Alder Creek Rockfall Storm Damage Project

Author:  Ryan Turner G.E., Office of Geotechnical Design North San Luis Obispo

Despite California’s historic drought, maintaining Highway 1 along the Big Sur Coast through winter is an annual challenge that keeps maintenance crews in Cambria, Willow Springs, Big Sur, and Monterey on high alert, and this winter was no exception. Even with record low seasonal rainfall totals and relatively few storms, rocks fell, landslides moved, drainage systems were overwhelmed, sections of the highway dropped, and the dedicated Caltrans employees that call the Central Coast home were working hard to keep the highway open and safe for the traveling public.

Rainstorms in December triggered rockfall at a new location near the Alder Creek Landslide. Engineers and engineering geologists from the SLO Geotechnical Design Branch responded quickly to a request from Willow Springs Maintenance Supervisor Greg De Alba Jr. to investigate. Alder Creek Landslide closed Highway 1 for three months in 2011, completely covering the highway with about 60,000 cubic yards of soil and rock. Large and frequent rockfalls at two new “chutes” to the south of the landslide were reported by the maintenance crews patrolling the area, prompting the District to initiate an emergency contract to address the rockfall. Accurate record keeping and documentation of rockfall events by the Willow Springs Maintenance crews were vital to justifying the project and designing the rockfall protection.

Workers suspended on a cable receive a 100-foot long panel of cable mesh from a helicopter while enjoying the view.

Workers suspended on a cable receive a 100-foot long panel of cable mesh from a helicopter while enjoying the view.

District 5 Maintenance Engineering and Geotechnical Design devised an innovative solution to mitigate rockfall and reuse salvaged materials to minimize cost and footprint. Steel cables and mesh salvaged from other local projects were used to construct a rockfall attenuator system. An attenuator system consists of cable or wire mesh panels suspended from cables across rockfall chutes and draped to the base of the slope. Falling rocks impact the suspended mesh and travel with very low energy under the drapery, where they can be safely removed by Maintenance at the roadside. Access Limited Construction, a specialty rockfall contractor based in San Luis Obispo, was hired to build the attenuator system due to their local experience working on the steep slopes and challenging access that characterize the Big Sur Coast.

The completed system spans two rockfall chutes and a large landslide, protecting the highway and public from rockfall.

The completed system spans two rockfall chutes and a large landslide, protecting the highway and public from rockfall.

Construction of the system required full-time rope access over 600 feet above the highway, the use of a helicopter to install the mesh, and hard work from the contractor and Caltrans climbers directing and inspecting the work. Caltrans climbers with rockfall engineering experience worked closely with the contractor to solve daily construction challenges, and the resulting finished product has unique features not used on any project in the past. The system is the longest constructed in California, spanning approximately 1000 feet. System designers used “tag lines” to create inflection points in the top support cable that allow the system to conform to the concave slope and horizontal curve in the highway, a feature that makes the Alder Creek Attenuator one-of-a-kind. Thanks to close working relationships and communication between District 5 employees, DES employees in the District, and local contractors, the project was a success and was completed in February on an expedited schedule.
Tag lines anchored above suspend the top cable and allow the system to conform to slope geometry

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