Please tell us more about your water tank, Eric.
I can tell you some, but I can't claim it is the right way to do it. Several tank manufacturers say a good water tank base can be made from sand, all it takes is something around the outside to keep it from moving out. The rabbits in this country love to burrow, so I had little option other than to make it from concrete.
I used broken bricks and old concrete for the bottom of the foundation, smashing them with a sledge hammer until I had built up a layer about 4 inches thick over what was already pretty solid ground. The rubble was blinded with sharp sand then vibrated in nice and tight with a whacker plate.
If I had been able to get ready mix concrete in, I would have poured a slab in one go. That was too much for me hand mixing (even with a bit of help from the Iseki's bucket), so I cast the perimeter first, outside the area where the tank would sit. The plinth is 6" deep and 10" wide with some good rebar, especially on each corner.
The middle of the slab where the tank sits is only 4" thick concrete (less to hand mix !), though I doubt making it much thicker would serve any real purpose. I went to the trouble of leaving out the aggregate for the top inch of the concrete, making that a pure sand/cement screed, which is a lot easier to tamp down flat. Before it hardened I smoothed that over that with a float, thinking it would be the best possible finish to leave with no chance of puncturing the liner. That was all a complete waste of time as when the tank parts came, the instructions say to put down a 2" layer of sifted sand over the concrete base inside the steel rings. Knowing that may save you some time.
If you need 30,000 gallons, that is a much larger tank than mine (I have the option to connect to a much larger tank in the next field). Mine is only 6' diameter and 6' high. 120,000 litres would cost here 3,130 pounds (+ tax) for galvanised steel rings, fixings and 1 mm butyl liner. With an EDPM liner that would come down to 2,433 pounds.
Steel Open Top Tank & Liner: 12 , ltr 1 .14m ∅ (33') | Butyl Products Ltd Group
