PURPOSE: To increase the bearing power of a soft ground and to reduce the settlement of the ground by restricting the soil just under a mat foundation by means of a continuous underground wall.

CONSTITUTION: A continous underground wall 4 is projected downward from the bottom of a mat foundation 3, in such a manner as to restrict the soil just under the mat foundation 3. When the building 2 is of ordinary dimensions, the thickness t of the wall 4 is preferably from 30 to 60 cm, while the depth D of the wall 4 is preferably from 0.1 to 0.4 times the length B of longer side of the foundation 3.