INJURED WAS KNELT DOWN ON HIS KNEES WITHH HIS BACK TO THE RIB WORKING ON HYDRAULIC CHAIN SAW WHEN RIB ROLL OCCURRED MEASURING 15"" BY 24"" BY 20'.RIB ROLL STRUCK HIS LOWER LEGS PINNING HIM TO TH E GROUND. THIS SECTION WAS PILLARING AND HE AND CO-WORKER WERE SETTING BREAKER POSTS IN CROSSCUTBETWEEN #7 AND #8 ENTRIES.