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Factors Affecting Mass Movement of Four Soils in the Western Cascades of Oregon¹

ABSTRACT

Four soils derived from tuffaceous rock in the Western Cascades were studied to determine relationships to slope stability. Two of the soils derived from greenish tuff and breccia were prone to slope failure. The other two soils were derived from yellowish and reddish tuff and breccia and were more stable.

Soils prone to slope failure were characterized by high amounts of smectite clay, absence of kaolin, and moderate amounts of free iron oxide. The more stable soils contained kaolin, more chlorite and chloritic intergrades, less smectite, and higher amounts of free iron oxide.

Pseudomorphs of clay were the major component of silt and sand fractions. The pseudomorphs seem to function mechanically as primary soil particles but the clay contributes to cation exchange and moisture retention. The ratio of clay calculated from 15-bar moisture retention to measured clay was highest for the most stable soils.

Stability of these soils did not appear to correlate with clay content, content of amorphous clay, or proportions of exchangeable cations. Color of the soils and of their source rocks was correlated with clay mineralogy, content of iron oxides, and landscape stability.

¹ Technical Paper no. 3043, Agr. Exp. Sta., Oregon State Univ., Corvallis 97331. Based on the senior author's Ph.D. thesis. Funds were provided largely by the USDA Forest Service, Pac. N.W. Forest and Range Exp. Sta.

² Former Graduate Research Assistant in Soils; Professors of Soils, Department of Soils, Oregon State Univ., Corvallis; and Soil Scientist, USDA Forest Service, Pac. NW Forest & Range Exp. Sta., Corvallis, respectively. Senior author is now Assistant Professor of Soils, American University of Beirut, Lebanon.

Received for publication March 15, 1971. Accepted for publication August 26, 1971.