First step would be a geotechnical investigation.

I agree with bimr. This sounds simple, but there is too much here that can be guessed wrong, leading to a bad design. Better to let a geotech engineer give you something closer to facts.

For my one petro tank project years ago, the geotech report (provided by the client) even included a recommended gravel foundation design. If you commission a geotech report, ask the geotech engineer to do the same.

Most of my tanks have been for potable water and used concrete ringwall foundations. The gravel foundation was a new one for me. IIRC, we had two 60,000 bbl and two 80,000 bbl tanks, 40 feet tall, and the gravel foundations in decent soil were something like 3 feet deep over the entire footprint of each tank and extending 5 feet outside the shell....something like that.

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

Without a professional to assist you, there might be a mistake, an item missed, etc. I'd go back to the "boss" and explain. While some guidance here might help, we do not have all the details. One thing fur sure, be sure that the earth or compacted fill outside that ring foundation is well compacted to a distance at least out from the foundation equal to the depth of that material below the ring foundation. Even though what the tank sits directly on might not compress, any weaker material outside and within that dimension mentioned above can allow the ring to settle and slip out down and away from the tank. Then a major failure is going to happen.

I never thought about the base until we put one in and it corroded out the tank bottom in about two years.

@JedClampett -
Years ago, before "oil" became a frightening word to environmental engineers, we used to thoroughly mix sand with oil to make the base pad for the tank bottom. That probably saved the lives of a lot of tanks from an early corrosion death!
Dave

Thaidavid