A slotted mandrel is used in displacement pier construction. Longitudinal slots are formed in the sides of the hollow mandrel which allow for the introduction of aggregate efficiently into the mandrel from a hopper through which the mandrel passes while being driven into the ground. Water may also be employed with the aggregate. Once in the ground the mandrel is raised to release the aggregate (and water if present) into the formed hole in lifts and a mandrel head then compacts the aggregate in the hole. This apparatus and method obviates the need for complicated and expensive hoppers that are raised with the mandrel or the use of expensive aggregate delivery systems.