Expanded polyethylene foam is invaluable as a void fill product in so many construction projects. The combination of being extremely lightweight with an excellent compressive strength means it fills space without adding weight. In the case of a swimming pool, geofoam from Universal Construction Products can also be shaped and formed into curves as well as providing void fill. The contractor installing a new swimming pool on the top floor of the Margaritaville Hollywood Resort meant used EPS geofoam as void fill and also to provide the contoured formwork for their gunite. Take a look here https://www.universalconstructionfoam.com/pastprojects/geofoam-used-for-filling-voids-and-shaping-pools/