HYDROLOGIC CONTROLS ON TRANSLATIONAL BEDROCK LANDSLIDES, COASTAL OREGON

Translational landslides in Tertiary sandstone and siltstone located along the Pacific coast north of Newport, Oregon are sporadically active and frequently damage US Highway 101 and residential and commercial property. These landslides often reactivate almost immediately following onset of heavy rainfall, even though they are typically 20-30 m deep with water tables that average about 15 m below the ground surface. Detailed studies of the Johnson Creek landslide by the USGS and state of Oregon and private partners may provide better understanding of the behavior of these landslides.

Borehole inclinometer monitoring at the Johnson Creek landslide suggests shear displacement along a discrete basal rupture surface that may follow bedding in the upper third of the landslide but transects bedding in the lower two-thirds. Back-analysis using limit equilibrium methods indicates an effective internal friction angle of the basal rupture of about 6.5° and no cohesion. Nearly continuous monitoring of cable extensometers has recorded seven movement episodes with generally consistent velocity of 0.1-0.3 mm/hr. Vibrating-wire-piezometer nests show nearly horizontal groundwater pressure transmission from the head of the landslide toward the toe, and suggest that the landslide basal rupture surface has no effect on groundwater flow. Groundwater is near the ground surface at the landslide head during wet winter months and gradually deepens to about 20 m below the ground surface within the lower third of the landslide. Rapid response of landslide displacement to rainfall appears to be due to infiltration of rainfall to the shallow saturated zone within the landslide head, followed by rapid groundwater pressure increase downslope through pressure diffusion or groundwater flow through downslope-oriented vertical fractures. Upcoming aquifer testing and groundwater modeling should clarify the mechanisms of this rapid pore-pressure response. Findings from these studies at Johnson Creek should be useful for mitigating hazards from similar translational bedrock landslides.