Abstract: The present invention provides a tool for forming an underground cast-in-situ pile having an enlarged base, the tool comprising: a first pile-forming member having a first base-forming member; a second pile-forming member having a second base-forming member, the pile-forming members extending collinearly and being movable in a longitudinal direction relative to one another between: a digging state wherein the base-forming members are proximate to one another so that the pile-forming members can be driven underground; and a base-forming state wherein the base-forming members are spaced from one another so as to form an underground void between the first and second base-forming members, the void being tillable with a flowable fill to form the enlarged base of the underground cast-in-situ pile; and jack screw means, rotation of which causes relative longitudinal movement between the pile-forming members.