Research Article Numerical Analyses of Earthquake Induced …ltu.diva-portal.org/smash/get/diva2:1074709/FULLTEXT01.pdf · 2017-02-16 · Research Article Numerical Analyses of Earthquake

Research ArticleNumerical Analyses of Earthquake Induced Liquefaction andDeformation Behaviour of an Upstream Tailings Dam

Muhammad Auchar Zardari1 Hans Mattsson2 Sven Knutsson2

Muhammad Shehzad Khalid3 Maria V S Ask2 and Bjoumlrn Lund4

1Department of Civil Engineering Quaid-e-Awam University of Engineering Science and Technology Nawabshah Sindh Pakistan2Department of Civil Environmental and Natural Resources Engineering Lulea University of Technology 97187 Lulea Sweden3Department of Urban Management Kyoto University Kyoto Japan4Department of Earth Sciences Uppsala University Uppsala Sweden

Correspondence should be addressed to Muhammad Auchar Zardari muhammadaucharquestedupk

Received 13 October 2016 Accepted 10 January 2017 Published 15 February 2017

Academic Editor Rui Wang

Copyright copy 2017 Muhammad Auchar Zardari et al This is an open access article distributed under the Creative CommonsAttribution License which permits unrestricted use distribution and reproduction in any medium provided the original work isproperly cited

Much of the seismic activity of northern Sweden consists of micro-earthquakes occurring near postglacial faults However largermagnitude earthquakes do occur in Sweden and earthquake statistics indicate that amagnitude 5 event is likely to occur once everycentury This paper presents dynamic analyses of the effects of larger earthquakes on an upstream tailings dam at the Aitik coppermine in northern Sweden The analyses were performed to evaluate the potential for liquefaction and to assess stability of the damunder two specific earthquakes a commonly occurring magnitude 36 event and a more extreme earthquake of magnitude 58Thedynamic analyses were carried out with the finite element program PLAXIS using a recently implemented constitutive model calledUBCSANDThe results indicate that themagnitude 58 earthquake would likely induce liquefaction in a limited zone located belowthe ground surface near the embankment dikes It is interpreted that stability of the dam may not be affected due to the limitedextent of the liquefied zone Both types of earthquakes are predicted to induce tolerable magnitudes of displacements The resultsof the postseismic slope stability analysis performed for a state after a seismic event suggest that the dam is stable during both theearthquakes

1 Introduction

This paper presents finite element analyses of dynamic behav-iour of the Aitik tailings dam in northern Sweden in responseto the impact of an earthquake It is one of the first attemptsto investigate the stability of a tailings dam in Swedenusing numerical methods to study dynamic loading Thepurpose of this study is to evaluate liquefaction potentialpermanent deformations and the stability of the dam underboth a commonly occurring Swedish earthquake and a moreextreme event

In Sweden approximately one earthquake with magni-tude above three occurs every year [1] Using the availabledatabase of earthquakes that have occurred in Sweden sincethe 19th century there are indications of approximately one

magnitude 5 event every century and one magnitude 6 eventevery millennium [2] Guidelines on tailings dams in Sweden[3] suggest that a tailings dam needs to be stable for at least1000 years In such a long time perspective it is thereforerelevant to consider the possibility of a large magnitudeearthquake [3]

TheAitik tailings dam ismainly raised using the upstreamconstruction method [4] In this raising method dikes aresequentially constructed on the previously deposited tailingsthat could be in a loose and saturated state In this type ofdam structure pore water in saturated tailings may not drainas rapidly as seismic shaking occurs As a result excess porepressures increase cumulatively with each cycle of loadingIf the excess pore pressures increase to such an extent thateffective stresses are reduced to near zero it could lead

Hindawi Publishing CorporationAdvances in Materials Science and EngineeringVolume 2017 Article ID 5389308 12 pageshttpsdoiorg10115520175389308

