PURPOSE: To economically design a double cofferdam without necessity of loading a residual hydraulic pressure upon designing, by embedding well points in sand filled between inner and outer steel sheet piles surrounding an underwater space so as to forcibly discharge water therefrom in order to hold the water at a low level.

CONSTITUTION: Steel sheet piles 3, 4 are driven into the bottom 2 of a river around a bridge pier 1 with a predetermined space therebetween so as to define two chambers. Then, the steel sheet piles are fastened together by tie-rods 5 at positions near to the top ends thereof, and then cut beams 7 are laid in the space, and further, packing sand 8 is filled between the sheet piles 7. Then, well points 9 are driven into the sand 8 so as to forcibly discharge water from the packing sand 8 so that the level of the water in between the sheet piles 7 is lowered from a high level (HWL) equal to the water level on the river side, to a low level (LWL), and accordingly, the level (WL) of the water is held due to the balance between the remaining quantity of the water in the packing sand 8 and the displacement. Accordingly, it is only necessary for strength calculation for the outer sheet piles to count an active earth pressure and a passive earth pressure, thereby it is possible to economically construct a double cofferdam.