Wash. Rev. Code § 78.12.010
Any person or persons, company, or corporation who shall hereafter dig, sink or excavate, or cause the same to be done, or being the owner or owners, or in the possession, under any lease or contract, of any shaft, excavation or hole, whether used for mining or otherwise, or whether dug, sunk or excavated for the purpose of mining, to obtain water, or for any other purpose, within this state, shall, during the time they may be employed in digging, sinking or excavating, or after they have ceased work upon or abandoned the same, erect, or cause to be erected, good and substantial fences or other safeguards, and keep the same in good repair around such works or shafts sufficient to securely guard against danger to persons and animals from falling into such shafts or excavations.
RCW 78.12.010