Mechanics and stability of the Lookout Creek earth flow

The Lookout Creek earth flow in the western Cascade Mountains ofOregon has moved an average of about 3.5 in. (8.9 cm) annually over thepast decade. The currently active slide mass which has been moving for atleast the past 80 years, overlies a 40,000-yr-old debris deposit. Monitoredsince 1975, measurable earth flow movement occurs only during the wetseason; piezometric level, which is at or near the ground surface, varies onlyabout 3 ft (0.9 m) between the wet and dry seasons. The base of the earthflow appears to be a shear zone about 10 in. (25 cm) thick located at a depthof 21.5 ft (6.5 m). Although earth flow movement does not correlate directlywith piezometric levels, it does correlate with shear-zone pore-water pres-sures computed with a finite-difference approximation to the Terzaghi theoryof one-dimensional consolidation; the finite-difference model uses piezo-metric pressure measured in the earth flow as a boundary condition. A risein pore-water pressure in the shear zone of about 85 psf (4.1 kPa), or 1.4ft (0.4 m) of water, is the calculated threshold value at which the earth flowbegins to move. However, because the earth flow has a high drainage ca-pacity, timber harvesting, which affects the ground-water regime, is unlikelyto induce a large increase in movement.