Root cellar with a roof pond

Got a large round stock tank to use as a pond.
Thought about digging it in,but then it occurred to me, it could be a huge waterproof thermal mass as a roof for a dug root cellar.
Turns out such a mass is called a roof pond.
During summer, I could shade it, during winter, floating pool insulation would moderate the temperature.

I am thinking of building with just enough headroom for a seated person, and putting a wheeled chair in the cellar.
This would avoid a few feet of digging.
I am considering 55 gallon drums as the walls.
A center column would reinforce the unsupported center.
Each barrels can support two filled barrels stacked on top of it.
That's about 900 pounds.
8 of them should hold a 10'diameter
stock tank filled 18" deep.

55 gallon drums are about 35" high, not enough for any reasonable access, and two drums would give almost 6', enough for most people to walk comfortably (tall people would have to bend their heads ).

You would probably want some kind of spreader to keep the center post from puncturing or denting the tank. A center post would make a wheeled chair impractical, as you would use up a lot of floor space for access. Standing/walking access should be no problem.

If you need to minimize height, you might dig the perimeter for the 35" drums as wall, and come in 2' and dig lower for floor space. Setting posts for shelf support and roof support at this edge could eliminate the need for a center post, giving a 6' unsupported diameter instead of 10'. (If the posts angle inward a bit, you could even reduce this to 5' or less.) Then a wheeled chair for access may become practical.

Thanks for the reply!
The soil on site is filled with urbanite, very hard to dig.
This is what sparked the idea of limited depth.
This may just be the wrong time and place for this, but the idea popped intent head so I thought I would throw it out there.