What type of surface is required for placement of a vertical tank?
Inspect the surface to ensure a smooth, level area that will supply full bottom support. A properly constructed concrete pad that is capable of supporting the contents of the tank is the best support and is recommended above 2,000 gallons. Other materials such as very flat blacktop, or contained, packed sand or pea gravel have been used successfully. All surfaces must maintain uniform support through freeze/thaw, erosion and other disturbances.
Source: FAQs - Peabody