During a final stage of the skirt penetration or after the installation of the foundation, some extra load is applied to the structure. This is called "preload" because of application of a load before the commencement of the service. The main purpose of the preload is to minimize the settlement or inclination in a lifetime, resulting in high level of serviceability and reduction of maintenance of the structure. In case of skirt suction foundation the preload is available by either a surcharge such as water ballasting or a load by reducing water pressure in the skirt compartments, so called suction. The soils just below and around the skirt tip are imposed by the preloading. The magnitude of the preload should not be less than the imposed load after the installation of the foundation. As the foundation is penetrated into sand/gravel strata, the application of preload in a short time is enough to satisfy the above purposes. The experiment has been conducted to evaluate the preload effect on the skirt suction foundation. This experimental approach investigates the difference in the preload effect between internal and external loading, which are suction load and jacking force respectively. The previous installation of the offshore concrete structure with skirt suction foundation shows that the preload during installation is valid because of no significant settlement after sand ballasting.
The skirt suction foundation consists of thin concrete and/or steel walls called "skirt" projecting down from bottom slabs. The skirt is penetrated into a required depth of a seabed due to not only a selfweight and a ballast but a suction applying inside of skirt compartments. The skirt penetration has the purpose to maintain the stability under extreme loads such as earthquake, storm and to control the settlement and inclination against ordinary loads during the service.