ALLEGED INCIDENT WAS SUPPOSED TO 0CCUR WHILE EMPLOYEE WAS BUILDING CEMENT BLOCK WALLS IN LAV. HE HAS A BAD BACK & THIS WORK AGGRAVATES IT. CAUSE OF INJURY--OVER-EXERTION. COMPLIANCE W/RULES NOT A FACTOR; PERSONAL PROTECTIVE EQUIPMENT, MINING EQUIPMENT/SYSTEMS, MINER PROFICIENCY ARE FACTORS.