Marina Fire Rockfall Protection: A rapid, practical response

摘要

The Marina Fire (July 2016) resulted in significant increase in rockfall risk to a portion of US 395 in Mono County, California. This corridor is the main north-south transportation artery on the east side of the Sierra Nevada providing access to Yosemite National Park, Mammoth ski area and Southern California. The roadway is cut into a slope comprised of colluvium and interspersed tufa from relict shorelines of Mono Lake. Due to rockfall events impacting the roadway immediately post-fire, safely keeping the road open to traffic was a priority for Caltrans. A rapid response was executed to assess the hazard and implement a mitigation strategy for this segment of roadway. Working in conjunction with Caltrans, Yeh engineering geologists evaluated the rockfall trajectories impacting the roadway and provided recommendations for installing a temporary flexible rockfall fence. Two Geobrugg flexible rockfall fences, 2,000 linear feet of GBE 500AR (3m height) and 1,500 linear feet of model GBE 500AR (4m height) were recommended. A significant constraint was the presence of a critical fiber optic utility at the base of the slope, which was installed via horizontal directional drilling. Due to the uncertainty of the precise location of this utility, excavation and drilling to install post support system and ground anchors for the post support were a primary concern. Design modifications were made to accommodate the site conditions. Of particular note was the design of the post support system. The fence posts did not have the typical concrete foundation type support systems but instead were supported globally by a wire rope support system and locally with number 10 threaded bar, 18 inches in depth. This design did not encroach on the buried utility and facilitated rapid construction, enabling the installation to be completed within a matter of weeks.