THREE EMPLOYEES WERE TURNING A 16"" DIAMETER, 28""LONG PIPE IN THE COLLECTION BOX FOR THE #129 RADIAL STACKER. INJURED WAS INSIDE THE BOX CLEANINGOUT, PICKED UP THE PIPE TO CLEAN UNDERNEATH IT. HE WAS LOWERING THE PIPE BACK INTO THE HOLE WHENHE FELT HIS BACK POP. HE TOLD HIS COWORKERS WHATHAD HAPPENED AND THEY HELPED HIM OUT OF THE BOX.