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Table 6. Slope stability analysis scenario conditions and factor of safety results. Permafrost in the Fairbanks region last formed dur- ing the Wisconsin glaciation beginning around 150,000

Table 6 Slope stability analysis scenario conditions and factor of safety results. Permafrost in the Fairbanks region last formed dur- ing the Wisconsin glaciation beginning around 150,000

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Figure |. Location of the study area with available lidar data extent in the FNSB in relation to Alaska, major cities, and transportatio1 systems (inset). Road data from ADOT&PF (2018), hydrology data from AGC (2020), lidar data from FNSB (2020), and ArcticDEM dat: (uniform gray areas indicate no data) from Porter et al. (2018).
Figure |. Location of the study area with available lidar data extent in the FNSB in relation to Alaska, major cities, and transportatio1 systems (inset). Road data from ADOT&PF (2018), hydrology data from AGC (2020), lidar data from FNSB (2020), and ArcticDEM dat: (uniform gray areas indicate no data) from Porter et al. (2018).
Figure 2. Landslide distribution within available lidar extent in the FNSB. Dots and triangles represent landslide occurrence and do not reflect landslide size. The Tanana 440 (T440) landslide location is indicated by the arrow and text. Letters (a) through (f) refer to the landslides presented in Figure 3. Road data from ADOT&PF (2018), hydrology data from AGC (2020), lidar data from FNSB (2020), and ArcticDEM data (uniform gray areas indicate no data) from Porter et al. (2018).
Figure 2. Landslide distribution within available lidar extent in the FNSB. Dots and triangles represent landslide occurrence and do not reflect landslide size. The Tanana 440 (T440) landslide location is indicated by the arrow and text. Letters (a) through (f) refer to the landslides presented in Figure 3. Road data from ADOT&PF (2018), hydrology data from AGC (2020), lidar data from FNSB (2020), and ArcticDEM data (uniform gray areas indicate no data) from Porter et al. (2018).
Figure 3. Examples of field-checked landslides of different material types and various ages: landslides in loess that are (a) prehistoric (inter- preted landslide extent marked by dashed lines), (b) historic, and (c) currently active; and landslides in bedrock that are (d) prehistoric, (e) historic (scarps of varying ages marked by dashed lines), and (f) currently active. In (a) and (e), there are adjacent landslides that are not delineated. Arrows indicate the direction of landslide movement. The background is a lidar-derived slope map. See Figure 2 for locations of these landslides.
Figure 3. Examples of field-checked landslides of different material types and various ages: landslides in loess that are (a) prehistoric (inter- preted landslide extent marked by dashed lines), (b) historic, and (c) currently active; and landslides in bedrock that are (d) prehistoric, (e) historic (scarps of varying ages marked by dashed lines), and (f) currently active. In (a) and (e), there are adjacent landslides that are not delineated. Arrows indicate the direction of landslide movement. The background is a lidar-derived slope map. See Figure 2 for locations of these landslides.
Figure 4. Tanana 440 (T440) landslide extent with test hole and stratigraphy locations and lidar-derived slope map background (FNSB, 2020). Radiometric age dates and depths are shown next to the corresponding test holes. The arrow indicates the direction of landslide movement. We used the profile A-A’ to extract the topography of the landslide cross section for the slope stability modeling.
Figure 4. Tanana 440 (T440) landslide extent with test hole and stratigraphy locations and lidar-derived slope map background (FNSB, 2020). Radiometric age dates and depths are shown next to the corresponding test holes. The arrow indicates the direction of landslide movement. We used the profile A-A’ to extract the topography of the landslide cross section for the slope stability modeling.
Table 1. Laboratory testing conducted for the T440 landslide.
Table 1. Laboratory testing conducted for the T440 landslide.
Table 2. Summary of the soil and rock properties used in the SLOPE/W modeling.
Table 2. Summary of the soil and rock properties used in the SLOPE/W modeling.
Figure 5. Schematic of (a) the longitudinal profile A-A’ (see Figure 4 for location) with reconstructed pre-landslide surface and (b) the Case 1 slope model with surficial loess and schist bedrock layers, piezometric surface, slip surface grid, and defined landslide entry and exit points.
Figure 5. Schematic of (a) the longitudinal profile A-A’ (see Figure 4 for location) with reconstructed pre-landslide surface and (b) the Case 1 slope model with surficial loess and schist bedrock layers, piezometric surface, slip surface grid, and defined landslide entry and exit points.
