you could throw a grenade into?
The only combat postions behind triple trench lines would be the artillery postions. Behind then would be storage areas, command infrastructure. Lodgments for troops not in the trenches (getting their share of rest) .
One could say that if the artillery positions were overrun there were no further fixed defenses. On the other hand, if the ennemy did break trough this line, a single hastely placed machine gun or light gun could easily stop an attack locally and gain time for new defenses to be improvised. This was excactly what happened during the late war German dpring offensives.
If the fixed front was breached, all was far from lost. Only a relatively small portion of the troops were norlally in the trenches. These being manned on a rotation scheme. The troops in rear areas could quickly be organised to firl new stp lines, offer local defenses etc...
The coming if the tank changed all this. The large rhomboids used ti break trough the front lines were imporant indeed, but the really novelty were the lighter mdedium tanks that could exploit the breaktrough.
One examle is a medium tank called Music box: a single medium tank that managed to get to the rear German areas after a breaktrough and spend hours rampaging around, shooting things up and taking quite some prisoners who just surrendered to a tank suddenly turning up, from which they had no effective defenses.