A gravity-based structure (1) for supporting offshore installations comprising a foundation (4). The foundation (4) has a basal portion (11). The basal portion (11) has a plurality of openings (10) for drainage of water. There is at least one first layer (7) of filter material, with a gradation suitable for draining water entrapped beneath the basal portion (11) to the openings (10). The layer (7) is installed on the seabed (8) beneath the foundation (4). The foundation has a downwardly extending skirt (5) that is adapted to penetrate into the seabed beneath the filter material layer (7).