Restoring Access: The Repair of Pilarcitos Lake's Landslide

Power Engineering Construction is wrapping up a crucial project for San Francisco Public Utilities Commission (SFPUC) repairing a landslide that cut off access to Pilarcitos Lake in the San Francisco Watershed. This landslide, triggered by the harsh storms of Winter 2023, blocked all vehicle routes to the reservoir below.

The repair process is complex and meticulous. It started with excavating the landslide material and then reconstructing the hillside using twelve tiers of 3-foot-high gabion baskets. These baskets follow the slope of the existing hillside and span approximately 40 feet wide. Each of the twelve tiers comprises 24 gabion baskets, each holding nearly 6 tons of rock. This significant weight meant that heavy equipment was crucial; however, the slope was too steep for it to maneuver up and down. The solution, guided by BAGG’s geotechnical slope stability analysis, was to construct a safe staging area for a long-reach excavator at the top of the slide, feeding material to a mini excavator at the bottom and placing the gabion baskets in a bottom-up fashion.

To overcome working along an active maintenance roadway with no space for staging, Power scheduled small and constant deliveries from the quarry and operated skid steers and site dumpers continuously. To prevent future soil saturation, a 6-inch perforated HDPE subdrain was installed after clearing the slide material, followed by a foundation of Class II aggregate base. This setup, combined with the tag-team method of using a large excavator and a mini excavator, ensured efficient construction and stability as the tiers were built up and completed.

As the project nears completion, Power will finish installing the gabion baskets, bringing the hillside close to its final grade. GSW Construction will then lay down a layer of asphalt concrete to restore the road, ensuring safe and reliable access to Pilarcitos Lake once again.