Slope Stability Model of the Questa Rock Pile Phase 1

Transcription

1 2 Prceedings Tailings and Mine Waste 2011 Slpe Stability Mdel f the Questa Rck Pile Phase 1 Murray Fredlund SilVisin Systems Ltd., Saskatn, Canada Rbert Thde SilVisin Systems Ltd., Saskatn, Canada Dirk van Zyl University f British Clumbia., Vancuver, Canada Abstract The Gat Hill mine rck piles at the Questa Mine in New Mexic have experienced natural debris flw and deep seated mvement requiring mitigatin measures be designed and implemented. This paper describes the slpe stability mdeling cmpleted using the SVSlpe finite element slpe stability sftware, as well as the methd with which material parameters were determined. The relative impact f parameters such as slip surface depth, frictin angle, chesin and Pissn s rati were als examined. It was fund that shallw failures will mst likely be caused by changes in the matric suctin due t rainfall events. Deeper slip surfaces required a crrespnding increase in chesin. The angle f internal frictin f the layers is deemed t be the determining factr in any slpe stability analysis. Intrductin The mine rck piles at the Questa mine extend t a height f 488 meters with slpe angles f apprximately 36. Natural failures within alteratin scars that prduce debris flws have been bserved in the vicinity f the mine rck piles. Furthermre, deep-seated mvement has ccurred in the Gathill Nrth (GHN) rck pile requiring excavatin and earthwrks t stabilize the slpes. This paper summarizes the numerical mdeling related t the stability f the Gathill Nrth rck pile. The questin t be addressed is whether further failures will ccur in the future within the mine rck piles as weathering f the mine rck prgresses with time r under variable cnditins assciated with climatic events. Slpe Stability Mdeling The scpe f the slpe stability study invlves a sensitivity analysis fr the Gathill Nrth (GHN) rck pile. The influence f a change in sil shear strength parameters as influenced by weathering factrs will be determined. The study will invlve the applicatin f finite-element based stability analysis as well as the applicatin f limit equilibrium methds. The SVSlpe slpe stability sftware is used in rder t study the influence f weak layers n the stability f the slpe. The pre-water pressure distributins were cmputed in a separate seepage study. These are used as input int the slpe stability analysis sftware t determine the influence f seepage scenaris n the stability mdel. A particular fcus f the study is the examinatin f ptential thin weak layers and their impact n the stability analysis. The weathering aspects f these weak layers and the resulting influence n stability will be determined thrugh a sensitivity analysis f the slpe stability mdel. Sme sil layers may increase with strength in time and thers may weaken with time. The influence f weathering n chesin and the angle f internal frictin will be examined in detail. Assumed variatins f these parameters are then used in the determinatin f resulting influences n verall stability.

