A primary earth penetrating mandrel formed of a hollow shell steel plate octagonal in cross-section has an upper end and a blunt lower end joined by an upwardly and outwardly tapered wall. The mandrel is driven downwardly in the earth to simultaneously form a vertical tapered cavity while compacting the sidewall of the cavity to provide structural integrity. The mandrel is then moved upwardly from the bottom of the cavity and aggregate is deposited in the bottom of the cavity following which the mandrel is lowered so that its blunt lower end engages the deposited aggregate and densifies the aggregate by vertical vibratory action and static force with these steps being repeated until the pier top is near the surface of the earth at which time the upper aggregate portions are densified by either the primary mandrel or a secondary mandrel having a substantially larger lower end surface than the lower end surface of the primary mandrel. A second embodiment includes a conduit in the primary mandrel for injecting concrete or grout into aggregate previously deposited in the cavity.