Mate, as much as I'm loathe to disagree with someone who's proven themselves so switched on (both in-game and in general discussion), I don't think I'd go so far as to call trenches ineffective.
German trench systems, which were far more established in the 1914-18 war than the Entente powers' I think were considerably effective against bombardment. The only thing preventing the Entente from building similarly effective trench systems was that they had to eject the Germans, whereas the Germans merely had to hold ground, thus developing fully effective trench systems was seen as a bad thing...