2 Advances in Materials Science and Engineering

to liquefaction (see eg [5]) In the liquefied state largedeformations could occur due to low stiffness of the tailingsThe deformations progressively increase with each cycle ofloading and may lead to failure of the dam It is relevant tomention here that several tailings dams located in differentparts of the world have failed due to seismic liquefaction(see eg [6])

Seismic liquefaction in a tailings dam is a complex phe-nomenon Therefore it is appropriate to investigate seismicbehaviour of the tailings dam with advanced numericalsoftware based on for example the finite elementmethod Tothe authorsrsquo knowledge there are just a few case studies of tail-ings dams published where such advanced numerical toolshave been used (see eg [7ndash10]) Some similar case stud-ies regarding seismic behaviour of geotechnical structures(mainly embankment dams) could be found in the literature(see eg [11ndash17]) More case studies of this type are neededin order to gain more experience of numerical modelling ofseismic liquefaction in tailings dams In this study numericalmodelling of seismic liquefaction of the Aitik tailings dam isperformed with the finite element program PLAXIS 2D [18]using a recently implemented advanced constitutive modelcalled UBCSAND (see [19 20])

The above information on seismicity in Sweden suggeststhat it is important to analyse liquefaction potential andstability of the Aitik dam by taking into account both normaland extreme seismic conditions in Sweden The modellingtechnique presented in this paper is not unique for the Aitiktailings dam and can be used for other tailings dam facilities

11 Seismicity of Northern Sweden Intraplate seismicity ischaracterized by few dispersed and irregularly occurringearthquakes In general these earthquakes tend to be rela-tively small in magnitude although there are many examplesof devastating intraplate earthquakes England and Jackson[21] report that sim100 out of the sim130 earthquakes that haveresulted in a thousand or more casualties over the last 120years worldwide took place in continental interiors There-fore they conclude that research efforts should focus onthe threat of unanticipated earthquakes located in intraplateregions

Sweden is located within the Baltic Shield far away fromplate boundaries Figure 1 shows that the micro-earthquakeactivity in northern Sweden is surprisingly high [22ndash24] andfocused along postglacial faults (eg [25]) It has been pro-posed that these faults hosted earthquakes with magnitudesbetween 7 and 8 when the ice disappeared after the latestglaciation some 10000 years ago (eg [26]) The roles ofthe ice in situ effective stresses and the nature of the crusthave been analysed in terms of fault instability (eg [27 28])The Swedish National Seismic Network [1] has recorded 9earthquakes larger than magnitude 3 in northern Swedensince 2000 and in 1983 a magnitude 41 earthquake occurredin Solberga in the southern part of the region [29]The largestearthquake with waveform data from south of Skelleftea wasused in this study

Near the Parvie fault the longest known postglacialfault in the world (cf (1) in Figure 1) a number of eventshave been reported by people living near the fault and

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Figure 1 Observed earthquakes from year 1398 and onwards fromthe joint Nordic catalogue at Helsinki University FENCAT (2011)and from 2000ndash2011 by the Swedish National Seismic Network(SNSN) at Uppsala University (eg [2]) The location of theAitik tailings dam is indicated by the circle Red dots show theepicentral location of the earthquakes Blue rectangles show thelocation of permanent Swedish seismic stations Postglacial faultsare highlighted by numbers (1) the Parvie fault (2) the Merasjarvifault (3) the Lainio-Suijavaara fault (4) the Stuoragurra fault (5) theSuasselka fault (6) the PasmajarviVenejarvi fault (7) the Lansjarvfault and (8) the Burtrask fault Modified from [22]

the instrumental records contain earthquake data of up tomagnitude 4 [30] Further to the southeast historical datafrom the Gulf of Bothnia area derived from newspaperreports suggest that nine earthquakes occurred during theperiod from 1757 to 1909 (eg [31 32]) The majority of theseevents have estimated magnitudes of 35 to 39 with a fewevents havingmagnitudes above 4 and the largest earthquakewith magnitude 47 recorded in the Gulf of Bothnia in 1898(eg [31 32]) Mantyniemi [32] proposed that earthquakeactivity in this region may occur in clusters over time Thelargest known historical earthquake near the study area hada magnitude of 53 and is proposed to have occurred in 1819in Luroslashy Norway [33]

