INDEPENDENT DRIVER WAS HAULING IN WAST-WASH OUT TO DUMP IN OUR PIT. HE LIFTED BED TO DUMP, PARTIAL LOAD GOT STUCK IN TOP OF TRAILER CAUSING WEIGHT OF UPLIFTED BED TO BE UNBALANCED AND ROLL TRAILER AND TRACTOR ONTO SIDE. DRIVER HAD SEAT BELT ON AND SUSTAINED SMALL CUT TO LEFT SIDE OF HEAD AND DISLOCATED SHOULDER