EE PARKED A BOOM TRUCK THEN EXITED AND WAS WALKING TOWARD A PIPE FUSION MACHINE THAT WAS 10' AWAY. AS HE WAS WALKING HE STEPPED ON A ROCK THAT WAS APPROX. 2""=3"" IN SIZE WITH THE INSIDE EDGE OF HIS LEFT FOOT, THE ROCK SHIFTED UNDER HIM CAUSING HIS ANKLE TO ROLL OUTWARD. THE GROUND WAS FLAT, COMPACTED AND HAD NO SNOW.