Abstract: Vibroflotation method is a comparatively matured means in the improvement of soft foundation. But it is seldom used abroad and firstly used by authors in china for consolidating soft foundation at the sea bottom which is affected by the tidal. Due to the effects of the tidal level, the following problems need to be solved in making piles at the sea bottom.1. Material-feeding SystemA system has been designed including protective barrels for material-feeding and pipe lines in feeding material. The system has been matched with automatic tide indicators in solving the problems with satisfaction.2. Position-fixingIt is required that the deviation of position for gravel piles, 1 kilometre off the seashore under the sea, should be controlled within ±10cm. And it is demanded too that the efficiency in the undersea position-fixing should not be lower than that on land. The cross position-fixing method has been adopted. It is shown that accuracy and efficiency have both met the requirements. Experiences on position -fixing in the night have been accumulated.3. Working Boat and Equipment(1) The working boat involved was a salvage tug, 79 metres long and 14.5 metres wide,with 3000 tons displacement. A 4.2×2.6m working platform and a cargo boom, 15 metres high and with a 15 tons of lifting capacity, were mounted on the tug.(2) Vibroflot is an eccentrical viberating vibroflot,driven by a 55 kW vertical hollow-axle underwater generator.Bearing capacity experiments with a 4×4m bearing plate on natural foundation and composite foundation with groups of undtrsea gravel piles, indicated that the bearing capacity of the latter increases by more than 200%.