Copied from one of my previous answers:

"It depends on where the trenches were. On land that was previously used for farming such as on the Somme and Arras battlefields, the trenches and craters were filled in and the land reverted back to grazing or crops. In other areas where the land is less useful such as wooded areas the trenches and shell holes were simply left to erode naturally, not worth the effort of filling in.

Some trench systems have been preserved as have heavily shelled areas simply because they are too dangerous to use, the most notable being the Vimy Ridge battlefield. There are other reasons of course, preservation for memorial reasons such as the Newfoundland Memorial Park where 1916 trenches are still present."