As you probably noticed in the trailer - there is no one in those trenches. This is what made World War I so tragic. When front could be moved it wasn't because the order wasn't given. And, worse, the orders were given when the attack was doomed.
Now, the War of the Trenches was fought in that manner. One day the Germans moves 100 metres, taking english trench, then month later the English/French counter attack taken it back moving even into German trench.
This is what the scene invoke. That they are in english-made trench previously occupied by Germans, now vacant. The longer the army stayed in the trench the more "naturilasied" it looked as each nation repaired and fixed them with their own methods. (From what I remember the English ones were more keen to use sandbags while the germans used wicker to weave the wall holders.)