It depended on where the trenches were. Trenches dug across land that was formerly used for farming were quickly filled in and the land reverted to farming (a vocation in northern France that remains quite dangerous to this day). Some trenches were preserved such as those at the base of Vimy Ridge in France, these now form part of the Vimy Ridge Memorial Park as well as those at the Newfoundland Memorial Park. Those that were dug on land that was of no use such as within forests, marshy areas etc were simply left to erode by themselves. You can Google First World War trenches and find photos of what remains of those trenches that went unfilled.