Chapter 7
Delineating Potentially Unstable Areas

This chapter explains two major goals and products of this study: an accurate record of all landslide features and geologic units displayed on The Engineering Geologic Map and based on this map and the stability analysis, the interpreted relative stability displayed on The Relative Stability Map.  The basic methods of preparing the Engineering Geologic Map (Plate 1) and the Relative Stability Map (Plate 2) included in this thesis was presented by Hoexter et al. (1978), Richards (1982), and Wieczorek (1984).

The Engineering Geologic Map

The Engineering Geologic Map is a record of all the landslide features, residual soils, geologic formations, and hydrogeologic features encountered during the field investigation portion of this study.  The landslide features are represented by differing line types corresponding to the type (deep and shallow-seated) and relative age (active, inactive-young, and inactive-mature) of the feature.  The residual soils and bedrock geologic formational contacts are represented by solid and dashed lines.  Hydrologic features are displayed as shaded areas (sag ponds and depressions) and points (springs and seeps).  This map also includes the locations of the bore holes and well logs examined throughout this study.  All of this explanatory information is located on the Engineering Geologic Map (Plate 1).

Although some slopes on the map of the study area do not depict slide features, these areas are not to be considered as totally stable (Richards, 1982).  This map is only a static representation of a dynamic system, as the Spady Landslide has proved.  However, slopes with landslide features can be expected to be either moving or critically stable (factor of safety at or close to one), and development should be planned with this in mind (Richards, 1982).  This Engineering Geologic Map is not a substitute for more detailed site-specific work.  It is intended as a tool for the geotechnical engineer and presents all the relevant information which may affect stability in the study area.

The Relative Stability Map

The Relative Stability Map is an interpretation of the stability of the study area based on all the information recorded within this thesis and the Engineering Geologic Map (Plate 2).  It is intended for land use planners, architects, and others involved in development.  This map delineates stable areas, potentially unstable areas, and moving ground with the following percents of the study area: moving ground (5%), potentially unstable ground (56%), stable ground (38%).  These categories are further differentiated into deep- and shallow-seated landsliding and areas of man-made fill and cutslopes of questionable stability.  This further subdivision is performed because of the potentially extreme difference in difficulty and cost of mitigation of shallow-seated landslides versus deep-seated landslides.  Since about half of the study area is located within Portland’s Urban Growth Boundary, it can be expected to be developed at some point in the future.  These categories also represent a recommendation for further investigation required prior to development.  To back up these interpreted delineated zones a stability analysis was performed on all of the residual soils.  The following is a brief description of each of the map categories delineated on the map and their associated recommendations and the residual soils stability analysis.

Stable Ground

These areas exhibit no current or past history of instability.  However, a geotechnical engineer should be consulted prior to development.  On steeper slopes, within the stable ground category, an engineer should design all slope and drainage modifications to insure that these modifications do not lead to instability (Richards, 1980).  A simple rule is that the only construction on a slope should be one that increases the stability of the slope, even if the slope could be considered stable in its current state.

Potentially Unstable Ground

Slopes which exhibit inactive-young and inactive-mature landslide features, hummocky topography, or other characteristics of past movement are categorized as potentially unstable.  Areas immediately adjacent to slide masses and areas having similar soil, hydrologic, and slope conditions are also included in this category.  These areas probably have a factor of safety above, but close to, one and can be considered critically stable.  These conditions are extremely susceptible to minor slope modifications which may initiate instability or renewed instability.

An example of this is the Highway 213, mile post 2.1 shallow-seated landslide, in which construction of the Highway through excavations (cutslope) and minor amounts of fill initiated movement.  Since the time construction was finished (early 1980s) and a large rock buttress and a caged/rock retaining structure were emplaced to stop movement, there does not appear to have been significant movement.  If this area had been initially recognized as potentially unstable, the design and construction plan could have taken this into account and possibly avoided these problems (Richards, 1980; Hoexter et al., 1978).

Before any development in areas delineated as potentially unstable ground, a geotechnical engineer and an engineering geologist should be consulted to investigate the site and surrounding area in order to make design recommendations.  In areas with potential for deep-seated landsliding, extreme caution should be used when planning for development.  Development in these areas should plan to increase the stability of the slope, the cost of which would most likely be far less than the cost of repairing damage and stabilizing a renewed deep-seated landslide (Richards, 1982).

Moving Ground

Areas delineated as moving ground are locations of active landsliding.  These areas should be stabilized prior to any development and extreme caution should be used prior to and during development of these areas.

Residual Soils Stability Analysis

A generalized stability analysis was performed on all of the residual soils in the study area to evaluate areas of potential instability and to confirm conditions in areas that contained mapped landslides.  This analysis was performed with the infinite slope equation (seepage parallel to slope, equation 3) to calculate the factor of safety.  After the factor of safety for all the residual soils was calculated, the program Arc View GIS was used to relate the data spatially (ESRI, 1997).

The eight soil series found in the study area were divided into three categories with relatively equal thickness, general slope angles, and soil properties (Table 6, Table 1 and Table 2)(Gerig, 1985).

Table 6: Generalized material properties for the three catagories of residual soils

The soil properties for these three categories were derived from the pre-failure stability analysis of the 1996 Spady Landslide, the Soil Survey of Clackamas County, and Taylor’s Stability Chart for soils with a friction angle (Gerig, 1985; Abramson et al, 1996).  With these three categories, the factors of safety were calculated for the generalized slope angles from Gerig (1985) and the slope angles needed to reach a factor of safety < 1.5 (projected) or a slope considered to be unsafe (Senneset, 1996).  Figure 24 is a graph of the values calculated for the three categories.  Next, Arc View GIS was used to calculate the actual slope angles in the study area from the 10 foot contours.  This was done with the spatial analysis tool package in the program with a grid spacing of 15 meters (Figure 25).  Finally, a map query was performed between the soil series layer and the slope angles layer.  This query consisted of the spatial location of any slopes in any of the three categories that had a factor of safety below 1.5 (Figure 26).  Examining this map, one can see that the locations calculated (zoned) with slopes with a F_s<1.5 coincide relatively strongly with the mapped, shallow-seated landslides.  A few mapped slides are not touching these zone areas, but I noticed that nine out of these ten slides occurred on creek cut banks with a local slope angle higher than depicted by the slope angles.

Figure 24: Graph of the factor of safety versus slope angles for the residual soils in the study area with a constant water table at a depth of 0.5 m.  Note the critical slope angles for each category, above which the factor of safety falls below 1.5.

Figure 25: Map of actual slope angles within the study area and the relation to the residual soils.

Figure 26: Map of residual soil series with factors of safety less than 1.5 and the approximated outlines of mapped shallow-seated landslides.

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