I am in the process of building a multi level deck where the second level would be low to grade. As such, i had to dig some trenches (about 10-12" deep) as part of the beams will be below grade. My issue is that since the ground is full of clay, this will cause a drainage issue within the trench whenever there is hard rain (see attached picture).
You will need to dig these trenches deep enough so that the supports for your deck are situated BELOW the frost line for your area. No amount of fill or drainage will be sufficient since it's the freezing of the moisture in the ground that causes frost heave.
Get in touch with your local building codes department and get their specifications for how deep things should be in your area.
I'm also not clear what you're doing here with this trench. Generally decks are supported by piers which would be a lot less excavation than what you're doing.
If you actually intend the beams for your deck to be below grade, I think you're going to have issues. You should probably rethink your plans here if it's not possible to excavate the area and grade it so that water drains away from the area.