When planning the construction of a rainwater harvesting system, one of the most critical aspects to consider is the ground beneath the future tank. Water weights 8.34 pounds per gallon. This can add up to tremendous weight under a tank:
Two extremes to look out for are soils with high clay or high sand content. Having a large amount of clay in a soil can result in shrinking and swelling. This can result in shifting tanks or broken pipes. A large amount of sand may be prone to erosion around the base of the tank.
An ideal base for tanks over 500 gallons would be concrete. However, you can also place a layer of pea gravel over a clay base to lessen the impact of swelling.