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A� <br /> Mr. Brandon Guite <br /> Henderson Mill <br /> March 29,2018 <br /> Page 5 <br /> AECOM Response 5 <br /> The FLAOD model was used at the suggestion of the TRB to model and review the stability <br /> and deformation influences on 3 Dam after an extreme earthquake event(10,000 year event) <br /> to more accurately simulate the actual geometry and better understand the effect of 3D terrain <br /> on seismic response. The FLAOD model does not show any development of tensile forces <br /> at any location within the slope,which would be indicative of potential for tension crack <br /> formation. However, if transverse cracks were to develop,either due to tension or to <br /> differential movement around the natural rock knob,the risk of release of process water is <br /> low considering that the decant pond is located a significant distance from the crest. In <br /> addition,the phreatic surface within the dam near the natural knob is below the toe of the <br /> setback slope, such that potential transverse cracks in this location would not be deep enough <br /> to experience seepage erosion. <br /> DRMS Comment 6 <br /> In the Results section of the AECOM report dated January 26, 2018, the Engineer states as <br /> shown in these figures the movement arrest and comes to equilibrium after motion. The <br /> stability analysis assumes an idealized uniform set of input parameters to represent <br /> variable conditions. Please perform a sensitivity analysis on the final closure conditions <br /> (Stage 4) to determine how much of a change it would take to generate a debris flow <br /> condition, if possible. Please include variable soil input parameters and variable phreatic <br /> level within a reasonable range to determine an actionable threshold during long-term <br /> monitoring. <br /> AECOM Response 6 <br /> While we agree that a sensitivity analysis is sometimes warranted when completing <br /> geotechnical analyses with limited information, we do not believe that applies to <br /> this project. The 3 Dam stability analyses are based on extensive investigations <br /> where a robust understanding of material stratigraphy, material types and <br /> corresponding characteristics, as well as phreatic levels have been established. <br /> Conservative values for the range of conditions measured have already been <br /> integrated in the stability analyses completed; therefore, further sensitivity analysis <br /> is unwarranted. The investigations and testing completed are summarized below: <br /> • The cross sections have been developed using various numbers of Cone Penetration <br /> Testing(CPTs), drill holes, core holes and geophysical investigations(refer to Figure <br /> 1 with the report)over time. <br /> • The material properties used in analysis have been developed using various <br /> laboratory testing including Isotropically Consolidated Undrained Triaxial (CIU), <br /> Direct Simple Shear(DSS), DSS with strain reversal, Consolidation, Cyclic CIU and <br /> M\DCSIPROJECTS\WTR\60551494_HEN3_BUTTR\500-DELIVERABLESIDRMS REPORTIDRMS COMMENTS\FINALIDRMS RESPONSE_AECOM_FINAL DOCX <br />