With what materials do you fill your 9 foot deep in-ground pool to permanently close it?

Concrete Hello, before you go filling a large hole with concrete, think about a few things: 1. that could be quite costly. 2. concrete has a density roughly in the range 2300kg/cu.m. to 2450kg/cu.m. Water has a density of 1000kg/cu.m. You might be increasing the load beyond what the foundations were designed to take. This could be dodgy for services passing near or under the pool. It might also be dodgy for buildings that share a foundation. Your poolside surface may also be affected by cracking. 3. Filling the pool to 500mm from the top with inorganic construction spoil, then 400mm of a quarry run gravelly sand compacted in two layers and 100mm of concrete with a mesh reinforcement 65mm from the top may work. Alternatively go 600mm from the top with spoil, 300mm with quarry run and 300mm of topsoil, which could be landscaped. It all depends on site conditions and what you want to achieve with the final look. MOST IMPORTANT. Get advice from a local professional, apart from technical issues, there will probably be planning and environmental issues with your local authority. Do consult a professional with indemnity insurance, probably a civil engineer . Your national engineering council will help, if you do not know anyone. They should be available with a quick search. By the way do not even think about suing me for that advice, unless you want my mortgage, my cat and my lab boots. Best of luck.