The Aitik tailings dam is located within about 20 km and100 km from the large undergroundmines of LKAB inMalm-berget and Kiruna respectively As the mining progress todeeper levels mining-induced seismicity has been increasedLKAB has established local seismic networks to monitormining-induced seismicity and the larger events are alsorecorded by the SNSN [1] Two magnitude 29 events wererecorded in 2007 and 2008 in Malmberget and Kiruna res-pectively

2 The Aitik Tailings Dam

Aitik is the largest open pit coppermine in SwedenThismineis situated near Gallivare in northern Sweden The annual

Advances in Materials Science and Engineering 3

Figure 2 Aerial view of Aitik tailings dam and impoundment [36]

production of the mine was expanded to 343 million tonnesof ore yearly in 2012

Tailings are the by-products which are generated in amine processing plant when minerals are extracted fromthe ore The tailings produced at Aitik are transported inslurry form to the tailings impoundment The tailings slurryis then discharged through spigots in the pipeline alongthe dam embankments The tailings which are depositedclose to the embankments can be classified as silty finesands according to the European standard [34] and as siltysands in accordance with the unified soil classification system[35]

Figure 2 shows Aitik tailings impoundment which occu-pies an area of 13 square kilometres The impoundmentcomprises four dams named A-B C-D E-F (including E-F2 extension) and G-H The clarification pond is situateddownstream of dam E-F

Stability of the dam E-F as well as the corner betweenthe dam parts E-F and G-H has already been analysed understatic loading conditions for gradual raising [37 38] Amongthe various dams at the Aitik impoundment the dynamicfinite element analyses have been performed on the dam E-F The consequences of a failure of the dam E-F are expectedto be more serious compared to failures of other dam partsat the Aitik impoundment The reason is that a failure of thedam E-F may also lead to a failure of the dam I-J which islocated downstream of the clarification pond (Figure 2) Sucha failure would lead to discharge of tailings and waste waterwhich might affect the surrounding environment

3 Finite Element Model of the Dam

The construction of a finite element model basically consistsof the following steps before calculation specify the typeof analysis and the type of elements input of geometryinput of boundary conditions input of material propertiesmesh generation input of initial conditions and definition ofcalculation phases

Figure 3 presents the finite element model of the Aitiktailings dam section named E-F The different material zonesare illustrated in the model The tailings material zones 2 35 6 7 and 8 are categorized as silty sands The bottom of thedam is at an elevation of 340m It has been assumed based onmine production plans that the tailings are deposited in theimpoundment at a rate of threemeters per year from a level of376mThe elevation of the dam in 2012 was about 390m andit is planned to raise the dam to the level of 409m in 2018 (cf

Figure 3)The gradual raising of the dam from level 376m to409m was simulated in eleven stages Each stage consistedof a raising phase over 10 days and a consolidation phase over355 days

The phreatic level of the dam was evaluated usingPiezometers The phreatic level for first raising is shown inFigure 4 It can be seen that the phreatic level is on the surfaceof tailings and then it is parallel to the downstream slope ofthe dam The same trend of the phreatic level was utilized infurther raising of the dam (see Figure 5)

A finite element program for two-dimensional modelsPLAXIS 2D [18] was utilized for numerical analyses of thedam A plane strain conditionwas adopted which is a suitableassumption for both static and dynamic analyses of the longand straight dam section E-F

The numerical analyses were conducted in two stepsIn the first step static analyses were performed to simulatethe sequential raising of the dam For this purpose coupleddeformation and consolidation analyses [18 39 40] werecarried out to estimate the development and dissipation ofexcess pore pressures as a function of time in the dam

