SAMPLER WAS WALKING TOWARDS THE LAB ALONG THE NORTH TRUCK AISLE OF THE CONCENTRATOR BETWEE LINE 8 & 9. THERE WAS A SMALL HOLE IN COBING CHUTE THAT RESULTED IN FEW SCATTERED ROCKS ON THE FLOOR . HE SLIPPED AND EXPERIENCED A BACK STRAIN. CHIROPRACTIC TREATMENT. THE HOLE WAS PATCHED AND THE AREA WAS CLEANED. THIS INCIDENT WAS NOT REPORTED TO MGT FOR 13 DAYS.