Unfortunately, I do not know what the electric company buried in the yard, as I simply contracted out the work, and they essentially ran what they use for underground residential 200-amp service. That may be overkill, but the price for the work was reasonable.
The gravel looked like pea-gravel and sand mixture. I thought it was called bihumideous fill (spell?), but again, the electric company did all the work. Whether you need this or not is, again, dependent on your municipality.
Besides, whatever was used for me may not be legal, or even available for your area. I would suggest contacting your local authorities, something you will need to do because you will probably need permits for the project. As you can tell, I‚Äôm big on permits, because I‚Äôve seen people get burned (both figuratively and literally) on work not properly documented and inspected, with regards to electrical work. In one case, the resulting fire ended up not being covered by the insurance policy.
Good luck, and keep posting those questions!