In the second step dynamic analyses were performeddirectly after the 11th raising of the dam (elevation 409mcf Figure 3) to evaluate the seismic behaviour of the dam fortwo different earthquake events In both steps slope stabilityanalyses were conducted to compute safety factors and toobserve the potential failures zones in the dam Slope stabilityanalyses for static loading (ie gradual raising) of the dam aredescribed in more detail in Ormann et al [37]

Figure 6 illustrates the finite element mesh adopted inthe analyses Fifteen noded triangular elements were utilizedin the mesh These elements give a fourth-order (quartic)interpolation for displacements [18]The elements used in themesh were 14077 in number Average size of an element wasabout 188mThe finite elementmeshwas sufficiently refinedas suggested by Kuhlemeyer and Lysmer [41] to ensurethat seismic waves are realistically transmitted through thenumerical model The horizontal width of the numericalmodel was large enough in order to minimize the influenceof seismic waves that might reflect back from the left verticalboundary

In both the static and the dynamic analyses ground-water flow andor consolidation can occur through all theboundaries except the left vertical boundary and the baseIn the static analyses displacements along the left verticalboundary were restrained in the horizontal direction whiledisplacements at the base were restricted in both the hori-zontal and the vertical directions In the dynamic analyses(i) an absorbent boundary was utilized on the left verticalboundary to absorb the seismic waves which otherwisewould reflect back into the dam body [18] and (ii) a pre-scribed horizontal displacement factor of 001m was appliedat the bottom of the model and vertical displacements are setto zero there The prescribed horizontal displacement factorof 001m at the bottom of the model is scaled to the actualmagnitudes of the displacements that vary with respect totime during an earthquake In the finite element programPLAXIS an input for a dynamic load can be given with eitherdisplacements or velocities or accelerations produced due to

4 Advances in Materials Science and Engineering

(1) Moraine (initial dike)(2) Soft sand tailings(3) Layered sand tailings(4) Moraine (dikes)

(5) Compacted sand tailings(6) Soft sand tailings(7) Compacted sand tailings (dikes)(8) Layered sand tailings

(9) Filter(10) Rockfill berms(11) Rockfill (external erosion

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protection)

Figure 3 Finite element model of dam E-F Dimensions are in meters

Phreatic level at 1st raising of dam

Figure 4 Location of phreatic level at first raising of the dam

Phreatic level at 11th raising of dam

Figure 5 Location of phreatic level at eleventh raising of the dam

a seismic shaking [18]The velocities or accelerations are thenconverted into displacements in the computation process

The foundation of the dam lies on a dense moraine layerThere is bedrock below the moraine layer The average depthof the moraine deposit is about 8m It was assumed that thefoundation of the dam on the moraine deposit is rigid andnonliquefiable so the moraine layer was not included in theanalyses

It is to be noted in the static analyses rockfill berms wereplaced on the downstream side of the dam to increase slopestability during various raising (see [37]) The rockfill bermsP Q R S T U V and W (Figure 7) were placed at the startof the second fourth fifth sixth seventh eighth ninth andtenth raising respectively

31 Constitutive Models and Material Properties In the staticanalyses the Mohr Coulomb model was used to representthe constitutive behaviour of all the material zones in thedam For the dynamic analyses (i) the UBCSAND model[19 20] was chosen to simulate the constitutive behaviourof the tailings material zones and (ii) the Mohr Coulombmodel was applied to the filter moraine and rockfill zones(cf Figure 3) which are assumed to be nonliquefiable Adetailed description of the two selected constitutive modelsis presented in the following sections

32 Mohr CoulombModel TheMohr Coulomb (MC) modelis a simple linear elastic perfectly plastic model whichrequires five input parameters These input parameters areYoungrsquos modulus Poissonrsquos ratio friction angle cohesionand dilatancy angle These parameters can be obtained frombasic soil tests [18] In the finite element program PLAXIS

Figure 6 Finite element mesh of dam E-F

some additionalmaterial parameters such as unit weights andhydraulic conductivities are also needed

