We have some of our sites in caliche clay areas. The hardpan is located under the clay soil so sometimes we can't get the depth we really want. When it comes to actually trenching through the soil it depends on how wet it is. You can still trench unless the area is totally flooded out. However, the trencher may leave jumbled up wet clay in the trench which means that a LOT of trench clearing of muck will be necessary. What it's going to boil down to is excatly how wet the clay is. You'll also want to have a water hose handy because you'll want to wash down your equipment as soon as practical when done. If not the clay will harden like rock and be next to impossible to remove from the equipment.