TWO WORKERS WERE REMOVING LIMESTONE FROM A RECENT SHOT. AFTER LOADING WAS COMPLETED USING A 235 EXCAVATOR, THE HAUL TRUCK DRIVER WALKED TOWARDS THE HOE PARELLEL TO THE TRUCK FROM THE FRONT TO REAR DIRECTLY BESIDE AND BELOW TO ABOUT THE REAR WHEELS OF THE TRUCK WHEN A PIECE OF LIMESTONE APPROX. 20 LB. FELL FROM THE BED STRIKING HIS HARD HAT AND SENDING HIM TO THE GROUND.