This engineering geologic investigation and interpretation of the relative stability of the southern half of Newell Creek Canyon covered an area roughly 3.1 km2. The results of this study are a set of detailed maps displaying areas of active and formerly active landslides, geologic bedrock and soil distribution, and a set of recommendations concerning future development and the possible stability conditions that may be encountered.
The mapping of morphological landslide features and associated
features resulted in the identification of 79 landslides in the southern half
of Newell Creek Canyon (Plate 1). These
landslides were divided into two classes related to the depth to the failure
plane resulting with 65 shallow-seated and 14 deep-seated landslides. All of these landslides were also classified
geomorphologically, totaling 65 earthflows, 3 earthflow/debrisflows, 9 slumps,
and 2 slump/earthflows (Easterbrook, 1993).
From the Engineering Geology Map combined with a stability analysis, the
Relative Stability Map was created resulting in three zones of relative
stability with the following percents of the study area: moving ground (5%),
potentially unstable ground (56%), stable ground (38%).
Engineering geologic investigations and interpretation of the relative slope stability serve as an essential first step in the evaluation of landslide hazards for regional and community land-use planning, decision making, and development. After this decision making has taken place, an experienced engineering geologist should complete a similar site-specific investigation including an engineering geologic map, relative stability map, and a report of observations and suggestions concerning development. This investigation should, in turn, be followed by a geotechnical study including a subsurface investigation such as drilling or trenching with insitu testing and sampling, testing of soil and bedrock samples, and stability computations.
The annual estimated $4 million cost of landslide damage in Oregon reveals the severity of the problem. In 1997, along with one of my professors, Scott Burns, and two other graduate students completed a landslide inventory of the entire Portland Metropolitan Region (Burns et al., 1998). This study focused only on landslides occurring in the past two years (1996-1997), yet 705 landslides were mapped, and a total of over $40 million for mitigation was estimated! However, I suggest that this study on Newell Creek Canyon and studies similar to it, followed by the recommended site-specific studies can reduce these costs immensely.
The purpose of the engineering geologic investigation is not to prevent development, but to promote educated decision making and wise development. Any area can be developed if the potential for instability is determined and the cost of stabilization and development meets the developer’s budget. Yet, some of these areas may be worth more to the community, socially and economically, if other land uses are implemented, such as parks.
As Portland continues to expand its suburbs due to continued population increases and as there is continued pressure to develop in geologically hazardous terrain, the need for the development of relative stability maps for educated decision making is increased. In a recent study by Burns et al. (1998), five zones of high risk for landsliding were delineated for the Portland Metropolitan Region. One of these zones is the fine grained facies of The Troutdale Formation, which covers hundreds of square kilometers in the southeast section of Portland. Newell Creek Canyon is located within this zone and most of the rest of this zone contains slides similar in type and density to these in Newell Creek Canyon.
Another concern within the Portland Metropolitan Region is the potential for earthquake induced and reactivation of currently identified landslides. Portland is in seismic risk zone 3, meaning the area can experience earthquakes of magnitudes up to VIII (Schlicker and Finlayson, 1979). The effects of an earthquake can be added to the simplified factor of safety equation through a second pseudostatic driving force component or earthquake thrust (Abel, 1997).
I suggest that before the area zoned as a high potential for landslides in the 1998 study by Burns, et al. becomes urbanized, a study similar to this one should be performed on this zone. Included with this study should be the evaluation for reactivation of earthquake induced landslides.
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