From the Leadville Herald Democrat (Ann E. Wibbenmeyer):
The reaction to the work was positive, with comments about how authentic the piles still looked. There was some discussion about the wood used for the new cribbing wall, and whether it should have been treated to look old. According to Kerry Guy, project manager with the U.S. Environmental Protection Agency, the wood was not treated at all. In this way, the boards will begin to look weathered sooner than with a treatment. The treatment on the surface of the wood could be done at a later time, he said, if that is what the community wants…
The work was done on the Denver City mine piles, which is owned by Leadville Silver and Gold. Bob Elder, local mining engineer, is the only remaining board member of this company and gave the EPA permission to use the piles [for the pilot study]…
Around the back of the Denver City piles, to the left, is the area that was covered with shotcrete. This is concrete shot onto the piles in varying shades to more closely resemble the rocks left on top of mining piles. Half of this was lined and the other half shot without a liner, to test the need for a liner to reduce the amount of acid mine drainage water into the Arkansas River.
More California Gulch/Yak Tunnel coverage here and here.