APPENDIX H. Limit Equilibrium Analysis of Dike 1 Abutments Prior to Failure

Transcription

1 APPENDIX H Limit Equilibrium Analysis of Dike 1 Abutments August 25, 2016

2 Appendix H Limit Equilibrium Analysis of Dike 1 Abutments TABLE OF CONTENTS H1 INTRODUCTION... 1 H2 ENGINEERING PROPERTIES OF THE TAILINGS... 3 H3 METHODOLOGY... 4 H3.1 Effective Stress Analyses (ESA)... 4 H3.2 Undrained Strength Analyses (USA)... 4 H4 RIGHT ABUTMENT... 5 H4.1 Model Setup... 5 H4.1.1 Geometry... 5 H4.1.2 Tailings Stratigraphy... 6 H4.1.3 Pore-Water Pressure Conditions... 6 H4.2 Results... 6 H5 LEFT ABUTMENT... 7 H5.1 Model Setup... 7 H5.1.1 Geometry... 7 H5.1.2 Tailings Stratigraphy... 9 H5.1.3 Pore-Water Pressure Conditions... 9 H5.2 Results... 9 List of Tables Table H2-1 Material parameters adopted in pre-failure stability analyses... 3 Table H2-2 Selected effective stress parameters... 3 Table H2-3 Selected undrained strength parameters... 4 Table H4-1 Right abutment model geometry November, Table H4-2 2D limit equilibrium results (Section AA) November, Table H5-1 Left abutment model geometry November, Table H5-2 Left abutment model geometry August, Table H5-3 Left abutment dike crest elevation February through November, Table H5-4 2D limit equilibrium results (Section 01) November, Table H5-5 2D limit equilibrium results (Section 01) August, August 25, 2016 Page H-i

3 TABLE OF CONTENTS (continued) List of Figures Figure H1-1 Stability sections location plan... 2 Figure H4-1 Right abutment model geometry... 6 Figure H5-1 Left abutment model geometry November, Figure H5-2 Left abutment model geometry August, Figure H5-3 2D limit equilibrium results (Section 01) field data ESA FOS tracking (February, 2015 to November, 2015) Figure H5-4 2D limit equilibrium results (Section 01) data ESA FOS tracking (February, 2015 to November, 2015) List of Attachments Attachment H1 Slope Stability Output August 25, 2016 Page H-ii

4 H1 INTRODUCTION This appendix summarizes static slope stability analysis undertaken on the right and left abutments of Fundão Dam for conditions prior to failure on November 5, Analyses were done to compare the drained and undrained static stability of the right and left abutments as one input to understanding why the failure occurred on the left abutment, not on the steeper right abutment. Section AA, one of the instrumentation sections described in Appendix E, was used for the right abutment, while Section 01, also one of the instrumentation sections, was used for the left abutment. Those sections are shown in plan on Figure H 1-1. The geometry of these sections, both the surface of the tailings and the original ground, are taken from the topographic compilation done by the Panel, as described in Appendix B. Key inputs into the stability analysis included tailings stratigraphy, engineering properties of the tailings and pore pressures. Tailings stratigraphy for both sections was based on the work described in Appendix B. Engineering properties of the tailings are based on the field data described in Appendix C and the laboratory data described in Appendix D. The pore pressures are based on the piezometers at the sections themselves, described in Appendix E, augmented with the 3D seepage analyses described in Appendix G. Section H4 presents the analysis undertaken on the right abutment of the Fundão Dam and Section H5 presents the analysis undertaken on the left abutment. August 25, 2016 Page H-1

5 Figure H 1-1 Stability sections location plan August 25, 2016 Page H-2

6 H2 ENGINEERING PROPERTIES OF THE TAILINGS There were a number of stability analyses conducted for the Fundão Dam during the design life of the structure by various parties. Table H 2-1 summarizes the material parameters that have been used in previous stability evaluations by others. Table H 2-1 Type Tailings Foundation Material parameters adopted in pre-failure stability analyses Material Total Unit Weight (kn/m 3 ) Effective Friction Angle (degrees) Effective Cohesion (kpa) Reference Sand VOGBR [64] Slimes Weathered Phyllite University of Ouro Preto [38] DAM [65] and VOGBR [64] The material parameters assumed in this work are listed in Table H 2-2 and Table H 2-3. The material parameters differ from previous assumptions as follows: A total unit weight of 22 kn/m 3 was adopted for all materials based upon the results of the field and laboratory investigations conducted by the Panel (see Appendix C and D). A friction angle of 33 was adopted for the loose sand tailings in the effective stress analyses based upon the results of the direct shear tests (see Appendix D). Undrained shear strength ratios were varied for the loose sand tailings and slimes, see Table H 2-3 and Section H3.2 below. Table H 2-2 Type Tailings Selected effective stress parameters Material Compacted Sand Tailings Unit Weight (kn/m 3 ) Effective Friction Angle (degrees) Effective Cohesion (kpa) Loose Sand Tailings Slimes Foundation Weathered Phyllite August 25, 2016 Page H-3

