Laserfiche WebLink
Interoffice Memorandum <br />Exhibit 23, Item 2 is a geotechnical study performed by Shannon & Wilson, Inc. (SW) for the revised Collom temporary <br />spoil fill. Location of the fill has changed compared to the original submittal of Exhibit 23, Item 1 which has moved up <br />slope within the same drainage. Based on the relatively small change of location between the old and the now proposed <br />spoil pile, the majority of SW's stability analysis and recommendations, and the site material properties remain valid. <br />The general spoil pile configuration has remained the same with a fill slope of 3h:1v and with the change of surrounding <br />topography, the maximum height of the new proposed spoil pile has been reduced by 75 feet. The new location does <br />have a change in the slope of the valley base. The change in slope is minor (+1.5 degrees) and has little impact to the <br />stability of the spoil pile. The southern side of the spoil pile has also been revised as it approaches the low wall for the <br />Collom Pit. <br />Revised recommendations for the spoil pile's underdrains were also included in SW's report. The revised spoil pile <br />underdrains meets all requirements by Rule 4.09 and no comments are necessary. <br />A total of three slope stability analyses were ran by SW. All results were for long term stability of the pile. A conservative <br />approach was used in SW's first two analyses by including a water table within the spoil 40 feet above the valley floor. <br />With the proposed underdrain, it is highly unlikely water should reach that height within the spoil. The three analyses <br />were for a mass failure along the main slope of the spoil pile (No. 1), a small failure along the toe of the slope (No. 2) and <br />failure along the slope of the south end of the fill (No. 3). Using the material properties provided with SW's Exhibit 23, <br />Item 1(material properties remain valid), long term slope stability safety factor has been calculated using GALENA slope <br />stability program. A total of six analyses were performed using GALENA compared to the three performed by SW. The <br />reason for the additional GALENA runs was due to the change in material properties of spoil used in SW's No. 3 analysis. <br />Analysis No. 3 used a unit weight of 130pcf compared to 110pcf used in analyses No.1 and No.2. This appears to be an <br />input error because no justification was included within SW's report. Due to the change in spoil unit weight, three <br />additional GALENA runs were performed using both unit weights. The GALENA results are attached. The following table <br />compares SW's results to GALENA's: <br />From the results, all safety factors and construction practices recommended by SW meet all standards required under <br />Rule 4.09. After the review of SW's Exhibit 23, Item 2, the Division has the following comment: <br />• It was observed by the Division that a different unit weight was used for south end of the fill. The unit weight <br />used for this analysis was 130pcf compared to 110pcf used in the analyses performed for the primary slope of <br />the spoil fill. Please provide the Division with an explanation for the change in unit weight for spoil or provide a <br />slope stability analysis using the correct unit weight. <br />This concludes my preliminary review of Colowyo's PR -4. If you have any questions please feel free to contact me. <br />Sincerely, <br />SW <br />Division <br />Anlysis <br />FS <br />FS <br />FS* <br />No. <br />1.86 <br />2.07 <br />2.07 <br />No. 2 <br />2.03 <br />1.81 <br />1.8 <br />No.3 <br />2.07* <br />1 2.54 <br />2.49 <br />* Unit Weight 130pcf <br />From the results, all safety factors and construction practices recommended by SW meet all standards required under <br />Rule 4.09. After the review of SW's Exhibit 23, Item 2, the Division has the following comment: <br />• It was observed by the Division that a different unit weight was used for south end of the fill. The unit weight <br />used for this analysis was 130pcf compared to 110pcf used in the analyses performed for the primary slope of <br />the spoil fill. Please provide the Division with an explanation for the change in unit weight for spoil or provide a <br />slope stability analysis using the correct unit weight. <br />This concludes my preliminary review of Colowyo's PR -4. If you have any questions please feel free to contact me. <br />Sincerely, <br />