EE ON HAUL ROAD; RAN OVER A DIP ON ROAD CAUSING SCRAPER TO BOUNCE & BOWL TO BOTTOM OUT STOPPING SCRAPER. THE FORCE OF THE SCRAPER STOPPING CAUSED EE TO GO FORWARD & HIT AGAINST STEERING WHEEL OF SCRAPER. WHEN HE HIT STEERING WHEEL IT THEN KNOCKED HIM OUT OF CAB OF SCRAPER. EE STATED HE BUCKLED HIS SEATBELT BUT EVIDENTLY IT WAS NOT SNAPPED IN THE LOCKED POSITION.