Following heavy spring rain in May of 2005, a significant landslide destroyed a pipeline that carried wastewater from homes in Cascade Shores to the treatment plant near Nevada City. Although the treatment facility was not severely damaged, continued instability of the 100-foot-tall cliffs above the plant posed a risk to buildings, equipment, and workers below. Nevada County Department of Sanitation retained Holdrege & Kull to evaluate the situation and develop a mitigation plan to prevent further damage. To facilitate protecting the plant prior to the start of the 2005/2006 rainy season, H&K designed a soldier beam wall, a Gabion wall, and rock fall protection netting on the cliff face to protect the plant. H&K also acted as construction managers for this fast-track project. The project was successful in protecting the plant from further damage and received the 2005 Project of the Year award from the Engineer’s Association of Nevada County.
To assess future risk to the treatment plant, H&K conducted an intensive slope stability analysis that included drilling and down-hole observations in several 30-inch-diameter borings. The findings resulted in a new industry-wide understanding of the instability risks of the Ione Formation, a common geologic unit in Northern California.