the second chamber of the cofferdam shell has been dewatered forms can be positioned to define the contour of the sill under construction concrete is poured into the forms to physically construct the sill segment after the concrete has been set the forms are withdrawn and the interior of the cofferdam is flooded with water next the cofferdam is repositioned along the length of the segment of the sill by pumping water out of the first chamber and floating the sill for lateral displacement of course in the event that the sill under construction is designed to have a hollow structure as shown for example in fig4 it is necessary to place bulkheads at the end of a completed sill segment prior to flooding the interior of the cofferdam shell once the shell has been repositioned to construct the next segment the interior of the cofferdam shell is evacuated or dewatered and the bulkheads are removed before the next sill segment is constructed in this manner the sill is constructed in segments as the mobile cofferdam is displaced laterally along the length of previously constructed sill segments as shown in fig1 and 4 for example approximately 100 feet of sill or wall are constructed during each cycle at the leading end approximately 50 feet of the structure will be built from driving piles if required through placing the concrete immediately behind that on the previously built 50 foot section the remaining structural appurtenances 30 wickets 32 and operating equipment 34 for the the sill are installed the mobile cofferdam is anchored to the section during the dewatered period to resist the unbalanced hydrostatic forces between the leading and trailing ends thereof using the inventive method and mobile cofferdam structure substantial.