MONITORING THE FERGUSON ROCKSLIDE, CENTRAL CALIFORNIA “WHAT WE HAVE LEARNED AND HAVEN'T LEARNED”

The Ferguson Rockslide failed in May 2006, with 45 feet vertical movement at the headscarp and deposition of 90,000 yds³ of rock talus that resulted in blocking Highway 140. A monitoring program was implemented after dealing with the initial movement and concern by the public and government officials that the slide could possible block the Merced River and flood downstream values. Implementation of the monitoring program included collaboration with USGS, CalDWR, CalTrans and Yosemite NP partners. The monitoring program consists of 3-Spider GPS units, 2-water level meters, approximately 45-survey bench marks, a rain gage, and a seismometer. The slide has moved up to 3 feet per year and moves from 1 to 2 inches per day during peak rainfall periods. Rainfall has been below normal since the initial failure and the slide has not been tested with a Pineapple Express Storm, which typically can drop 6 -8 inches of rain. There is still concern that the slide could fail and block the Merced River. In situ engineering failure properties of the slide have not been determined; however several assumptions can be made including failure envelope properties. Scenario modeling suggests that the slide could block the Merced River under various conditions. Preliminary results of isotopic dating of the slide suggest that the slide is less the 1000 years old. The slide is sensitive to rain and moves as a direct result of rain storms. Monitoring will continue for at least another year.