The influence of mineralogy, chemistry and physical engineering properties on shear strength parameters of the goathill rock pile material




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THE INFLUENCE OF MINERALOGY, CHEMISTRY AND PHYSICAL ENGINEERING PROPERTIES ON SHEAR STRENGTH PARAMETERS OF THE GOATHILL ROCK PILE MATERIAL,

QUESTA MOLYBDENUM MINE, NEW MEXICO


by


Luiza Aline Fernandes Gutierrez


Submitted in partial fulfillment of the requirements for the

Degree of Master of Science in Mineral Engineering

with Specialization in Geotechnical Engineering


New Mexico Institute of Mining and Technology

Department of Mineral Engineering


Socorro, New Mexico

March, 2006


ABSTRACT


This thesis develops information regarding the engineering characteristics (direct shear, Atterburg Limits, particle size) of the Goathill North (GHN) rock pile material at the Questa molybdenum mine in New Mexico and examines correlations with chemistry, mineralogy, particle size distribution and weathering indexes (SWI – simple weathering index, WPI – weathering potential index, MI – Muira index). Results of peak internal friction angle () ranged from 40º to 47º and residual friction angle varied between 37º and 41º. These high values of peak internal and residual friction angle are attributed to grain shape (subangular to very angular) and relative density of the test specimens. Correlations of  from GHN samples with chemistry, and mineralogy are shown to be weak or absent. Correlation of f with lithology was not observed. Negative correlations f were observed for %Fines, liquid limit, plasticity index, LOI (lost of ignition), SWI, WPI, and MI. The f decreased as these parameters increased.

Direct shear tests were performed using a 2-inch square shear box and air-dried samples with a maximum particle size of 3.36 mm (U.S standard sieve No. 6). A displacement rate of 0.5 mm/min (0.02 in/min), and normal stress varying from 159 to 800 kPa (23 to 116 psi) were adopted for all the tests. These tests were conducted on disturbed samples. The  values determined from these tests should not be used for slope stability considerations.

GHN rock pile samples were classified according to the United Soil Classification System (USCS as poorly- to well-graded gravel with fines and sand). The percent of fines (silt + clay size) and percent of clay varied from 3 to 19 and 0.3 to 6, respectively. Most of the fines were identified as CL-group (inorganic clay with low swell potential).


ACKNOWLEDGEMENTS


Support for this research was provided by the Molycorp corporationCorporation. in the form of a Research Assistantship, by the WAAIME (The Woman’s Auxiliary to the American Institute of Mining, Metallurgical, and Petroleum Engineers), and by the New Mexico Bureau of Geology and Mineral Resources. I gratefully acknowledge this support.

I would like to express my sincere appreciation to Dr. McLemore and Dr. Aimone-Martin for providing guidance, insight, and support throughout the course of this research. Appreciation is also extended to Dr. Mojtabai who is on the thesis advisory committee and who encouraged me to do my thesis research at New Mexico Tech.

I would like to thank many people who provided insight and suggestions on this research: Dr. Virgil Lueth, Kelly Donahue, Erin Phillips, Fernando Junqueira, Dr. Fakhimi, Dr. Gundiler, Prof. Ward Wilson, Mike Smith, and other members of the Molycorp project weathering study. I especially want to thank Rick Lynn, Lynne Kurilovitch, Farid Sariosseri, Pedro Martin Moreno, Erico Tabosa, Vanessa Viterbo, Heather Shannon, Claudia Duarte, Alexandre, Igor, Armando and Jario for assistance with the laboratory testing program. Last but not least, I would like to thank Remke van Dam for all his support by reviewing my thesis thousand times, and literally going through this experience with me.

This thesis is dedicated to my parents Marta Gutierrez and Zenon Gutierrez, my sisters Norma Gutierrez Ventura, Kelly Gutierrez Alves, Adela Gutierrez Branco, and to my lovely husband.