2 Prceedings Tailings and Mine Waste Mdeling Apprach The cmbinatin f seepage and slpe stability mdels will be first used t quantify mdel inputs. The effects f each mdel input n the resulting factr f safety are then examined. Given the current large number f mdel input parameters, it is f utmst imprtance t determine the relative influence f each f these input parameters n the factr f safety calculatin. There is a cmplicatin that arises frm a study in which s many input parameters are varying and it invlves the determinatin f the relative influence f each parameter. It is als preferable t determine the relative influence f each mdel input parameter in a prbabilistic manner. The Alternate Pint Estimate Methd (APEM) was utilized t quantify variability in this study. The purpse f the numerical mdeling f Phase I f this prject was t identify parameters sensitive t the slpe stability numerical mdeling as well as the seepage numerical mdeling study. Subsequent phases f the prject will address the variatin f mdel parameters in a mre cmprehensive manner. The fcus f the Phase I numerical mdeling prgram is n the valuatin f shallw r intermediate class slip surfaces. Deep failure surfaces may be cnsidered at a later time. Sil Parameters A cnsiderable amunt f field and labratry infrmatin has been cllected. It was prpsed that the best apprach t the numerical mdeling study was t take a snap sht f the data at a particular time and use this as a starting pint fr the numerical mdeling prgram. Subsequent field and labratry results can allw further numerical mdel refinement in the future. Data was btained frm the University British Clumbia Team, UBC, r extracted frm the New Mexic Tech, NMT, database n Nvember 24, Samples cllected by NMT were btained frm the lcatins in the trenches created during destructin f the waste rck pile. Slpe Stability Results Phase I f the numerical mdeling slpe stability study started with the use f a selected generic gemetry. Subsequent t the generic slpe study, a series f scenaris were examined based n a particular tw-dimensinal slice taken thrugh the Gat Hill Nrth gemetry. The study invlves an analysis f ptential intermediate and shallw failure surface mechanisms. An infinite slpe analysis prvided an analysis f shallw slides. The effect f Pissn s rati n the depth f the slip surface generated using SVSlpe was als examined. A specific slice thrugh the GHN pre-destructin gemetry was selected fr further analysis. Analyses perfrmed include the evaluatin f a hmgeneus slpe, the evaluatin f a layered slpe, and the analysis f a mre rigrus versin f the tw-dimensinal slice thrugh the rubble and clluvium layers. Layered Analysis with Varying Pissn s Ratis The influence f Pissn s Rati n the lcatin f the slip surface and the value f the factr f safety prmpted further study regarding pssible limits f variatin. A mdel was therefre selected which had layered stratigraphy in a reccurring weak and strng sequence. The strng layers were then left unchanged and the sil parameters f the weak layers were changed. Three scenaris were cnsidered in which the angle f internal frictin (phi) was varied between degrees. The Pissn s Rati values fr the three scenaris are 0.32, 0.42 and The angle f internal frictin (phi) was changed fr three Pissn s Rati values. The results are presented in Figure

3 4 Prceedings Tailings and Mine Waste The effect f varying the angle f internal frictin can be seen t be essentially linear in each scenari. A cnstant slpe angle f 38 degrees was cnsidered fr all layered analysis scenaris Pissns Rati: 0.32 Pissns Rati: 0.42 Pissns Rati: 0.48 Factr f safety Frictin Angle (degrees) Figure 1: Effect f Changing the Angle f internal frictin at three Pissn's Ratis The lcatin f the slip surface des nt seem t change significantly when varying the angle f internal frictin. The results presented in Figure 2 illustrate this pint. It can als be seen in Figure 3 that the lcatin f the critical slip surface des nt vary significantly. The nly difference between Figure 2 and Figure 3 is that the Pissn s Rati is changed frm 0.32 t It shuld als be nted that when the angle f internal frictin f the weak layer is reduced t a value less than the angle f internal frictin f the cntiguus layer the slip surface tends t fllw the weak layer. Slip surface pints abve the grund are a numerical artifact f the numerical methd and can be ignred Grund Surface w s w s w s w s w s Weak Layer: Phi=40 ; c=0 ; PR=0.32 ; FS=1.58 Weak Layer: Phi=30 ; c=0 ; PR=0.32 ; FS=1.33 Weak Layer: Phi=20 ; c=0 ; PR=0.32 ; FS=1.09 Y (m) X (m)

4 Prceedings Tailings and Mine Waste Figure 2: Effect f varying the angle f internal frictin when Pissn's Rati is equal t Slpe angle is 38 degrees. 80 Weak Layer: Phi=40 ; c=0 ; PR=0.42 ; FS= Weak Layer: Phi=30 ; c=0 ; PR=0.42 ; FS= Grund Surface w s w s w s w s w s Weak Layer: Phi=20 ; c=0 ; PR=0.42 ; FS=0.85 Y (m) Figure 3: Effect f varying the angle f internal frictin when Pissn's Rati is equal t Slpe angle is 38 degrees. X (m) Figure 4 presents the results f varying the angle f internal frictin with a cnstant Pissn s Rati f 0.48 and a chesin f 10 kpa. The variatin in the lcatin f the slip surface can be seen t be minimal but the value f the factr f safety varies between 0.97 and Grund Surface w s w s w s w s w s Weak Layer: Phi=40 ; c=10 ; PR=0.48 ; FS=1.32 Weak Layer: Phi=30 ; c=10 ; PR=0.48 ; FS=1.16 Weak Layer: Phi=20 ; c=10 ; PR=0.48 ; FS=0.97 Y (m) Figure 4: Effect f varying the angle f internal frictin when Pissn's Rati is equal t 0.48 and chesin is equal t 10 kpa. Slpe angle is 38 degrees. X (m)

