Influence of soil aggregation on slope stability in the Oregon Coast Ranges

Two major cohesionless soil series of the central Coast Ranges of Oregon were examined for soil and hydrologic properties. Although derived from different parent material, the Bohannon and Klickitat series exhibited nearly identical values of soil and hydrologic properties. Aggregation in both soils was found to be the most important property, for it influences shear strength and subsurface water movement, prime components of slope stability. The unusually high angle of internal friction of both cohesionless soils was 40° to 41°. The angle of internal friction was affected significantly by the mode of wetting. Subsurface water movement was predominantly by unsaturated flow. The influence of soil aggregation on pore-size distribution and arrangement created conditions where unsaturated flow was an effective means for water dispersal in both soils during most observed storm events. Considering the large area covered by these two soil series, aggregation appears to be an important slope-stability property with regional significance.