Laserfiche WebLink
HuJJle.lnn-Brut <br />3.5 Discussion <br />As indicated above, the results of the analyses yielded factors -of -safety for gob <br />pile #2 above 1.5. However, it is important to note that the observed factors -of -safety are <br />heavily dependent upon the presence of the piezometric surface modeled based upon the <br />available pore pressure data. In order to evaluate the impacts of changes in the <br />piezometric surface on the stability, HBET conducted sensitivity analyses. In general, <br />the available data suggests that the pore pressures are not increasing near the toe of gob <br />pile #2. As a result, the piezometric surface near the toe was not changed as part of the <br />sensitivity analyses. The piezometric surface was changed by adjusting the pore <br />pressures in the two `active' piezometers near the top of the existing gob pile, VWP-06 <br />and VWP-08. <br />The results of the sensitivity analyses, included in Appendix B, indicate that the <br />stability of gob pile #2 is currently most sensitive to changes in the pore water pressures <br />in VWP-08. An increase in the pore pressure in VWP-08 of 6 psi (equivalent to <br />approximately 14 feet of head) would result in a factor -of -safety of less than 1.5. At <br />VWP-06, and increase in the pore pressure of 20 psi (equivalent to approximately 46 feet <br />of head) would be necessary to reduce the factor -of -safety below 1.5. <br />In addition to the sensitivity with regard to the piezometric surface, HBET <br />evaluated sensitivity analyses with regard to the shear strength of the gob. As indicated <br />in the referenced documents, previous sensitivity analyses completed by Buckhorn <br />included friction angles (�') of 30° and 32°. However, the triaxial shear testing <br />completed in 2005 showed frilly drained friction angles of between 33.6° and 34.31. In <br />addition, triaxial shear testing completed in October 2013 showed a drained friction angle <br />of 34.5°. <br />None of the shear strength testing shows a drained friction angle as low as 30°. <br />HBET does not believe that this value is representative of the gob and as a result, <br />sensitivity analyses using this value is not warranted. However, for conservatism, HBET <br />conducted sensitivity analyses considering a drained friction angle of 32°. The results <br />indicate that Gob Pile #2 will still have a factor -of -safety above 1.5. The results of the <br />analyses are included in Appendix B. <br />3.6 Conclusions <br />As indicated previously, the analyses indicated that gob pile #2 will be stable to a <br />full buildout height of 350 feet. However, as indicated previously, changes in the pore <br />water pressures will affect the results of the analyses. <br />X Q003 ALL PROJECTS O0439 -1F Sto.er Associates Int 00439.0003 Booie .%Imei!00 - Gto'Gob 2, Full Build—t 00439-GM3 R073014 doe 5 <br />