PURPOSE: To relieve earth pressure in the case where frost heave occurs to the ground by providing foamed resin members to the sides whereto the earth pressure is applied from lateral sides of a built-up stepped ditch, U-shaped ditch or the like, and by making the upper ends covered with fire-resistive material.

CONSTITUTION: A drainage ditch 1 is formed over a foundation 4. The ditch is made up of concrete blocks made from U-shaped blocks 5, and the outside of the block is covered with a foamed resin member 6 up to its upper end faces. The upper parts of the foamed resin members 6 are then covered with fire-resistive layers 10 made of a fire resistive material. Thereby earth pressure in case of frost heave can be relieved, while damage by burning in case of burning the dead grass in field can be prevented.