Mechanics and movement of the Lookout Creek earthflow

The Lookout Creek Earthflow is located in the Cascade Mountain Range in western Oregon. The Cascade Mountains are mainly volcanicin origin, and deposits in and around the slide have a complex geomorphic history, affected by glacial, mass movement, and fluvial processes. The currently moving land mass is about 1600 ft. long and 300 to 800 ft. wide. The average depth to the failure surface is around 20 to 25 ft., and the average surface inclination is 9 degrees.
Movement takes place mainly in the wet winter months, from late fall to early spring. Average surfical movement is about 4 in. (10 cm) per year, with a maximum recorded velocity of 0.16 in/day (0.4cm/day). There is some variation in movement rates measured up anddown the slope.
Movement is best described by a shear failure of soil at residual shearing strength. Rheological models cannot explain the variations in movement rates with very slight changes in stress levels. Limit equilibrium methods with infinite slope and two-dimensional non-circular procedures are used in backanalysis for strength parameters. For both methods, back-calculated phi is about 15 degrees, much lower than phi determined from laboratory testing.
Phreatic level observed in three continuously recording piezometers does not correlate well with the annual start of slide movement, or movement rates. The annual start of movement is better explained by the pore pressure-volume change relationship in a 10 in. thick relatively impermeable shear zone.
The Lookout Creek Earthflow is covered by coniferous forest vegetation which is equivalent to a surcharge load of 10 psf or less. There is no effect of root strength along the slip surface of this slide. Removal of vegetation should have very little affect on stability of the Lookout Creek Earthflow.