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Scott pit' <br />6WISi 4031 E[vddtsieo-IIcsn <br />M-•?•{?$ •#, ?? L1..L-m 7s•.MilC <br />With regard to liquefaction, HBET it, unclear rewarding the definition used by Sclimuser Gordon <br />Meyer, Inc. In geoteehnical terminology, liquefaction of the so-11 is a process whereby pore water <br />pressures increase rapidly as a result of cyclic, typically sei.mie, 'loading_,. Based upon the <br />context of Sehinvser Gordon Meyer's discussion, HRET anticipates that they are speaking about <br />'quick' conditions in the soil. <br />In order to develop quick conditions in the soil, the pore pressures must be high enough to reduce <br />the eflectivu stTesses in the soil to zero. However, as discussed preVlously, due to the hivt <br />hydraulic conductixity of the soils and mass of the large particles in the soils, HBE•T does not <br />believe that large enough pare pressures can develop to Cause quick conditions in the native <br />,-ravel and cobble soils. <br />Slum st"hi[if <br />In addition to the concerns relating to the impacts of flooding oil the buffer zone, HAFT <br />understands that there is concern about the stability of the buffer zone over time after mining and <br />rcctatnation. As a result, I•IBET conducted limit equilibrium slope stability analyses on the 100- <br />foot butter zone using the GSTABL 7 computer software package. The results of the analyses <br />are included in Appendix C. <br />As sho%wi on the output from the slope stability analyses, HBET modeled the 100 foot buffer to <br />include the shallow overburden silty sand soils above the dense gravel and cobble soils. In <br />addition, while seePa,-e fare{s Were not considered for the reasons prc;viously outlined, a sloping <br />water table was included in the model for uonsc vatisrin. Also, the soil strength parameters <br />selected for the silty sand soils and gravel and cobble soils tivere conservative. The :actual <br />strength of these soils is likely higher than modeled. <br />The slope stability analyses considered two case. The first case was for a deep seated failure <br />where a siecnificant portion of the buffer zone would be impacted. 'the second case considered <br />shallow failures in the face of the buffer zone. Both cases consider the critical, pit side of tho <br />buffer. <br />As shown in the results included in Appendix C, a factor-of-safety oiF2.175 was determined for <br />the deep seated failure case and a factor-of-safety of 1.875 was determined for the shallow <br />failure case. In general, a fucior-of-safety of 1.3 to 1.5 is appropriate for long terrsn stability. <br />'t'herefore, the 100 foot buffer is anticipated to be stable. <br />General Notes <br />The conclusions and recommendations included above are based upon the results of the <br />subsurface investigation and on our local experience. These umclusions and recommendations <br />should only be comsidered valid in the context of the limits of the investigation. <br />W tX.')X ALL PRO11:e71S IM12'S6 - tlaitr iC<<mvr_?es'•1X?? Vi?GiUQi Scua Pit-201? • Gru 40V 564NI LR43Ln-lip 3