5 6 Prceedings Tailings and Mine Waste 2011 The preceding figures have illustrated the changes in factr f safety that can be assciated with varying the angle f internal frictin. Figure 5 presents the results f varying the chesin in weak layers. The angle f internal frictin f the weak layers was made t be 20 fr the material with a Pissn s Rati f 0.42 and 40 fr the material with a Pissn s Rati f The figure shws that there is a muting effect n the chesin f a sil as the Pissn s Rati is lwered. The relative influence f increases in chesin can be seen by the slpes f each line. The break in the slpe represents the cnditin where the final slip surface mves frm ne weak layer t the adjacent weak layer Pissns Rati: 0.42 Pissns Rati: 0.48 Factr f safety Chesin (kpa) Figure 5: Variatin in factr f safety fr weak layer chesin One f the bservatins that can be made is that a Pissn s Rati less than r equal t 0.44 will mean that chesin will have minimal effect n the cmputed factr f safety fr the slpe. As Pissn s Rati decreases the effect f the chesin n the cmputed factr f safety is minimized (i.e., when using SVSlpe). In Figure 6 and Figure 7 the lcatin f the critical slip surface can be bserved as chesin is varied. It can be seen that the lcatin f the slip surface generally mves frm ne weak layer t anther Grund Surface w s w s w s w s w s Weak Layer: Phi=20 ; c=0 ; PR=0.42 ; FS=0.85 Weak Layer: Phi=20 ; c=5 ; PR=0.42 ; FS=0.91 Weak Layer: Phi=20 ; c=10 ; PR=0.42 ; FS=0.99 Weak Layer: Phi=20 ; c=15 ; PR=0.42 ; FS=1.04 Weak Layer: Phi=20 ; c=20 ; PR=0.42 ; FS=1.10 Y (m) X (m)

6 Prceedings Tailings and Mine Waste Figure 6: Variatin in factr f safety and the lcatin f the critical slip surface fr weak layers with varying chesin and an angle f internal frictin = 20 degrees. Slpe angle is 38 degrees Grund Surface w s w s w s w s w s Weak Layer: Phi=40 ; c=0 ; PR=0.48 ; FS=1.00 Weak Layer: Phi=40 ; c=2.5 ; PR=0.48 ; FS=1.167 Weak Layer: Phi=40 ; c=5 ; PR=0.48 ; FS=1.309 Weak Layer: Phi=40 ; c=10 ; PR=0.48 ; FS=1.32 Y (m) Figure 7: Variatin in the cmputed factr f safety and the lcatin f the critical slip surface fr weak layers with varying chesin and an angle f internal frictin = 40 degrees. Slpe angle is 38 degrees. GHN 2D Crss-Sectin A hmgeneus 2-D crss-sectin has been selected thrugh the stable prtin f the GatHill Nrth site. The prximity f the stable and unstable prtins f the GatHill Nrth site may be seen in Figure 8. X (m) Figure 8: Gathill Nrth rck pile befre re-grading and lking east. Slid line indicates apprximate lcatin f trenches cmpleted in summer-fall 2004; dashed line indicates the bundary between the stable and unstable prtins f the rck pile (after McLemre et al., 2006)

