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<br />1 <br /> <br /> <br />1 <br /> <br /> <br /> <br />1 <br /> <br /> <br />1 <br />1 <br /> <br /> <br /> <br />1 <br />1 <br />SECTIONTHREE Stab1111C11 Analyses <br />A stability analyses was performed to evaluate the embankment stability for the currf:nt and <br />ultimate height conditions with the raised phreatic surfaces resulting from the PMP (Probable <br />Maximum Precipitation) event. Both static undrained and post-seismic loadings conditions <br />were evaluated. The phreatic surfaces estimated from the seepage analyses, presented in Section <br />2, were used in the stability analyses below. <br />3.1 PRIOR STUDIES <br />The stability of the Henderson Tailing Dam, under normal operating pond conditions, was <br />previously evaluated by Woodwazd-Clyde in 1995 with the results presented in the report dated <br />April 2000. That report found that the current dam stability was acceptable. Based on ow <br />current understanding of a higher decant pond resulting from the PMP event, the stability of the <br />Henderson Tailing Dam was re-evaluated with the resulting estimated higher phreatic: surface <br />conditions within the embankment. <br />A liquefaction potential evaluation of the Henderson Tailing Dam was performed using Standard <br />Penetration Test (SPT) blow counts and published coaelations. Results from this evtduation <br />were summarized in ow report dated April 2000. Based on the results of ow liquefaction <br />analyses, the liquefaction potential of the saturated tailings was judged to be low. However, <br />some loss in the sheaz strength of the tailing could still be expected following an earthquake <br />event. Therefore, apost-earthquake stability analysis was performed to estimate the factor of <br />safety against a failwe of the dam following seismic shaking. Ow analysis conservatively <br />assumes that liquefaction occws and remolded undrained strengths are used for the ssltwated <br />tailing. Based on the results of the liquefaction analysis, the actual post-earthquake strengths <br />would likely be higher than the remolded strengths used in the analysis. <br />3.2 MODEL DEVELOPMENT AND ASSUMPTIONS <br />The geometry of the study section used in the stability analyses for the existing and ulltimate <br />height conditions is the same as the cross-section used for the seepage analyses. The crest <br />elevations for the existing and ultimate height conditions were assumed to be 8810 feet and 8900 <br />feet, respectively. <br />The undrained shear strength of the tailing material was estimated from the results of triaxial <br />shear tests performed on "undistwbed" and remolded soil samples. A detailed discussion of the <br />shear strength chazacterization of the various embankment materials is presented in our report <br />dated Apri12000. A summary of the material properties used in the stability analyses is <br />presented in Table 2. <br />Stability analyses were performed using the limit equilibrium program UTEXAS3 with <br />Spencer's method of slices. Both circular and non-circulaz shear surfaces were evaluated. <br />3.3 ANALYSES CASES <br />Stability analyses results presented in this section correspond to the various seepage conditions <br />evaluated eazlier in Section 3. The stability analyses cases presented in this section include: <br />• Existing height embankment under post-seismic loading condition using the phre~Ric surface <br />corresponding to 6 weeks after the PMP event. <br />C N:\PROJEGTSYaB14618_HEHDERSON MILL_BMGlSUB_W~6.0 PROJ DELrvw~anLL R3.DOI~NUL-01\\ 3-1 <br /> <br />