7 Table H 2-3 Selected undrained strength parameters Material Loose Sand Tailings/Slimes Unit Weight (kn/m 3 ) s u /σ v 22 Varied (Backscarp) 0.22 (Horizontal) 0.14 (Breakout) Description s u /σ v varied to determine the lower-bound of the available undrained yield strength in Section AA (FOS=1.2) Derived from Cone Penetration Tests on blended Sand and Slimes Tailings (Fugro campaign September, 2014 to March, 2015, CPTu-F01 through CPTu-F05 (Appendix C)) Derived from Cone Penetration Tests (Fugro campaign September, 2014 to March, 2015, CPTu-F01 through CPTu-F05 (Appendix C)) Post-liquefaction strength derived from Cone Penetration Tests (Appendix C) Material Loose Sand Tailings/Slimes 1. FOS = Factor of Safety Unit Weight (kn/m 3 ) φ (degrees) Description Peak total stress friction angle (collapse surface) determined from consolidated-undrained triaxial tests on loose sand samples (Appendix D) H3 METHODOLOGY H3.1 Effective Stress Analyses (ESA) Effective stress parameters from Table H 2-2 were used in the effective stress analyses (ESA) to assess the stability of the dam under static conditions. The analysis scenarios included: right abutment for November, 2015; left abutment for November, 2015; left abutment for August, 2014; and left abutment Factor of Safety (FOS) tracking on a monthly basis prior to the failure from February, 2015 to November, H3.2 Undrained Strength Analyses (USA) Undrained strength analyses (USA) were conducted with the dam geometry and pore-water pressure conditions as they existed in August, 2014 and November, These two times were selected to August 25, 2016 Page H-4

8 help understand the drop in factor of safety from August, 2014 to failure in November, The analyses were done using parameters in Table H 2-3. Loose tailings materials above the piezometric surface were modeled with ESA parameters while loose tailings materials located below the piezometric surface were modeled with USA parameters. The following variants of the USA parameters were analyzed: Right abutment undrained strength ratio was varied to determine the lower-bound of the available undrained yield strength (to obtain a FOS of 1.2). The resultant ratio was applied to analyses on the left abutment. Right abutment from an analysis of CPT data (Appendix C) an undrained strength ratio of 0.25 was adopted. Left abutment anisotropic USA properties based on data from cone penetration testing of the blended loose sand and slimes tailings (Appendix C) were used. Left and right abutments a loose sand collapse surface analysis was undertaken. The analysis utilized a friction angle of 16 o based on data determined from consolidated undrained triaxial tests conducted on loose sand samples (Appendix D). Left and right abutments from analysis of CPT data (Appendix C) a post liquefaction undrained strength ratio of 0.07 was adopted. H4 RIGHT ABUTMENT H4.1 Model Setup H4.1.1 Geometry The stability analyses reported for the right abutment were done along Section AA whose location is shown on Figure H 1-1. The idealized stability model from Section AA is shown on Figure H 4-1 with a geometric summary in Table H 4-1. Table H 4-1 Right abutment model geometry November, 2015 Right Abutment (Section AA) Crest Elevation Height (1) Downstream Slope (overall) 1. Height measured from the dam crest to the dam toe. 900 m 71 m 3.2H:1V August 25, 2016 Page H-5

9 Figure H 4-1 Right abutment model geometry H4.1.2 Tailings Stratigraphy The tailings stratigraphy at the right abutment of the Fundão Dam comprises sand tailings in most of the section with mixed sands and slimes below El. 830 m. There are no slimes between El. 830 m and El. 850 m as described in Appendix B. This is a key difference between the right and left abutments. The bedrock comprises weathered phyllite. The slimes below El. 830 m did not affect FOS because there was bedrock between the slimes and the toe on Section AA as shown on Figure H 4-1. The assumed properties of the sand tailings and the phreatic conditions governed the stability. As shown on Figure H 4-1, there is thin compacted shell of sand tailings on the upstream face. This zone was assumed to be 20 m wide and assigned a higher friction angle, 35 versus 33, than the looser beach tailings. H4.1.3 Pore-Water Pressure Conditions The pore-water pressure conditions within the right abutment of the dam were based on piezometers at Section AA (see Appendix E). Also, piezometric conditions were taken directly from the 3D seepage modeling results described in Appendix G. The 3D seepage model was calibrated against the piezometers. H4.2 Results Slope stability analyses were undertaken using SLOPE/W software. The Morgenstern-Price method was used to calculate the Factor of Safety (FOS) for the critical slip surfaces. The results are summarized in Table H 4-2. The stability analysis outputs by Analysis Number in the first column of Table H 4-2 are given in Attachment H1. August 25, 2016 Page H-6

