Abstract: A double-walled thickener tank has an outer shell bottom and an inner shell bottom spaced to form interconnected or a discrete void space therebetwee. An incompressible load-carrying permeable material such as oil-impregnated sand is compacted in the void space(s) to provide a support for the inner shell bottom and confined fluid in the tank. Any leakage of fluid from the tank through the inner shell bottom is initially confined in the void space and conveyed down a slight slope on the top surface of the outer shell bottom so the leaking fluid drains into an interior space of a double-wall or single-wall discharge cone at the center of the tank where the leak and void spaces are accessed to detect any leaking of fluid from the tank.