Figure 6. Stratigraphic columns for test holes (TH) 1 through 4. Vane shear test (VST) results are adjacent to the tested depth, with residual shear values in brackets. Dates in boxes are radiometric dating results for the corresponding organic layer. BOH = bottom of hole.
Figure 6. Stratigraphic columns for test holes (TH) 1 through 4. Vane shear test (VST) results are adjacent to the tested depth, with residual shear values in brackets. Dates in boxes are radiometric dating results for the corresponding organic layer. BOH = bottom of hole.
Figure 7. Stratigraphic columns for test holes (TH) 5 through 8. Vane shear test (VST) results are adjacent to the tested depth, with residual shear values in brackets. Dates in boxes are radiometric dating results for the corresponding organic layer. BOH = bottom of hole.
Figure 7. Stratigraphic columns for test holes (TH) 5 through 8. Vane shear test (VST) results are adjacent to the tested depth, with residual shear values in brackets. Dates in boxes are radiometric dating results for the corresponding organic layer. BOH = bottom of hole.
Figure 8. Peak (a) vane shear strength and (b) residual vane shear results with depth. Residual shear strength values from the direct shear tests are plotted at depths corresponding to the applied normal stress.
Figure 8. Peak (a) vane shear strength and (b) residual vane shear results with depth. Residual shear strength values from the direct shear tests are plotted at depths corresponding to the applied normal stress.
Table 3. Summary of laboratory test results. — Indicates no data. TH = test hole; SM = silty sand; ML = silt or sandy silt; STRAT = stratigraphic column.
Table 3. Summary of laboratory test results. — Indicates no data. TH = test hole; SM = silty sand; ML = silt or sandy silt; STRAT = stratigraphic column.
Table 4. Direct shear testing results.
Table 4. Direct shear testing results.
Table 5. Radiocarbon dating results. TH = test hole.
Table 5. Radiocarbon dating results. TH = test hole.
Figure 10. Grain-size distributions for T440 samples. The blue lines show upper and lower boundaries, and the dotted gray line is the approximate average grain-size distribution of all hydrometer-tested samples (N = 18) within the shaded blue region.
Figure 10. Grain-size distributions for T440 samples. The blue lines show upper and lower boundaries, and the dotted gray line is the approximate average grain-size distribution of all hydrometer-tested samples (N = 18) within the shaded blue region.
Figure 11. Variation of (a) friction angle and (b) cohesion with moisture content from the direct shear test results. Note that the cohesion of 16 kPa from one of the undisturbed tests is considered an outlier and was not included in the average.
Figure 11. Variation of (a) friction angle and (b) cohesion with moisture content from the direct shear test results. Note that the cohesion of 16 kPa from one of the undisturbed tests is considered an outlier and was not included in the average.
Figure 12. Modeling results with failure surface shapes for Case | (a) scenario | and (b) scenario 2; Case 2 (c) scenario 3 and (d) scenario 4; Case 3 (e) scenario 5 and (f) scenario 6; and Case 4 (g) scenario 7. The green region is the failure shape and slices; refer to Figure 5b for explanation of the other features. Each figure has 1:1 vertical and horizontal scales, and an indication of scale is provided in (g).
Figure 12. Modeling results with failure surface shapes for Case | (a) scenario | and (b) scenario 2; Case 2 (c) scenario 3 and (d) scenario 4; Case 3 (e) scenario 5 and (f) scenario 6; and Case 4 (g) scenario 7. The green region is the failure shape and slices; refer to Figure 5b for explanation of the other features. Each figure has 1:1 vertical and horizontal scales, and an indication of scale is provided in (g).
Figure 13. Schematics illustrating possible T440 landslide event models (not to scale). For the single landslide model, (a) the ~31,000-year-ol paleosol was transported within a competent block that formed the inner toe and then (b) subsequent erosion and gullying deposited sedimen that buried the ~8,000-year-old charcoal. For the multiple landslide model, (c) an older landslide event formed the inner toe prior to ~31,00( years ago and then (d) Event 1 was overrun by a younger event (Event 2) ~8,000 years ago that formed the outer toe, followed by erosion anc eullying.
Figure 13. Schematics illustrating possible T440 landslide event models (not to scale). For the single landslide model, (a) the ~31,000-year-ol paleosol was transported within a competent block that formed the inner toe and then (b) subsequent erosion and gullying deposited sedimen that buried the ~8,000-year-old charcoal. For the multiple landslide model, (c) an older landslide event formed the inner toe prior to ~31,00( years ago and then (d) Event 1 was overrun by a younger event (Event 2) ~8,000 years ago that formed the outer toe, followed by erosion anc eullying.
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