RCW 49.24.130: Air chambers—Hanging walks.

(1) Whenever the air pressure in a tunnel heading exceeds twenty-one pounds per square inch above atmospheric pressure, two air chambers shall always be in use, except for such time as may be necessary when headings are being started from shafts; and whenever practicable the pressure in the outer chamber shall not exceed one-half the pressure in the heading;

(2) In all tunnels sixteen feet in diameter or over, hanging walks shall be provided from working face to nearest lock. An overhead clearance of six feet shall be maintained and suitable ramps provided under all safety screens.