7 8 Prceedings Tailings and Mine Waste 2011 The lcatin f the crss-sectin was selected because f its prximity t the data btained in the field trenching prgram. The crss-sectin went apprximately thrugh the middle f the field trenches as shwn in Figure 6. The crss-sectin prceeds dwn t the te f the GatHill Nrth site. The data cllected in the field prgram will be applied t this crss-sectin. Figure 9: Lcatin f the selected 2-D crss sectin n the GatHill Nrth site The 2-D crss-sectin was determined by persnnel frm New Mexic Tech by interplating intersectin pints n tplgy maps. The upper surface was determined using a 2003 tplgy map extracted frm an AutCAD file prvided by Mlycrp. The lwer surface was interpreted frm a 1962 AutCAD tplgy file. It is assumed that the 1962 file represents the tplgy f the site prir t any dumping f waste rck. Fr the hmgeneus slpe it is assumed that the bttm surface represents the interface with the impermeable bedrck material. It is als assumed fr the hmgeneus mdel that the slip surface cannt be lwer than the bttm surface. The dimensins f the selected 2-D crss-sectin can be seen in Figure 10. The slpe angle f the surface f the waste rck at the site is apprximately 38. Figure 10: cnditin. Crss-sectin extracted frm the GatHill Nrth site representing pre-destructin

8 Prceedings Tailings and Mine Waste GHN Intermediate SVSlpe Statistical Analysis A tw layer numerical mdel was generated in rder t study the effects f a ptential weak layer thrugh the rubble r clluvium znes. The traffic zne was als ignred. A statistical analysis was perfrmed fr the factr f safety based n data in Table 1. The results f the slpe stability analyses are shwn in Figure 11 and summarized in Table 1 and the fllwing bservatins can be made: The angle f internal frictin f the rubble zne has by far the mst significant influence fllwed clsely by the angle f internal frictin f the waste rck zne. The influence f pre-water pressures and chesin culd nt be prperly quantified as the variatinal distributin fr this data was nt available. Given the measured distributin f phi angles f the clluvium zne by Nrwest it appears statistically reasnable that a failure culd be triggered by an assumed weakness in the lwer znes. Table 1: Sil parameters fr intermediate SVSlpe analysis. Rubble zne frictin angles are taken frm the presented Nrwest weak clluvium layer. Zne Parameter Mean Standard Deviatin Surce WR1 Phi Nrwest WR1 Chesin Assumed WR1 PWP (kpa) NMT tensimeter data WR1 Unit Weight (kn/m^3) NMT data Rubble Phi Nrwest Rubble Chesin Assumed Rubble PWP (kpa) Assumed Rubble Unit Weight (kn/m^3) NMT data Figure 11: Crss-sectin extracted frm the GatHill Nrth site representing pre-destructin

9 10 Prceedings Tailings and Mine Waste 2011 Phi (Rubble) Phi (WR1) Unit weight (WR1) Chesin (WR1) Chesin (Rubble) Pre-water pressure (Rubble) Unit weight (Rubble) Pre-water pressure (WR1) Factr f Safety, Fs Figure 12: Trnad diagram fr the factr f safety fr the slpe with tw different layers fr waste rck (90th percentile value) Cnclusins The fllwing cnclusins can be made based n the analyses perfrmed in the weathering study. The cnclusins will be presented categrized accrding t the material remved. Generic Slpe The cnclusins drawn frm the analysis f a generic slpe are presented under the shallw and intermediate sectins. Intermediate Chesin is needed t cause a slip surface t g deep in a hmgeneus slpe. Frm this analysis it can be estimated that = 30 and a chesin f apprximately less than 4 kpa wuld be required t prduce failure in a hmgeneus slpe. The lcatin f the assumed slip surface influences the resulting factr f safety in a significant way. Fr a hmgeneus slpe with a chesin f 10 kpa and an angle f internal frictin f 34 degrees, it is impssible t find a slip surface with a factr f safety belw 1.0. The depth f the slip surface and the resulting factr f safety are influenced by Pissn s Rati. A Pissn s Rati f 0.48 is mst cnsistent with a traditinal limit equilibrium analysis. In a weak/strng layered analysis varying the Pissn s Rati between 0.42 and 0.48 requires an angle f internal frictin between 20 t 30 in the weak layer in rder t achieve failure cnditins. In a weak/strng layered analysis varying the angle f internal frictin f the weak layer will nt affect slip surface lcatin. Varying the chesin f the weak layer will affect the slip surface lcatin and cause the slip surface t skip between adjacent weak layers.

