2009 Portland GSA Annual Meeting (18-21 October 2009)
Paper No. 191-7
Presentation Time: 9:00 AM-6:00 PM


PRIEST, George R.1, ALLAN, Jonathan2, NIEM, Alan3, NIEM, Wendy A.3, and DICKENSON, Stephen E.4, (1) Oregon Department of Geology and Mineral Industries, Newport Coastal Field Office, PO Box 1033, Newport, OR 97365, george.priest@dogami.state.or.us, (2) Coastal Field Office, Oregon Department of Geology and Mineral Industry (DOGAMI), P.O. Box 1033, Newport, OR 97365, (3) Pacific Northwest Geology, LLC, 6325 B Avenue, Otter Rock, OR 97369, (4) School of Civil and Construction Engineering, Oregon State University, Corvallis, OR 97331-2302

The Johnson Creek landslide is a translational slide in seaward dipping Miocene siltstone and sandstone (Astoria Formation) and an overlying Quaternary marine terrace deposit. The slide terminates in a sea cliff and has a hummocky to nearly horizontal ground surface. The basal slide plane, however, slopes subparallel to the dip of the Miocene rocks, except beneath the back-tilted toe blocks where it curves upward. The siltstone and sandstone have low estimated permeability but cores and field mapping reveal an extensive fracture system within the slide mass. The slide mainly moves in response to groundwater pressure and coastal erosion of the toe. Limit-equilibrium stability analyses indicate that 3 m of erosion at the toe would destabilize the slide for most of the wet season, although no movement could be directly attributed to erosion in the 5 years of observation. Intense rainfall events raise pore-water pressure throughout the slide in the form of pulses of water pressure traveling from the headwall graben down the axis of the slide at rates of 1.4-2.5 m/hr in the upper part, and 3.5 m/hr to virtually instantaneous in the middle part. Infiltration of meteoric water was only ~50 mm/hr. Slope of the water table exceeds topographic slope from the head to the toe of the slide, so infiltration was too slow to directly raise head in 90 percent of the slide mass where the saturated zone is deeper than a few meters. Only at the headwall graben was the saturated zone shallow enough for rainfall events to trigger pulses of water pressure through the entire saturated zone. When a pressure pulse reached the threshold pressure for movement in the central part of the slide, the whole slide began slow, creeping movement. As head became larger and larger than the threshold for movement in more of the slide mass, movement accelerated and differential displacement between internal slide blocks became more pronounced. These findings suggest that dewatering the shallowest part of the saturated zone in this type of slide will stop these rapid pressure pulses, thereby stopping or greatly reducing seasonal movement. If slides are also subject to continual removal of material from the toe, especially where there are back-tilted toe blocks, then some type of buttress or tied-back shear pile wall may be the only effective long term remediation.

2009 Portland GSA Annual Meeting (18-21 October 2009)
General Information for this Meeting
Presentation Handout (.pdf format, 30685.0 kb)
Session No. 191--Booth# 350
Landslides in the Pacific Northwest: Advances in Research and Practice (Posters)
Oregon Convention Center: Hall A
9:00 AM-6:00 PM, Tuesday, 20 October 2009

Geological Society of America Abstracts with Programs, Vol. 41, No. 7, p. 497

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