Dam Safety Assessment Mount Nansen Tailings Facility Near Carmacks, YT
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Dam Safety Assessment Mount Nansen Tailings Facility near Carmacks, YT Project No: 0201-00-14618 May2002 EBA Engineering Consultants Ltd. DAM SAFETY ASSESSMENT MOUNT NANSEN TAILINGS FACILITY NEAR CARMACKS, YUKON Prepared by: EBA ENGINEERING CONSULTANTS LTD. Whitehorse, Yukon Submitted To: Water Resources Division Department of Indian Affairs & Northern Development - Yukon Region Whitehorse, Yukon Project No.: 0201-00-14618 May2002 Calcite Business Centre, Unit 6, 151 Industrial Road, Whitehorse, Yukon, Canada Y1A 2V3 Telephone: (867) 668-3068 - FAX: (867) 668-4349 - e-mail: [email protected] - http/lwww.eba.ca 0201-00-14618 -1- May2002 TABLE OF CONTENTS 1.0 INTRODUCTION ....................................................................................................................................... 1 1. 1 PROJECT DESCRIPTION .......................................................................................................................... 1 1 .2 SCOPE OF WORK ..................................................................................................................................... 2 1.3 AVAILABLE INFORMATION .................................................................................................................... 4 2.0 MOUNT NANSEN TAILINGS FACILITY .............................................................................................. 4 2.1 FACILITY DESCRIPTION ......................................................................................................................... 4 2.2 FACILITY DESIGN & CONSTRUCTION ................................................................................................... 6 2.3 FACILITY OPERATION ............................................................................................................................ 9 2.3.1 Operational Phase ............................................................................................................................... 9 2.3.2 Shutdown Phase ................................................................................................................................. 10 2.4 CONSEQUENCE CLASSIFICATION AND EXTREME EVENT CRITERIA ............................................... 12 3.0 FIELD EV ALUATIONS ........................................................................................................................... 12 3.1 SITEVISITS ............................................................................................................................................ 12 3 .2 FIELD PROGRAMS ................................................................................................................................. 13 4.0 SEEPAGE CONTROL DYKE ASSESSMENT ...................................................................................... 14 5.0 SPILLWAY ASSESSMENT ..................................................................................................................... 14 5.1 HYDROLOGY REVIEW .......................................................................................................................... 15 5.2 CHANNEL FLOW CAPACITY AND CONDITION ................................................................................... 15 5.2.1 Diversion Channe/ ............................................................................................................................. 16 5.2.2 Emergency Spillway ........................................................................................................................... 16 5.2.3 Diversion Spillway ............................................................................................................................. 17 5.3 DIVERSION SPILLWAY UPGRADING ................................................................................................... 18 5.3.1 Upgrading Design .............................................................................................................................. 18 5. 3. 2 Upgrading Activities .......................................................................................................................... 18 5.4 FINAL SPILLWAY CONDITION ............................................................................................................. 19 6.0 TAILINGS DAM ASSESSMENT ............................................................................................................ 19 6.1 DAMPERFORMANCE ............................................................................................................................ 20 6.1.1 Permafrost Thaw ................................................................................................................................ 20 6.1. l. l Pond Level ................................................................................................................................................ 23 0201-00-14618 -11- May2002 6.1.1.2 Phreatic Levels .......................................................................................................................................... 24 6.1.1.3 Seepage ..................................................................................................................................................... 26 6.1.2 Settlement & Deformation .................................................................................................................. 28 6.2 LIQUEFACTION AND STABILITY ASSESSMENT .................................................................................. 31 6.2.1 Seismic Hazard ................................................................ ,................................................................. 32 6.2.2 Liquefaction Assessment .................................................................................. ,................................. 33 6.2.2.1 Liquefaction Assessment Procedure ........................................................................................................ .33 6.2.2.2 Liquefaction Assessment Results ............................................................................................................. .35 6.2.3 Stability Analyses ............................................................................................................................... 36 6.2.3.1 Thawed Depth .......................................................................................................................................... .37 6.2.3.2 Bulk Unit Weight and Soil Strength Parameters ....................................................................................... 37 6.2.3.3 Phreatic Surface within the Dam .............................................................................................................. 39 6.2.3.4 Excess Pore Water Pressure in Foundation Soils ..................................................................................... .40 6.2.3.5 Residual Strength of Liquefied Soils ....................................................................................................... .40 6.2.3.6 Results of Stability Analyses ................................................................................................................... .41 6.3 SOUTH ABUTMENT SUBSIDENCE ........................................................................................................ 43 7.0 CONCLUSIONS AND RECOMMENDATIONS ................................................................................... 46 7.1 CONCLUSIONS ....................................................................................................................................... 46 7.2 RECOMMENDATIONS ............................................................................................................................ 47 7.2.1 Maintenance and Surveillance ........................................................................................................... 48 7.2.2 Extreme Event Studies ........................................................................................................................ 49 8.0 CLOSURE .................................................................................................................................................. 50 Figures Appendix A: Terms and Conditions Appendix B: Spillway Assessment Appendix C: Thermal Analysis Appendix D: Seepage Assessment Appendix E: Seismic Hazard Assessment 020 l-00-14618 -m- May2002 EXECUTIVE SUMMARY EBA Engineering Consultants Ltd. was retained by the Yukon Region of Indian and Northern Affairs Canada to complete a dam safety assessment of the abandoned tailings impoundment at the B.Y.G. Natural Resources Inc. mine site, west of Carmacks, Yukon. The dam safety assessment was to consider the physical condition and stability of the earthfill tailings dam, and the adequacy of both the spillway and the seepage control dyke associated with the tailings dam. The evaluation of these items was to consider both the current state of these facility elements as well as a projected condition five years in the future. During the course of this assessment, site visits, site characterization studies, and reviews of available geotechnical data were completed. Detailed reporting of these activities is included in a Summary Data report that is presented under separate cover. Moreover, specific safety deficiencies and inadequate performance of the spillway and the seepage control dyke were identified at an early stage in the assessment process and upgrading and repair measures were enacted in the fall of 2000 and the