EE CAME OFF J-21 MAIN HAUL ROAD GOING AT APPROX. 45 MPH FROM K MINE FACILITIES, MADE A TURN GOING TO J-21 EXPLOSIVE YARD, APPROACHED THE EAST GATE GOING ABOUT 15 MPH & TRIED TO APPLY THE BRAKES. BRAKE PEDAL JUST FELL TO THE FLOOR, STEERING WHEEL LOCKED UP. FRONT OF VEHICLE WENT UNDER THE GATE - WINDSHIELD SHATTERED, GATE OPEN BROKE SIDE POST.