Are forms needed/recommended for heavy duty foundation or will using the dug out pit as the form enough?

I’m digging and pouring several foundations for solar trackers. The plan calls for an inverted “T” dug down approximately 80” and approximately 3.5 yards of concrete poured in each.

I’m wondering if a form is needed for the bottom part which is approximately 80”x80”x30” or would it be just fine to fill the pit with 30” of concrete and only form the vertical center section?

AHJ doesn’t care how it’s done as long as it passes electrical inspection.