Details:

Creators:
Priest, George R. ; Allan, Jonathan C. ; Niem, Alan R. ; Niem, Wendy A. ; Dickenson, Stephen E.
Priest, George R. ; Allan, Jonathan C. ; Niem, Alan R. ; Niem, Wendy A. ; Dickenson, Stephen E. Less -
Corporate Creators:
Oregon. Dept. of Transportation. Research Unit
Corporate Contributors:
United States. Federal Highway Administration
Subject/TRT Terms:
[+]
Geological Events Geology, Structural--Oregon--Lincoln County. Geotechnical Engineering Landslides Landslides--Oregon--Lincoln County
Publication/ Report Number:
FHWA-OR-RD-08-13
Resource Type:
Tech Report
Geographical Coverage:
Oregon
Edition:
Final report.
Corporate Publisher:
Oregon. Dept. of Geology and Mineral Industries
Abstract:
A five-year study indicates that the Johnson Creek landslide moves in response to intense rainfall that raises 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 to 2.5 m/hr in the upper part and 3.5 m/hr to virtually instantaneous in the middle part. Vertical

arrays of piezometers measured infiltration at rates of only 50 mm/hr, so infiltration is too slow to affect saturated water

pressure except in the headwall graben. The hydraulic gradient through the slide mass is small and groundwater flow

appears to be nearly horizontal, roughly parallel to the slide plane. These observations and the rapidity of pressure

transmission are consistent with a high effective hydraulic conductivity throughout the slide mass. Westward slope of the

piezometric surface is consistent with better drainage in the western part of the slide. Movement episodes proceed by en

masse movement when threshold pore pressures are reached followed by faster and faster movement of the middle portion

of the slide when pore water pressure there rises above ~9.4 to 10.8 m head above the slide plane. In January 2003, slide

velocity increased by an order of magnitude when head above the slide plane at the middle observation site reached 11.4 m

while the western site reached ~9 m, ~2 m above its maximum for the following four winter seasons. Antecedent rainfall

correlating with this accelerated movement was mean precipitation of 0.84 m in the previous 60 days and 2.1 mm/hr in the

62 hours immediately before the movement. Antecedent deformation correlating with the accelerated movement was

extension of 1 cm in the lower part of the slide, possibly raising effective hydraulic conductivity there. This increased

hydraulic conductivity may have caused a uniquely rapid pore pressure response in the lower part of the side and the unique

2-m increase in head. With respect to engineering solutions for slide mitigation, the reduction of water pressures at the

headwall graben by dewatering (e.g., drains or pumps) should be effective given the inferred high hydraulic conductivity of

the slide and sensitivity to pressure change at the graben. Limit equilibrium stability analyses indicate that 3 m of erosion

would destabilize the slide for most of the winter season. This finding suggests that buttressing the toe of the slide is an

effective long-term remediation option.


Format:
PDF
Funding:
SPR 356
Collection(s):
US Transportation Collection
Main Document Checksum:
[+]
urn:sha256:59aaadac5a37947d4e2d50e9330b82ac9c5b063b74f60a2ac7e745f1f6a8dd28
Download URL:
https://rosap.ntl.bts.gov/view/dot/21832/dot_21832_DS1.pdf
File Type:

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