A hydraulic jack assembly is interposed between at least a pair of adjacent sections of the pile, and a hollow center is defined in a central region of the hydraulic jack assembly, thereby allowing communication between adjacent sections of the pile. Thus, concrete can be filled into the shaft without creating any voids, in particular around the hydraulic jack assembly. Therefore, the integrity of the pile is not compromised by the presence of the hydraulic jack assembly, and the testing device in no way detracts from the performance of the pile. The hydraulic jack assembly may consist of an annular arrangement of individual hydraulic jacks or may consist of a single cylinder device defining an annular cylinder chamber between a pair of coaxial, mutually slidable cylinders. By placing a pile consisting of at least three sections in an earthen shaft, and interposing at least a pair of hydraulic jack assemblies each between an adjacent pair of the pile sections; it is possible to simultaneously actuate the pair of hydraulic jack assemblies so as to test the load supporting capacity of the pile section located between the hydraulic jack assemblies without being interfered by axial forces or displacements of other pile sections.