EE & CO-WORKER ARRIVED IN THE PIT TO REPAIR A LOOSE BUCKET TOOTH RETAINING PIN ON A P&H BUCKET. THEY PARKED THE SERVICE TRUCK PARALLEL TO THE BUCKET. INJURED WAS PERFORMING THE WELDING, HE WAS KNEELING IN FRONT OF THE BUCKET. A HAUL TRUCK WAS PARKED APPROX. 200'FROM THE BOTTOM OF THE RAMP. IT BEGAN MOVING, STRIKING THE SVC.TRUCK THENJACKKNIFING AND STRIKING EE AND THE BUCKET.