J'ai finalement trouvé, en faisant la recherche en anglais : The game has a built-in mod "Unlimited Soil - Landscaping without limits", which enables the player to add or remove as much soil as desired with the landscaping tools. The cost of moving soil is still in effect. Note that activating this built-in mod will disable Achievements, unless they are re-enabled by a mod from the workshop.
Donc c'est bien pour terraformer sans limite