Electricians on FABO - I want to add a 60Amp sub panel

I don't know what your local codes dictate, but I think 18" would be deep enough if it is in conduit. Go deeper if driving on it. Also, keep your grounds and neutrals separate in your sub panel. Your neutrals and grounds should be bonded together at only ONE point in your system, and that should be in your 200a main panel. Completely remove the green bonding screw in the sub panel and have separate buss bars for your neutrals and grounds.