Shallow Landslides In the Tyee Formation of the Coast Range, Oregon

Landslides are common in soils underlain by the Tyee Formation located in the central Coast Range, Oregon. The Tyee Formation is Eocene in age and consists of massive micaceous arkosic sandstone with thin siltstone interbeds. The Coast Range is characterized by wet climate and rugged terrain. Frequent timber harvesting, and shallow, coarse-grained, low-cohesion soils overlying relatively low permeability sandstone and siltstone beds create an ideal setting for frequent landslides. Water routed along the bedrock surface issuing from the ground as springs in locations where the colluvium thins leads to instability in overlying soil when groundwater levels are elevated. Slope failures are typically less than 1 meter thick and occur on slopes that average 39° ± 4°.

A field-based analysis of 61 landslide initiation sites in soils derived from the Tyee Formation reveal two distinct failure types based on the porosity and fine-grained content of the soil. The two failure types consist of (1) debris flows and (2) landslides that do not mobilize as debris flows. Debris flows initiate from loose soils with porosities greater than about 0.5 (cm³/cm³), whereas soil slumps and rotational slides initiate from soils denser than 0.5. Soils near the border of the two types of failures that contain a relatively greater amount of fine-grained sediment are more prone to initiate as debris flows. The presence of springs in debris-flow headscarps and the general lack of springs in the headscarps of landslides that did not mobilize as debris flows suggest that these springs may play a role in debris flow mobilization.