Stabilization By Drainage Trenches Of Two Major Earth Flows In Western Oregon, Usa

Shallow earth flows occur in tuffaceous siltstone lithology in the Coast Range and the Cascade Range Physiographic Provinces of the State of Oregon, U.S.A. Abundant rainfall, between 90 to 165 cm per year, creates a soil profile typically composed of clayey silt residual soils with a relatively pervious relict structure, and a zone of highly plastic clay (CH) at depth overlying weathered bedrock. The depth of the plastic clay zone is often relatively shallow, varying between 3 and 5 meters. Large earth flows are periodically activated by critical storm events which are characterized by high antecedent rainfall over a period of 4 to 12 days, followed by a high intensity rainfall event of 10 centimeters or greater in 24-hours. Movement continues for periods of days to months until ground water levels decrease. The earth flow movements may range between 6 and 8 meters annually, with the basal failure plane within the plastic clay (CH). Stabilization of two large earth flows in Lane County, Oregon, Figure 1, affecting rural arterials, has been achieved through the construction of a network of drainage trenches. The area of southern Lane County is in the transition zone between the Coast Range and Western Cascade physiographic provinces of Oregon. Shallow earth flows are common occurrences in the residual soils overlying weathered tuffacous siltstone. The first earth flow (Fox Hollow Landslide) had a long history, since 1972, of movements ranging from 3 to 6 m during the wet, winter months. The large-scale movement required periodic maintenance of the arterial to remove encroaching soil debris. The second earth flow (London Road Landslide) was a dormant landslide that was reactivated by an intense storm in November 1996, with the movements causing upheaval and closure of the road at the toe.