MONITORING DISPLACEMENT OF THE ACTIVE MISSION PEAK LANDSLIDE IN FREMONT, CALIFORNIA

The Mission Peak Landslide is a massive (9 million m³) slope failure that is a partial reactivation of an enormous (370 hectare) ancient landslide complex in Fremont, California. In March 1998, 35 hectares of ground began advancing as an earthflow towards a recently completed residential subdivision. A pronounced landslide toe developed across four lots and also constricted flow along the local Aliso Creek. Over the last 13 years, the City of Fremont has funded installation and monitoring of 14 inclinometers in the toe vicinity of the landslide to track landslide movement, help assess risks and guide community response decisions.

From September 2001 to present, we have conducted annual monitoring of initially installed inclinometers, replacement of sheared-off inclinometers, deployment of supplemental inclinometers with pneumatic piezometers, and general landslide geologic inspections. Ongoing inclinometer monitoring has revealed that incremental landslide movement continues to advance 13 years after the 1998 reactivation. Thrusting forces exerted by the pronounced landslide toe have resulted in lateral displacement of an adjoining reactive block of ground. An existing residence located on this reactive block is still being translated on a shear surface located at a depth of approximately 15m. More than 7cm of lateral movement has been recorded by inclinometers in the reactive block. However, a clear surface geomorphic signature of ground movement has not been detected in this area located beyond the prominent landslide toe.