Tabletop Games had a ruleset called 'Trench', how original!
It had rules for large battles but also included a trench raid set as well, along with ideas on making a trench board. You had to plot star shells, artillery etc so you could end up bringing down a barrage or illuminating your own troops.
If you fancy them and cannot get hold of a set then I can try to dig mine out as I have not used them for years.