TABLE OF CONTENT


List of Tables………………………………………………………………...…………...iv


List of Figures…………………………………………………………………...………...v

1. INTRODUCTION 1

2. REVIEW OF STUDIES AT GOATHILL NORTH ROCK PILE 9

3. LITERATURE REVIEW OF CONCEPTS RELATED TO THIS RESEARCH 20

4. METHODOLOGY 38

5. RESULTS AND DISCUSSIONS 55

6. CONCLUSIONS AND RECOMENDATIONS 79

REFERENCES 81

APPENDIX A – SAMPLE LOCATION 86

APPENDIX B – GRAIN SIZE DISTRIBUTION curves and summary table 88

APPENDIX C – DIRECT SHEAR STRESS DIAGRAMS 118

APPENDIX D – MOHR COULOMB DIAGRAMS 156

APPENDIX E – Description of geologic units, SUMMARY OF GEOLOGICAL AND GEOTECHNICAL DATA USED FOR CORRELATIONS 176

APPENDIX F – STANDARD OPERATING PROCEDURES FOR petrographic analyses 186

1.0 PURPOSE AND SCOPE 187

2.0 RESPONSIBILITIES AND QUALIFICATIONS 187

3.0 DATA QUALITY OBJECTIVES 188

4.0 RELATED STANDARD OPERATING PROCEDURES 188

5.0 EQUIPMENT LIST 188

6.0 PROCEDURES 189

APPENDIX F – TERMINOLOGY 191



LIST OF TABLES

Table 2.1. Summary of geotechnical properties at GHN rock pile 10

Table 2.2. Summary of friction angles of Molycorp mine rock piles and the “weak zone” at GHN and their gradation results. 10

Table 3.3. This table shows the weathering field survey used to characterize the weathering sequence in the gneiss (after Calcaterra et al., 1998). Weathering grades I, II, and VI were unavailable to survey. 32

Table 3.4. This table shows the main engineering-geological features of weathered horizons near Acri (after Calcaterra et al., 1998). 32

Table 3.5. This table shows the shear strength parameters of sedimentary residual soil with weathering grades varying from III to V. The lower end is for less weathered material and the higher end is for more weathered material 34

Table 3.6. This table shows the grain size distribution of rock piles from around the world. 35

Table 3.7. This table shows a summary of mine rock friction angle and cohesion data from around the world. 37

Table 4.8. This table shows the minimum specimen size required for particle size analysis according with the diameter of the largest particle (U.S. Army Corps of Engineers, 1970). 41

Table 4.9. This table is a summary of results of the 3 methods for particle size analyses. 43

Table 4.10. Summary of the results from direct shear tests using different maximum particle size and shear box size. 50

Table 4.11. This table summarizes the results from direct shear test using different maximum particle sizes. 52

Table 4.12. This table is a summary of direct shear test results for samples at dry and moist states. 52

Table 5.13. This table is a summary of particle size analyses for samples from GHN indexed by geologic unit. 56

Table 5.14. This table is a summary of Atterberg limit results for samples from GHN indexed by geologic unit. 57

Table 5.15. This table is a summary of moisture content and paste pH results for samples from GHN indexed by geologic unit. 57

Table 5.16. This summary table shows direct shear test results of samples from GHN indexed by geologic unit. 60



LIST OF FIGURES



Figure 1.1. This image shows the location map of Molycorp’s Questa molybdenum mine, which is in northern Taos County, New Mexico. 4

Figure 1.2. This image shows an aerial photo of Questa mine showing the nine rock piles adjacent to the open pit. 4

Figure 1.3. This image shows Goathill North rock pile before re-grading, looking east. Solid line indicates approximate location of trenches completed in summer-fall 2004; dashed line indicates the boundary between the stable and unstable portions of the rock pile (after McLemore et al., 2006). 6

Figure 1.4. This image shows one of the trenches (LFG-003) excavated on the stable portion of the Goathill North rock pile during the re-grading. 6

Figure 2.5. This graph shows the friction angle of GHN mine rock based on triaxial test results for samples from Sugar Shack rock piles that have gradations lying towards the finer range of materials sampled at GHN rock pile (from Norwest Corporation, 2004). 12

Figure 2.6. This graph shows the grain size distributions from triaxial samples from Sugar Shack rock piles and typical Goathill North mine rock (from Norwest Corporation, 2004). 13

Figure 2.7. This graph shows friction angle versus confining stress showing a decrease in friction angle as confining stress increases (from Norwest Corporation, 2004). 14

Figure 2.8. This figure shows an example of a geologic map like those created for each trench at GHN rock pile. Geologic map of trench LFG-009 (from McLemore et al., 2005). 15

Figure 2.9. This figure shows a geologic cross section of bench 9, trench LFG-006 showing the identified subsurface units. See Appendix E for description of the subsurface units. 15