10 Prceedings Tailings and Mine Waste Gat Hill Nrth The fllwing cnclusins may be derived frm the analysis f the Gat Hill Nrth gemetry scenaris. Shallw In a limit equilibrium analysis f a hmgeneus slpe the factr f safety will drp belw 1.0 with a = 36 and a chesin less than 1 kpa. Additin f 5 kpa f chesin will result in a lwering f = 33 required t achieve stability. Based n the statistical analysis f a shallw failure mde the fllwing cnclusins can be drawn: The angle f internal frictin has the mst significant influence n the factr f safety. The influence f unit weight n the cmputed factr f safety appears negligible. The influence f chesin n the factr f safety is f imprtance and needs t be further studied. Realistic variance f suctins due t climatic events needs t be determined. Given the present field-measured sil prperty statistical distributins it appears unlikely that shallw failure cnditins can be achieved if the material is assumed t be hmgeneus. Intermediate Based n the current analysis it has been shwn that a weak layer with an angle f internal frictin less than 30 and a chesin f zer may cause failure cnditins. The statistical analysis prvides further insight int the ptential behaviur f the system as nted by the fllwing cnclusins: References The angle f internal frictin f the rubble zne has by far the mst significant influence fllwed clsely by the angle f internal frictin f the waste rck zne. The influence f pre-water pressures and chesin culd nt be prperly quantified as the variatinal distributin fr this data was nt available. Given the measured distributin f phi angles f the rubble zne by Nrwest it appears statistically reasnable that a failure culd be triggered by weakness in the rubble zne. Duncan, J. M.,Wright, S. G., 2005, Sil Strength and Slpe Stability, New Yrk : Jhn Wiley & Sns, Inc. Fredlund, D.G., Rahardj, H., 1993, Sil Mechanics fr Unsaturated Sils, New Yrk : Jhn Wiley & Sns, Inc. Gutierrez, L., 2006, The Influence Of Mineralgy, Chemistry And Physical Engineering Parameters On Shear Strength Parameters Of The Gathill Rck Pile Material, Questa Mlybdenum Mine, Thesis M.Sc., New Mexic Institute f Mining and Technlgy, Department f Mineral Engineering, Scrr, NM Nrwest Crpratin, 2004, Gathill Nrth Slide Investigatin, Evaluatin and Mitigatin Reprt, Vlume 1 Main Reprt, Submitted t Mlycrp, January Nrwest Crpratin, 2005, Questa Radside Rckpiles: 2005 Operatinal Getechnical Stability Evaluatin, Final Reprt, Submitted t Mlycrp, December 23

11 12 Prceedings Tailings and Mine Waste 2011 SilVisin Systems Ltd, SVSlpe User s Manual finite element slpe stability analysis, Versin 1 (sftware package). Saskatn, Saskatchewan, Canada: SilVisin Systems Ltd. SilVisin Systems Ltd, SVFlux User s manual finite element seepage analysis, Versin 5 (sftware package). Saskatn, Saskatchewan, Canada: SilVisin Systems Ltd. SilVisin Systems Ltd, SVSlid User s manual finite element stress/defrmatin analysis, Versin 4 (sftware package). Saskatn, Saskatchewan, Canada: SilVisin Systems Ltd. Silvisin Systems Ltd, SVSlpe User s manual limit equilibrium slpe stability analysis, SilVisin Systems Ltd. Saskatn, Saskatchewan, Canada

12 Prceedings Tailings and Mine Waste T H E U N I V E R S I T Y O F B R I T I S H C O L U M B I A Science & Engineering, UBC Library Irving K. Barber Learning Centre 1961 East Mall, Rm 414 Vancuver, BC Canada V6T 1Z1 Phne Authrs are required t sign the shrt licence belw which allws UBC t depsit the papers in the repsitry. Fax Appendix 1: License that every authr has t sign: I authrize the depsit f the item(s) listed belw int circle. I have read and agree t circles Nn- Exclusive Distributin License as presented n: This license cvers the fllwing item(s): Tailing and Mine Waste 2011 papers in the UBC Institutinal Repsitry circle Authr Name(s): Title(s): Signature f Authr(s):