Abstract: 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.