I did and arty test with 81mm vs a platoon and found that men hiding in the foxholes had fewer casualties then in the trenches.It makes sense when you think about it, hiding in a foxhole offers 360 degree protection,men in trenches are extremely vulnerable to the sides.If a round hits 5m to the right of your foxhole, no effect, 5m to the right in a trench and it lands in the trench, that's a bad thing for those men.
Ran the test 5 times with exact conditions and the same results,men hiding in foxholes suffered fewer casualties than men hiding in trenches.
Not saying its absolute or conclusive, just my results, do your own tests.