which bears a spade 23b designed to anchor into the seabed the periphery of base 6 is provided with rolls 24 onto which are rolled porous sheets 25 manufactured under the brand name filter x by the american firm carthage mills inc cincinnati ohio united states and distributed in france by the sindic company 16 rue jean mermoz paris these sheets 25 are made of porous plastic material which retain particles with a dimension in excess of 0 088 mm in a turbulent water stream and retain even finer particles in a laminar flow of water such porous sheets 25 are commonly used to allow the foundations of constructions on land to drain without causing undermining by water the jack cylinders 22a can be fed with hydraulic fluid through flexible lines not shown long enough to enable the jacks 22 to be actuated from the surface of the sea after the base 6 has been set down on an inclined seabed depicted schematically by reference numeral 20 in fig9 the jacks 22 are operated from a craft not shown floating on the surface 21 of the water in order to position the base 6 horizontally a team of divers then unwinds the porous sheets 25 and secures their ends to the seabed 20 thereby to enclosing the space 26 between base 6 and seabed 20 concrete is then injected into the space 26 through a pipe leading from the craft on the water surface 21 porous sheets 25 allow any surplus water to drain from the concrete yet maintain the.