Figure 2.10. This graph shows a plot of QSP hydrothermal alteration intensity (defined by the percentage of hydrothermal alteration minerals that have replaced primary minerals) across bench 9, trench LFG-006 (from McLemore et al., 2005). Refer to Figure 2.5. for geologic units. 17

Figure 2.11. This graph shows a plot of authigenic gypsum across bench 9, trench LFG-006 (from McLemore et al., 2005). Refer to Figure 2.5. for geologic units. 18

Figure 2.12. This graph shows the results of paste pH and NAG pH across bench 9, trench LFG-006 (after Tachie-Menson, 2005). 18

Figure 3.13. This figure shows examples of sphericity and roundness charts (a) from (Cho et al., 2004) and (b) from AGI (American Geological Institute) data sheet 18.1 comparison chart for estimating roundness and sphericity, by Maurice C. Powers, copyright 1982. These charts were used for this project. 23

Figure 3.14. This graph shows the effect of particle shape on internal friction angle for sand ( from Cho et al., 2004). Open circles and closed circles are for sand with sphericity higher than 0.7 and sphericity lower than 0.7, respectively. 24

Figure 3.15. This graph shows the correlations between the effective friction angle and the relative density for different soil types (from Holtz and Kovacs, 2003). ML: Silt, SM: Silty sand, SP: Poorly graded sand, SW: Well-graded sand, GP: Poorly graded gravel, GW: Well-graded gravel. 25

Figure 3.16. This graph shows the variation of peak internal friction angle with effective normal stress for direct shear tests on standard Ottawa sand (from Das, 1983). 26

Figure 3.17. This image shows the progressive physical break up boulders by the transformation of anhydrite to gypsum common at the Questa mine site. 29

Figure 3.18. This image shows evidence of chemical weathering process (oxidation of iron ) at Questa mine site. 30

Figure 3.19. This graph shows the correlation of weathering grade with dry density and porosity. High/mean/low values are plotted for each grade (from Thuro and Scholz, 2003). 33

Figure 4.20. This figure shows a generalized cross section of GHN with the location of the samples analyzed for this thesis project. 39

Figure 4.21. This graph shows a comparison of the grain size distribution for the three different approaches tried for the estimation of particle size distribution using sample GHN-LFG-0003. 43

Figure 4.22. This image shows the manual direct shear equipment used, with views showing the 2-inch square shear box, displacement dials and load frame. 46

Figure 4.23. This graph shows an example of a shear stress versus shear strain plot. The test was conducted at four different normal stresses 159, 356, 562, and 754 kPa. The arrows indicate the peak shear strength and the residual shear strength. 47

Figure 4.24. This image show an example of a shear diagram showing the best fit line for the peak internal friction angle and the residual internal friction angle. 47

Figure 4.25. Shear box size effects on direct shear test (a) for sample GHN-KMD-0056 with dmax = 4.76 mm, (b) GHN-LFG-0003 with dmax = 4.76 mm. 49

Figure 4.26. This figure shows the results of the effect of particle size on direct shear tests using a 2-inch shear box for (a) sample GHN-KMD-0056 with maximum particle sizes (dmax) of 9.52, 4.76, and 3.36 mm and (b) sample GHN-LFG-0003 with dmax of 4.76 and 3.36 mm. 51

Figure 4.27. This figure compares the results of the influence of moisture on the direct shear test for samples (a) GHN-KMD-0017 and (b) GHN-KMD-0018. 53

Figure 5.28. This graph shows the range of grain size distribution for samples from the GHN rock pile. 58

Figure 5.29. This graph shows the distribution of samples from the GHN rock pile on the plasticity chart. 58

Figure 5.30. This graph shows direct shear test results for a dry sample versus a sample with gravimetric moisture content of 12.4%. 61

Figure 5.31. This graph shows the Mohr-Coulomb diagram for sample GHN-KMD-0014. Data points generated by both the automatic Ele and the manual NMT shear box machines are included. 62

Figure 5.32. This graph shows the Mohr-Coulomb diagram for sample GHN-KMD-0017. Data points generated by both the automatic Ele and the manual NMT shear box machines are included. 63

Figure 5.33. This graph shows the Mohr-Coulomb diagram for sample GHN-KMD-0027. Data points generated by both the automatic Ele and the manual NMT shear box machines are included. 64

