Soil Properties for Slope Stability Analysis; Oregon and Washington Coastal Mountains

Abstract Soils from 11 sites in the Oregon-Washington coastal region were collected and tested to determine their index properties and effective strength parameters. The index properties were natural moisture content, liquid limit, plastic limit, specific gravity, grain size distribution, moisture-density relationship, and in situ density. Effective strength parameters were determined by consolidated-undrained triaxial tests with backpressure saturation. The soils tested can generally be classified as slightly cohesive silty sands. The average angle of internal friction and saturated unit weight were 35.4 degrees and 1.86 g/cm³, respectively. For the soil population represented by this study it was estimated that 95 percent would have angles of internal friction exceeding 26.2 degrees and saturated unit weights exceeding 1.56 g/cm³. These statistics may be useful for slope analysis in the region. Forest Sci. 29:823-833.