MONITORING RAPID MOVEMENT IN A LARGE LANDSLIDE

Understanding landslide movement behavior is critical for assessing hazard. During the wet winter of 1997, we began radio telemetered, real-time monitoring of the large, active Cleveland Corral landslide in the Sierra Nevada of California, in hopes of capturing the transition from slow creep to rapid failure. Nearby large slides moved rapidly in 1983 and 1997. The monitored slide, 450 m long with slopes typically ranging between about 10 and 30 degrees, incorporates rocky colluvium and older landslide deposits. Over the ensuing five years, we recorded hydrologic conditions and landslide movement using precipitation gages, pore-water pressure transducers, surface extensometers, and ground vibration sensors. In addition, we determined 3-D movement patterns using repeated high-precision surveys, and we mapped in detail surface deformation structures.

In this slide, we observed three modes of movement behavior related to different topographic and hydrologic settings: episodic slow movement, localized slumping, and localized rapid debris-flow initiation. The slide remains stationary during the dry summer months and moves slowly (0.5 to 2 cm/day) after about 800 mm of winter precipitation elevates subsurface pore-water pressures. Movement timing and patterns vary from year to year and within a wet season; landslide pieces that moved dramatically one year may have modest movement the following year. Once movement begins, other failure behaviors can occur. Steeper sections (~30 to 35 deg) where groundwater is concentrated can slump locally, causing highly disrupted deposits of sub-meter sized blocks. When the slide is active, materials in the toe are progressively steepened (~35 to 42 deg), pervasively cracked, and visually dilated. Here, small, shallow, secondary slides mobilize into debris flows that travel down adjacent stream channels. We monitored one secondary failure where movement accelerated from about 10 cm/day to over 100 cm/day during several hours prior to mobilizing into a flow. Three factors appear to influence the initiation of debris flows from the toe and margins of the landslide complex: 1) locally steepened ground created by dynamic landslide movement, 2) locally elevated pore-water pressures and abundant soil moisture, and 3) locally dilated materials, allowing rapid collapse.