Washington Revised Code Title 49. Labor Regulations § 49.24.270. Shields to be provided

Wherever, in the prosecution of caisson work in which compressed air is employed, the working chamber is less than twelve feet in length, and when such caissons are at any time suspended or hung while work is in progress, so that the bottom of the excavation is more than nine feet below the deck of the working chamber, a shield shall be erected therein for the protection of the workers.