Laserfiche WebLink
Henderson 3 Dam Buttress Design <br />January 26, 2018 <br />Page 4 <br />the summer 2017 to improve the undrained stability of 3 Dam for current operating conditions. This <br />design report is focused on the seismic stability performance of 3 Dam based on the seismic design <br />criteria described below. <br />Seismic Design Criteria <br />Regulations of tailings dams fall within the jurisdiction of the Colorado Division of Reclamation, Mining <br />and Safety. The stability criteria for the Henderson tailings dam follows the relevant industry <br />guidelines and/or professional standards including the Colorado State Engineer's Office for water <br />dams and other state regulators (New Mexico and Utah), Australian National Committee on Large <br />Dams (ANCOLD), Canadian Dam Association (CDA) and the International Congress on Large Dams <br />(ICOLD). The maximum design earthquake (MDE) selected was a 10,000 year earthquake. <br />The seismic stability criteria selected for the dam was based on the state of practice, which uses <br />dynamic deformation analyses to demonstrate acceptable deformations. Deformations are <br />considered tolerable if the following conditions are met: <br />• The structural integrity of the slope is maintained with no release of tailings or process water. <br />• The slope does not undergo excessive differential movement with strain levels above those <br />that would suggest progressive failure or secondary potential failure modes, which could lead <br />to cracking or piping and release of tailings. <br />• Embankment movement is stabilized after shaking. <br />Two dimensional (213) deformation analyses are used to confirm that the TSF embankment will <br />perform effectively under various static and dynamic loading conditions. There are cases in narrow <br />valleys, basins or with significantly undulating foundation terrain where 2D plane strain conditions do <br />not simulate conditions particularly well. In narrow valleys, higher strength foundations along the <br />steep sided perimeter may provide significant additional strength along the critical failure surface. In <br />these cases, such as 3 Dam, it is beneficial to assess the stability and deformation of the TSF slope <br />in three dimensions (3D) to simulate the seismic performance more accurately. <br />The seismic design criteria for 3 Dam considers both 2D and 3D stability analyses. A 2D post - <br />earthquake design factor of safety of 1.1 was selected, and supported by three-dimensional dynamic <br />deformation analyses that demonstrate acceptable deformation. <br />Seismic Hazard Update <br />An updated site-specific seismic hazard analysis of the Henderson tailings dams was performed in <br />2015. The analysis incorporated new information on seismic sources, recent data on foundation <br />shear -wave velocities (V,) and updated ground motion prediction models. The primary objective of <br />this study was to estimate the levels of ground motion that could be expected at specified annual <br />frequencies (or return periods) at the sites of 1 Dam and 3 Dam, by performing a probabilistic seismic <br />hazard analysis (PSHA). Seven sets of two -component time histories were spectrally -matched to the <br />mean 10,000 -year return period uniform hazard spectrum (UHS) to establish the MDE ground <br />motions. <br />M=SftwEctgWtReaa14M MK_ RMS SOFT 3W4 BURRE690--LEM%REPORT, 1W e_FWI.LWC)L <br />