THE WEST TIDEWATER EARTHFLOW, 1994, NORTHERN OREGON COAST RANGE

The West Tidewater earthflow, one of the largest in Oregon's history, occurred in December of 1994. It is located 15 km north of Jewel, Oregon near the summit of the northern Oregon Coast Range Mountains. The earthflow is nearly 900 m long and 250 m wide, giving it a surface area of 9 ha or 22 acres. The volume is 3.5 million m3. The earthflow occurred in low strength, well bedded clayrich mudstone of the Miocene Northrup Creek Fm.. The soil clay fractions contain up to 90% smectite with indications of halloysite. This earflow is a reactivation of a 650 year old landslide as determined by C-14 dating of buried trees. The failure mode is examined using a Janbu slope analysis and may include a double wedge failure near the headscarp. High soil pore water pressure is one of the major causes of this slope failure. Rainfall levels in October, November and December of 1994 were twice the previous five year average. Present day groundwater level within the basin is less than one meter below ground surface. The earflow is partially controlled by two faults of regioinal extent that dissect the basin near the headscarp in NW-SE and NE-SW directions, Soil in the basin may have been further weakened due to a loss of root strength following timber harvest on the site in 1991. Soil liquid limits range from 42-95% with PI values from 2-77%. Soil clay content ranges from 18-30%. Direct shear tests on the mudstone in both drained and undrained conditions produced internal friction angles of 14-18 degrees with cohesion values of 4-8 kPa. Back calculations of study area soil strength using the modified Bishop method results in a residual friction angle of 20.7 degrees. The failure mode of the earthflow is thought to be from the headscarp downward.