10 Table H 4-2 2D limit equilibrium results (Section AA) November, 2015 Analysis Number H.H1-1A/ H.H1-1B H.H1-2 H.H1-3 H.H1-4A/ H.H1-4B H.H1-5A/ H.H1-5B H.H1-6A/ H.H1-6B 1. WT = Water Table Description ESA Base Case USA below WT USA below WT USA below WT USA below WT with Low Strength Loose Tailings and Slimes below WT USA below WT PWP Conditions Tailings Strength Factor of Safety Assumptions Above WT Below WT Field Data 3D Field Data 33 s u /σ v = N/A 33 s u /σ v = 0.34 N/A s u /σ v = Φ = 16 o s u /σ v = H5 LEFT ABUTMENT H5.1 Model Setup H5.1.1 Geometry The stability analyses reported for the left abutment are located along Section 01 whose location is shown on Figure H 1-1. Stability analyses were done for the conditions that existed in August, 2014 and just prior to failure in November, The stability model for November, 2015 is shown on Figure H 5-1 with key dimensions listed in Table H 5-1. The stability model for August, 2014 is shown on Figure H 5-2 with key dimensions shown in Table H 5-2. Table H 5-1 Left abutment model geometry November, 2015 Left Abutment (Section 01) Crest Elevation Height (1) Downstream Slope (Overall) Setback (Elevation) Setback (Distance) Berm (Distance) 1. Height was measured from the dam crest to the dam toe. 901 m 56 m 6.2H:1V 862 m 160 m 100 m August 25, 2016 Page H-7

11 Figure H 5-1 Left abutment model geometry November, 2015 Table H 5-2 Left abutment model geometry August, 2014 Left Abutment (Section 01) Crest Elevation Height (1) Downstream Slope (Overall) Setback (Elevation) Setback (Distance) 1. Height was measured from the dam crest to the dam toe. 884 m 39 m 7.7H:1V 862 m 160 m Figure H 5-2 Left abutment model geometry August, 2014 An additional analysis set was undertaken to track the stability of the left abutment as the dike was raised through the months prior to failure in November, The left abutment was modeled from February through to November, The assumed dike crest height is summarized in Table H 5-3 by month. More details on the construction sequence are given in Appendix B. Table H 5-3 Left abutment dike crest elevation February through November, 2015 February 2015 May 2015 June 2015 Left Abutment (Section 01) July 2015 August 2015 September 2015 October 2015 November August 25, 2016 Page H-8

12 H5.1.2 Tailings Stratigraphy The stratigraphic model of the left abutment is described in Appendix B. Key to this stratigraphy is that slimes were deposited between El. 830 m and El. 850 m. The assumed continuity of the slimes layers as the downstream toe is approached is also described in Appendix B. For the purposes of stability analyses, slimes material properties were used where slimes could have been present. The slimes areas are in red in the model sections shown above. The sand tailings were divided into two types: loose sand and compacted sand. An approximately 20 m wide compacted sand layer was modeled parallel to the downstream face of the dam. H5.1.3 Pore-Water Pressure Conditions The pore-water pressure conditions within the left abutment of the dam were based on piezometers at Section 01 (see Appendix E) and augmented with the output data from the 3D seepage modeling described in Appendix G. Data was extracted from the seepage model at the dates defined for each analysis listed in Section H3.1. H5.2 Results Slope stability analysis was undertaken using SLOPE/W software (GEO-SLOPE 2012). The Morgenstern-Price method (Morgenstern and Price 1965) was used to calculate the Factor of Safety (FOS) for the critical slip surfaces. The slope stability analysis results are summarized in Table H 5-4 and Table H 5-5 and Figure H 5-3. The stability analysis outputs by Analysis Number in the first column in Table H 5-4 and Table H 5-5 are given in Attachment H1. Table H 5-4 2D limit equilibrium results (Section 01) November, 2015 Analysis Number Description H.H1-7A/ ESA Base Case H.H1-7B H.H1-8 USA below WT H.H1-9 USA below WT H.H1-10A/ USA Heterogeneous H.H1-10B below WT ESA with Low Strength H.H1-11A/ Variant for Loose H.H1-11B Tailings and Slimes below WT H.H1-12A/ ESA above WT / USA H.H1-12B below WT 1. WT = Water Table PWP Conditions Tailings Strength Assumptions Factor of Safety 3D Above WT Below WT Field Data s u /σ v = 0.34 N/A 1.87 Field Data 33 s u /σ v = N/A 33 Backscarp: s u /σ v = 0.30 Horiz. Zone: s u /σ v = 0.22 Breakout Zone: s u /σ v = Φ = 16 o s u /σ v = August 25, 2016 Page H-9

13 Table H 5-5 2D limit equilibrium results (Section 01) August, 2014 Analysis Number H.H1-13A/ H.H1-13B H.H1-14A/ H.H1-14B 1. WT = Water Table Description ESA Base Case USA Heterogeneous below WT PWP Conditions Tailings Strength Assumptions Factor of Safety Above WT Below WT Field Data 3D Backscarp: s u /σ v = 0.30 Horiz. Zone: s u /σ v = 0.22 Breakout Zone: s u /σ v = Factor of Safety Date Figure H 5-3 2D limit equilibrium results (Section 01) field data ESA FOS tracking (February, 2015 to November, 2015) August 25, 2016 Page H-10

14 Factor of Safety Date Figure H 5-4 2D limit equilibrium results (Section 01) data ESA FOS tracking (February, 2015 to November, 2015) August 25, 2016 Page H-11