What material should the oil storage tank base be constructed of and what dimensions does it need to be?

Types of base materials, as listed in British Standard 5410: Part 1: 1997 to achieve this protection, include concrete of at least 100mm thickness, paving stones of at least 42mm thickness or stonework of at least 42mm thickness (all laid on a hardcore base to give an imperforate base). The overall size of the base should be larger than the oil storage tank and any integral oil storage tank bund, so that when the oil storage tank is installed, the base has a clear projection of a minimum of 300mm around all sides of the oil storage tank.