PURPOSE: To increase horizontal effective stresses of the ground acting on a pile, and obtain a large peripheral surface supporting force of the pile by forming the outer peripheral surface of a ready-made pile with ribs into a tapered surface.

CONSTITUTION: The outer peripheral surface of a pile P made of steel pipe or concrete is formed into a tapered surface T having a very small inclination in the axial direction. The required number of enlarged diameter ribs R horizontally projected are provided on the outer peripheral surface in the axial direction. When the pile is displaced vertically, the tapered surface and upper and lower enlarged diameter ribs R displace the surrounding ground in the horizontal direction to increase horizontal effective stresses of the ground.