2011-02-04_PERMIT FILE - M2010071

Home Browse Search

Slope Stability - Goose Haven Reservoir No. 2 Page 3 February 4, 2011 b. Please provide slope stability analyses such as computer output, spreadsheets or manual calculations for all five conditions. 7. South Slope: a. Figures C-12 through C-16: i) The approximate scale (1"=200') appears to be off by an order of magnitude. According to that scale, the slurry wall is about 200 ft from the crest of the slope and is 340 feet deep. Based on Drawing 3, the slurry wall is about 10 feet from the crest of the slope. Based on borehole logs, the slurry wall should only be about 30 feet deep. Please correct or clarify the scale. Also indicate whether the vertical scale is exaggerated or the same as the horizontal scale. ii) The light blue gravelly sand and darker blue sandy gravel layers shown above bedrock do not correspond with the TH-24 borehole log shown on Figure E-6. TH-24 indicates there is about nine feet of clayey sand on top of the claystone bedrock, with a seven- foot layer of clay on top of that, and 12 feet of silty or clayey sand above that to the surface. Please either correct the analysis to reflect the borehole log properties or explain what data was used to select materials for the slope stability analyses. iii) The slip/failure surfaces shown in Figures C-12 and C-16 suggest surface sloughing only. The Division is concerned with slope failures that may affect the overall slope and what if any structures may be near the top of the slope failure, such as those shown in Figures C-13 through C-15. Please provide factors of safety for significant slope failures. iv) Please indicate where the phreatic surface elevations are obtained. b. Please provide slope stability analyses such as computer output, spreadsheets or manual calculations for all five conditions. 8. North Slope: a. Figures C-17 through C-21: i) The approximate scale (1"=200') appears to be off by an order of magnitude. According to that scale, the slurry wall is about 200 ft from the crest of the slope and is 340 feet deep. Based on Drawing 3, the slurry wall is about 10 feet from the crest of the slope. Based on borehole locations (e.g., Figure 4) there is not a definitive hole to represent the north slope. However, the borehole logs for holes TH-13 and TH-17 (Figure A-1) may be suitable. Borehole TH-36 (Figure A-6) and Figure 4 suggest bedrock is several feet higher in the northwest corner of the proposed reservoir than at the aforementioned locations. Based on the TH-13 and TH-17 logs, the slurry wall should only be about 32 feet deep. Please correct or clarify the scale. Also indicate whether the vertical scale is exaggerated or the same as the horizontal scale. ii) The light blue gravelly sand (SP) and darker blue sandy gravel (GP) layers shown above bedrock for the North Slope figures do not correlate well with either TH-13 or TH-17 borehole logs shown on Figures E-18 and E-19. TH-17 indicates there is no SP material in that location and TII-13 shows the layer overlying the claystone bedrock is primarily SP-SM or SP-SC, not (GP). Please either correct the analysis to reflect the borehole log properties or explain what data was used to select materials for the slope stability analyses. iii) The slip/failure surfaces shown in Figures C-17, 18, 20, and 21 suggest surface sloughing only. The Division is concerned with slope failures that may affect the overall slope and what if mAmin\tc1\m-2010-071 goose haven\stabilitycomments_mem04feb11.docx