PURPOSE: To efficiently remove water from soil by mutually disposing a plurality of T-shaped finned projections onto the top face of a flexible polymeric core sheet so as to form rows and columns at spaces.

CONSTITUTION: A polymeric core sheet 12, in which a plurality of T-shaped finned projections 14 are extruded from the surface of the sheet, is formed by extrusion-molding a thermoplastic substance. The core sheet 12 is cut by a saw or the like drawn in a cross direction Y at periodic intervals along the machine direction X that the sheet is extruded continuously, a portion of the finned projections 14 is removed, and the rows and columns of the finned projections 14 are formed. A water-penetration fabric is bonded with the top face 22 of the finned projections 14.