This study was initiated with an extensive review of the literature pertaining to any aspect of landslides in the Oregon City area. Environmental Assessment of Newell Creek Canyon, Oregon City, Oregon and Geology and Geologic Hazards of Northern Clackamas County, Oregon, both provided a historical perspective on landsliding in the study area (Burns, 1993; Schlicker and Finlayson, 1979).
The geological literature was consulted and several reports provided excellent Tertiary to Quaternary geologic history of the complex stratigraphy located in the study area (Schlicker and Finlayson, 1979; Tolan and Beeson, 1984). This geologic history literature was coupled with reviewing any well-water log reports, boring logs, and soil pit data the author could acquire to examine the local stratigraphy within the formations. Many of these well log reports were collected from the Water Resources Department in Salem, Oregon (appendix 1). Soil properties were derived from Oregon Department of Transportation boring logs, the Soil Survey of Clackamas County, field evaluation of materials, and laboratory tests performed by fellow students during the evaluation of the Spady Landslide (Oregon Department of Transportation, 1998; Gerig, 1985; James, Burns, and Braibish, 1996).
To prepare a plan for the investigation, several techniques for evaluating and constructing engineering geology and relative slope stability maps were consulted including: Wieczorek (1984), Richards (1982), McCalpin (1974), and Soeters and Van Westen (1996). These reports helped build an excellent framework for which the rest of this study was built upon.
After the initial literature review was complete, the acquisition of digital data from the City of Oregon City helped immensely for creation of the base maps to record field data. The City of Oregon City provided digital data of contours at ten feet and two feet intervals, streets, structures, fences and walls, and many more layers in the form of CAD drawings, projected in State Plane, Oregon North Zone. These drawings were imported into the geographical information system program, Arc View GIS, and then converted to shape files (ESRI, 1997). I then created a detailed base map at a scale of 1:250 with a ten to two foot in places contour interval. This base map proved excellent for this type of study.
The initial field mapping began in April 1996 and lasted until November 1996. This mapping consisted of completing the following tasks:
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walking or viewing every square meter of the study area, |
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recording all features indicating instability onto the base map, |
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filling out a landslide inventory worksheet, |
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recording vegetation change and response, |
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measuring or calculating (from topography) amount of vertical and horizontal displacement, |
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noting any hydrologic features (springs, ponds), |
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estimating relative age of landslide features, and |
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taking photographs. |
Also recorded were broader things like changes in slope from linear to hummocky topography and changes in soils and geologic formations. This field mapping also included a set of detailed maps of the Spady Landslide, representative of many of the slides in the area, at two different stages in its development. Final field work was performed during December 1997 and consisted of taking soil samples from the Spady Landslide and walking into areas that were once covered in vegetation too heavy to cover during the summer months.
Other field work performed consisted of mapping the Spady Landslide. This landslide is considered a typical slide in the study area and was mapped in detail at a scale between 1:100 to 1:200 at two different stages in its development, first in May 1996 and then in May 1997.
The first mapping of the Spady Landslide (1996) was performed with a plane table at a scale of 1:120, while the second mapping (1997) was done with a Sokkia electronic total station at a scale of 1:100.
The grain size distribution for soil samples collected at the Spady Landslide were determined by sieve and hydrometer analysis. Atterberg limits and unconfined compressive strength tests were also performed on samples from the 1996 failure surface (James, Burns, Braibish, 1996; Al-Khafaji and Andersland, 1992). These data proved helpful for a stability analysis on this slide. The stability analysis was performed using Janbu’s Method calculated with a Visual Basic subroutine (Janbu, 1973). Stability analysis also included using the Infinite Slope Equation and XSTABL analysis on the residual soils and on some of the larger deep-seated landslides, located elsewhere in the study area to see how stable they are and what conditions are needed to reactivate these landslides (Sharma, 1994).
The data collected in the field were digitized from the paper copy to a digital copy using Arc View GIS (ESRI, 1997). Several different themes or layers were digitized including scarps, shear zones, toes, sag ponds and local depressions, areas of development and man-made fill, creeks, and geology. This digitizing was performed at a scale ranging from 1:200 to 1:1000 depending on the size and detail of features. To accompany these polylines and polygons, a database, linked to the shapes, was created which includes data such as relative age, vertical and horizontal displacements, and mitigation of the landslide features.
The final phase of the investigation consisted of preparing an Engineering Geology Map and a Relative Stability Map. The Engineering Geologic Map (Plate 1) includes information collected on the base map and on field notebooks/worksheets, including landslide features and bedrock geology. From the information on The Engineering Geology Map, an interpretive Relative Stability Map (Plate 2) was created (Hoexter et al., 1978; Richards, 1982). This Relative Stability Map consists of a map of the study area with zones of relative stability marked on it, including active, inactive-young, inactive-mature, and deep-seated and shallow-seated landslides. Accompanying these zones of relative stability are suggestions for further study needed before any further development or other changes to the land take place (Hoexter et al., 1978; McCalpin, 1974; Richards, 1982; Weickzorek, 1984). These two maps represent the results of this study of the Engineering Geology and Relative Stability of the Southern Half of Newell Creek Canyon, Oregon City, Oregon.
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