Investigation of White Bluffs Landslides in Washington State

摘要

The objective of this study was to conduct experimental and numerical investigations to evaluate potential failure mechanisms of the White Bluffs landslides. Increased landslide activity in the 1970s and 1980s has been attributed to irrigation activities behind the bluffs, which altered moisture conditions and caused seepage and piping. Soil instability is believed to originate in weakly cemented clays and silts of the Ringold Formation. The experimental analysis included triaxial compression tests on undisturbed and remolded specimens from the Ringold Formation, to evaluate anisotropy, wetting, and remolding effects. Strength anisotropy manifested an effective cohesion intercept (c') that is approximately 2.7 times larger for the vertical orientation (46 kPa) compared to the horizontal; however, similar effective stress friction angles (Φ' ≈ 20-21°) were determined in both orientations. Larger Φ' and c' were determined for air-dried specimens compared to saturated specimens in both orientations. A larger Φ' and smaller c' were determined for the remolded saturated specimens, indicating that remolding altered the soil fabric. The numerical slope stability analyses indicate that landslide activity may be attributed to a combination of factors, i.e., strength reduction and pore pressure development with soil wetting, coupled with anisotropic shear strength.