a submerged portion of sls 3 in the landing state and a represents the area under installation of sls 1 sls 3 can be prevented from floating up to the surface on condition that the quotient thus obtained is positive irrespectively of a variation of the water level when the upper structure 2 is constructed on sls 1 to accomplish the construction of sls 3 in this case u03b3 w represents the unit weight of water and v represents the volume of the submerged portion of sls 3 as described above the contact pressure p is found according to the expression of w 1 w 2 w 3 u03b3 w u00b7 v a the ground can be prevented from settlement due to consolidation on condition that the sum of the increment u03b4 u03c3 of the subsurface stress caused by the contact pressure p and the own weight stress u03c3 0 of the subsurface part determined by the self weight of the soil is smaller than the consolidation yield stress u03c3 y i e u03c3 0 u03b4 u03c3 lt u03c3 y when v varies with the variation of the water level the ground can be prevented from settlement on condition that the sum of u03c3 0 and the increment u03b4 u03c3 max of the stress corresponding to the maximum value of the contact pressure p is smaller than the.