Figure 5.34. This diagram shows the stratigraphic positions of the geologic units for bench 9 (Trench LFG-006). 65

Figure 5.35. These graphs show cross plots of internal friction angle versus paste pH. Plot on the left side includes samples only from bench 9. Plot on the right side includes all GHN samples tested in this study. 65

Figure 5.36. These graphs show cross plots of internal friction angle versus NAGpH. Plot on the left side includes samples only from bench 9. Plot on the right side includes all GHN samples tested in this study. 65

Figure 5.37. These graphs show cross plots of internal friction angle versus percentage of fines. Plot on the left side includes samples only from bench 9. Plot on the right side includes all GHN samples tested in this study. 66

Figure 5.38. These graphs show cross plots of internal friction angle versus plasticity index. Plot on the left side includes samples only from bench 9. Plot on the right side includes all GHN samples tested in this study. 66

Figure 5.39. These graphs show cross plots of internal friction angle versus liquid limit. Plot on the left side includes samples only from bench 9. Plot on the right side includes all GHN samples tested in this study. 67

Figure 5.40. These graphs show cross plots of internal friction angle versus percentage of Amalia Tuff. Plot on the left side includes samples only from bench 9. Plot on the right side includes all GHN samples tested in this study. 67

Figure 5.41. These graphs show cross plots of internal friction angle versus percentage of Andesite. Plot on the left side includes samples only from bench 9. Plot on the right side includes all GHN samples tested in this study. 68

Figure 5.42. These graphs show cross plots of internal friction angle versus quartz-sericite-pyrite (QSP) alteration. Plot on the left side includes samples only from bench 9. Plot on the right side includes all GHN samples tested in this study. 69

Figure 5.43. These graphs show cross plots of internal friction angle versus propyllitic alteration. Plot on the left side includes samples only from bench 9. Plot on the right side includes all GHN samples tested in this study. 69

Figure 5.44. These graphs show cross plots of internal friction angle versus LOI (lost of ignition). Plot on the left side includes samples only from bench 9. Plot on the right side includes all GHN samples tested in this study. 70

Figure 5.45. These graphs show cross plots of internal friction angle versus percentage of epidote. Plot on the left side includes samples only from bench 9. Plot on the right side includes all GHN samples tested in this study. 70

Figure 5.46. These graphs show cross plots of internal friction angle versus percentage of illite. Plot on the left side includes samples only from bench 9. Plot on the right side includes all GHN samples tested in this study. 71

Figure 5.47. These graphs show cross plots of internal friction angle versus percentage of MgO. Plot on the left side includes samples only from bench 9. Plot on the right side includes all GHN samples tested in this study. 71

Figure 5.48. These graphs show cross plots of internal friction angle versus percentage of CaO. Plot on the left side includes samples only from bench 9. Plot on the right side includes all GHN samples tested in this study. 72

Figure 5.49. These graphs show cross plots of internal friction angle versus percentage of Al2O3. Plot on the left side includes samples only from bench 9. Plot on the right side includes all GHN samples tested in this study. 72

Figure 5.50. This figure shows plots of the WPI and MI weathering indexes with distance across bench 9, trench LFG-006. The outer oxidized edge is at Hfrom=0 and the inner zone of the bench is at Hfrom=105. Weathering increases towards the left. 75

Figure 5.51. This figure shows plots of WPI and MI vs. paste pH for all GHN samples. WPI and MI are explained in Figure 5.23. 76

Figure 5.52. This figure shows a cross plot of Friction angle versus simple weathering index (SWI) for bench 9 samples, trench LFG-006. Weathering intensity increases towards the left. 77

Figure 5.53. This figure shows a cross plot of Friction angle versus weathering potential index (WPI) for samples from bench 9, trench LFG-006. The weathering intensity increases towards the left. 78

Figure 5.54. This figure shows a cross plot of Friction angle versus Miura Index (MI) for samples from bench 9, trench LFG-006. The weathering intensity increases towards the left. 78



This thesis is accepted on behalf of the

Faculty of the Institute by the following committee:


_________________________________________________________

Research Advisor


__________________________________________________________

Academic Advisor


__________________________________________________________

Committee Member


___________________________________________________________

Date


I release this document to the New Mexico Institute of Mining and Technology.


_____________________________________________________________

Student's Signature Date


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