Keep the artillery going for longer. Sooner or later the entrenched German army would surrender due to lack of supplies. Bomb til the mud is settled and no man's land is passable, and if they still don't surrender, do what the British eventually thought of: send in infantry, covered by artillery fire a bit in front of them. Concentrate a large force on one specific area, while holding back plenty of reserves to try and hide the fact. Once the trench is broken into, a strike into the communications centre to eliminate the leaders. Then, do what would not be expected by the Germans and attack from behind. Of course, by this point, it's quite likely something's gone wrong and this force is destroyed.