Rainwater Havesting installation

 So you make a big hole in the ground for the rain water harvesting tank, pour some concrete to make a slab (about 6″ thick is fine) making sure you place some steel loops to strap the tank down on to the slab so that it does not float up when you put more concrete on it to hold it down. We are on very free draining sandy soil but with the heavy flooding of recent times, the idea of the tank rising up out of the ground if it were empty prior to heavy rain fall was something we wanted to avoid. Our tank supplier said there have been a few stories of people who just backfilled with pea shingle or similar having such uplifting experiences so for £500-600 of concrete (about 8 cubic metres) it did not seem worth taking the risk.