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r� Mr. Brandon Guite Henderson Mill March 29, 2018 Page 3 study sections at 3 Dam where the phreatic surface continues to increase, as expected by design. The rate of the phreatic surface rise depends upon several factors including the impoundment raise rate,pond location,tailings permeability and drainage. When deposition ceases,the impoundment will consolidate and eventually achieve a lower static phreatic surface. The 3 Dam Henderson drainage system consists of the original foundation drain system installed during the initial dam construction. Supplemental drainage has also been provided through horizontal drains which will continue to be installed as needed. The horizontal drains and outfalls of the foundation drain system are monitored as part of regular inspections. The phreatic surface is monitored through piezometers which provide an indication of drainage system performance. Henderson monitored the piezometers on a monthly basis until installation of the Canary automated telemetry system in October 2014. Currently the Canary system records all piezometric levels at least twice daily.Alarms are set on the system to alert Henderson and AECOM of any large changes of piezometer levels or if an established piezometer threshold value is reached. DRMS Comment 4 In the Results section of the AECOM report dated January 26, 2018, the Engineer states for Stage 4 with crest elevation of 8,900 feet the calculated displacements for the dam crest with final stage buttress using the selected median 10,000 year earth quake time history is less than 4.5 feet. The Division believes 4.5 feet of displacements is significant. Please explain why the Engineer believes this is acceptable displacement. AECOM Response 4 The estimated 4.5 feet of displacement under the final Stage 4 is following an extreme seismic 10,000-year event at the final stage of construction assuming approximately the highest phreatic surface expected at the end of deposition. As discussed in the report, deformations are considered tolerable if the following conditions are met: • The structural integrity of the slope is maintained with no release of tailings or process water. • The slope does not undergo excessive differential movement with strain levels above those that would suggest progressive failure or secondary potential failure modes, which could lead to cracking or piping and release of tailings. • Embankment movement is stabilized after shaking. The dynamic deformation analyses completed and presented within the report support that the integrity of embankment is maintained after the seismic event and there would be no M OCSIPROJECTSIWTR%MS1494_HEN3_BUTfRXSO4DELNERABLESIDRMS REPORTIMMS COMMENTSIFINALIDRMS RESPONSE_AECOM_FINAL DOCX