Specially-shaped blocks of foam for use underground as aggregate. Because of protuberances on the faces of the blocks, the blocks fit closely together in only a small number of ways. In filling a volume, the overwhelmingly more likely arrangements provide a reasonably certain percentage void. Thus, the blocks can be transported to an application arranged without any significant void, and then used to fill a volume, providing a reasonably well known percentage void and compressive strength. Use of these foam blocks can provide thermal insulation, cushioning and various combinations of compressive strength and void, depending on the size and shape of the blocks and the size and shape of the protuberances relative to the blocks.