Fortunnel sections withgoodsurrounding rocks, the U-shapedrock-bolted section is selectedas afinaloptimized section instead of the originalcircularreinforcementconcretelining section.
It was caused by a private landowner clearing vegetation with a weed trimmer, which may have hit a rock and produced a spark that ignited the dry brush, U.S. Forest Service spokeswoman Linda Christman said.