Johnson Creek Landslide Research ProjectThe Johnson Creek Landslide Project was begun July 1, 2002, with support from Oregon Department of Transportation (ODOT) and funds from the Federal Highway Administration. The project was supervised by ODOT and managed by DOGAMI. A final report summarizing findings was released as DOGAMI Special Paper 40 [Buy CD from Nature of the Northwest]. The following are some illustrations, the abstract from Special Paper 40, and a summary of the ODOT research problem statement.
Recent deformation of Highway 101 on the south side of the Johnson Creek landslide.
Northeast corner of Johnson Creek landslide showing recent clear cut and translational block failure at headwall. Note exposure of Pleistocene marine terrace sand that caps the Astoria Formation in this area. Headscarp is about 6-8 m high. Absolute elevation at the top of the headscarp is about 30 m above sea level. The north-south trending headwall graben is about 9-10 m wide. The east-west trending graben/fissure is 15 m wide and bounds the southern margin of the landslide in this area. Jonathan Allan of DOGAMI is on the ladder measuring the section in the right-hand picture.
Landslide toe at the sea cliff on Beverly Beach (looking north). Note deformed white bed above the soft, wet slide breccia/mylonite that forms the dark gray layer. Original dip of bedding in Astoria Formation is west but note that in far background beds dip steeply east. The small slide block at the toe is thus rotated back into the slope. There is no evidence of such large backward rotations in the larger slide blocks in the eastern part of the landslide.
Abstract of Special Paper 40
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.
ILLUSTRATION OF SPEED OF WATER PRESSURE TRANSMISSION
FROM THE HEAD OF THE SLIDE IN RESPONSE TO A RAINFALL EVENT
(Above) Cross section through the Johnson Creek Landslide illustrating vertical percolation of groundwater and lateral water pressure transmission through the saturated zone after a rainfall event in February of 2007. Isochrones (black lines) are in 2-hr intervals for first response of grouted piezometers to pressure increase. Baseline is the time when the increase occurred at the LT-3p site at the head of the slide. Blue numbers are the first response time for grouted piezometers; black numbers separated by a back slash are response time for half of the total pressure response; red numbers are the same data for adjacent sand packed piezometers. Blue boxes list travel velocity of pressure response; (i) = virtually instantaneous response between the two monitoring sites. Green dotted lines are isochrones for downward infiltration of groundwater, assuming a mean rate of 50 mm/hr derived from observations of piezometers in the unsaturated zone. Gray vertical lines are observation boreholes; horizontal black bars are depths of piezometers; bold dark gray line is the base of the basal shear zone; brown line is the slide surface; blue dash-dotted line is approximate piezometric surface which approximates the water table. Vertical exaggeration is 1.6. Graphic is from Figure 50 of Special Paper 40.
Landslide Technology, 2004, Geotechnical investigation Johnson Creek Landslide, Lincoln County, Oregon: Oregon Department of Geology and Mineral Industries Open-File Report O-04-05, 115 p, published on CD.
SPR RESEARCH PROGRAM
200 Hawthorne SE,
Salem OR 97301-5192
phone (503) 986-2700
fax (503) 986-2844
DETAILED GEOTECHNICAL ANALYSIS OF LARGE TRANSLATIONAL LANDSLIDES IN SEAWARD-DIPPING SEDIMENTARY ROCKS
Uplift of coastal mountain ranges in the Pacific Northwest over the last several million years has inclined sedimentary rock bedding planes toward the coastline, causing widespread translational landslides measuring thousands of feet in length and width. The Johnson Creek Landslide and others of similar nature (large translational slides in seaward dipping Tertiary sedimentary rock) pose an ongoing threat to public safety and are a continual, very expensive highway maintenance issue. In the Beverly Beach-Moolack Beach area alone there are 5 of these landslides, 4 of which constantly damage US Highway 101. To date remediation of most of these landslides was thought to be more expensive than continual repaving, but this decision is not based on a detailed understanding of the forces causing movement. Continual addition of asphalt to the highway may in fact be exacerbating the problem by adding load. An important policy issue in Oregon is the need to reduce the impact on the public beach of any remediation solution. For example, reduced impact might mean minimizing the footprint of a buttress and the associated factor of safety. Reducing the factor of safety requires a highly refined understanding of landslide geometry and relative importance of forcing mechanisms (pore pressure, wave erosion, etc.). If the main forcing mechanism were pore water pressure, then refined understanding of the hydrogeology (e.g., relative importance of permeable sedimentary layers versus water bearing fractures; orientation of fractures) might lead to more accurate and cost-effective drilling of horizontal, inclined or vertical wells. An effective dewatering scheme might avoid buttressing entirely and be more cost effective than continual repair of the highway. If forces promoting movement were small enough or could be reduced through dewatering, then a buttress with a small footprint might suffice. If wave erosion were the chief forcing mechanism, then reducing wave energy at the toe of the slide would be the only effective remediation alternative. Obtaining a refined understanding of the failure mechanisms and driving forces for these large landslides is therefore critical to making sound, environmentally responsible decisions on remediation, maintenance, and future highway design in the coastal zone.
The overall objective is to provide information and a process that can be used as a model for investigation and remediation of large translational landslides that are affecting the coastal transportation system here and in other parts of the US. The Oregon Department of Geology and Mineral Industries (DOGAMI) is uniquely positioned to help ODOT by supplying needed geological expertise, research personnel, and a recognized outlet for published, reviewed publications.
This pilot project will:
(1) Obtain preexisting and new geotechnical data on the Johnson Creek landslide;
(2) Determine landslide geometry by detailed mapping and drilling;
(3) Determine hydrogeology of the landslide by geologic mapping, drilling and monitoring of piezometers;
(4) Determine correlation of slide movement to wave erosion, rainfall and groundwater pore pressure changes by monitoring for 5 years wave erosion, slide movement, rainfall, and pore water pressure in cracks and rock or soil layers;
(5) Develop remediation options for the most environmentally sustainable, economically feasible solution, and
(6) Explore a new and evolving partnership between DOGAMI and ODOT that capitalizes on the technical and institutional strengths of both agencies.
NOTE: ODOT will have technical oversight of the project, being available to DOGAMI for technical advice.? ODOT will have authority to make needed adjustments to the project plan, if needed.?? DOGAMI will manage the project, handling all aspects of subcontracting and data collection.
TASK 1: Project management, monitoring, and technical reporting by DOGAMI:
TASK 2: Subcontracted topographic survey of the slide at project start (year 1) and end (year 5).
TASK 3: Subcontracted detailed stratigraphic correlation in drill holes and outcrops.
TASK 4: Subcontracted geotechnical drilling, instrumentation, and interpretation (see attached map):
TASK 5: Technical oversight and review plus sample storage by ODOT:
TASK 6: Research Administration
*These tasks will be done by ODOT personnel, if they have time, but they reportedly will not have time for at least the next few years; hence, these tasks are shown as subcontracted. Appropriate budget will be allocated for ODOT in the event that their personnel become available in future years.
Year 1: Geotechnical instrumentation, drilling, surveying, excavation of slide toe, geologic mapping, stratigraphic anlaysis, and monitoring of water pressure, rainfall, erosion and slide movement
Year 2: Interim report, after monitoring of water pressure, rainfall, erosion and slide movement
Year 3: Monitoring of water pressure, rainfall, erosion and slide movement
Year 4: Monitoring of water pressure, rainfall, erosion and slide movement
Year 5: Final report after final survey and monitoring of water pressure, rainfall, erosion and slide movement NOTE: Subcontractor will be used for quantitative landslide analysis only if ODOT personnel are not available.
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