The values of Youngrsquos modulus friction angle cohesionunit weights and hydraulic conductivities for tailings mate-rial zones were determined from field and laboratory testsAppropriate values of these parameters for moraine filterand rockfill zones were obtained from the available literature[3 42ndash45] The abovementioned parameters for all the mat-erial zones in the dam are presented in Table 1 It is to beremembered that for the dynamic analyses the MC model isjust utilized for the nonliquefiable zones 1 4 9 10 and 11 (cfFigure 3)

The value of Poissonrsquos ratio is assumed to be 033 forall the materials in the dam This value of Poissonrsquos ratio isconsidered to be suitable for this type of analysis [18] For allmaterial zones in the dam the value of the dilatancy angleis assumed to be zero This is a convenient assumption Thereason is that unrealistically high negative pore pressuresmay occur due to the use of a positive value of the dilatancyangle whereas unreasonably large positive pore pressuresmay develop if the value of the dilatancy angle is negative [18]

33 UBCSAND Model UBCSAND is an effective stresselastic-plastic model which is capable of simulating theliquefaction behaviour of sands and silty sands under seismicloading [20] The name UBCSAND implies that this modelwas developed at the University of British Columbia for pre-diction of liquefaction behaviour of sand An earlier versionof the UBCSANDmodel [46 47] was used in a case study ofdynamic analyses of Mochikoshi tailings dam in Japan andthe results of these analyses were consistent with the observedfailure pattern of the dam induced due to seismic liquefaction(see [9 10]) The UBCSAND model [20] with some modifi-cations has been implemented as a user defined soil model inthe finite element program PLAXIS [19]The PLAXIS versionof the UBCSANDmodel is utilized in this study

The material parameters required for the UBCSANDmodel are constant volume friction angle 120601cv peak friction

Advances in Materials Science and Engineering 5

Table 1 Parameters of the Mohr Coulomb model [3 42ndash45]

Material zone Material type 120574unsatkNm3

120574satkNm3

119896119909ms

119896119910ms

119864kNm2

1198881015840kNm2

1205931015840∘

1 Moraine (initial dike) 20 22 1 times 10minus7 5 times 10minus8 20000 1 352 Soft sand tailings 18 18 1 times 10minus7 1 times 10minus8 9800 6 183 Layered sand tailings 17 19 5 times 10minus7 5 times 10minus8 9300 10 224 Moraine (dikes) 20 22 5 times 10minus8 1 times 10minus8 20000 1 375 Compacted sand tailings 16 19 1 times 10minus6 1 times 10minus7 8800 13 266 Soft sand tailings 18 18 1 times 10minus7 1 times 10minus8 3000 6 187 Compacted sand tailings (dikes) 16 19 1 times 10minus6 1 times 10minus7 7200 13 268 Layered sand tailings 17 19 5 times 10minus7 5 times 10minus8 3900 10 229 Filter 18 20 1 times 10minus3 1 times 10minus3 20000 1 32

10 11 Rockfill (berms + externalerosion protection) 18 20 1 times 10minus1 1 times 10minus1 40000 1 42

Note 120574unsat is the unit weight above phreatic level 120574sat is the unit weight below phreatic level 119896119909 is the hydraulic conductivity in horizontal direction 119896119910 is thehydraulic conductivity in vertical direction 119864 is Youngrsquos modulus 1198881015840 is the effective cohesion and 1205931015840 is the effective friction angle

angle 120601119901 cohesion 119888 elastic shear modulus number119870119890119866 plas-tic shear modulus number 119870119901119866 elastic bulk modulus number119870119890119861 elastic bulk modulus index 119898119890 elastic shear modulusindex 119899119890 plastic shear modulus index 119899119901 failure ratio 119877119891atmospheric pressure 119875119860 (100 kPa) tension cut-off 120590119905 (0 kPa)densification factor 119891achard corrected standard penetrationtest value of the soil (1198731)60 and postliquefaction factor 119891post

The constant volume friction angle the peak frictionangle and cohesion were evaluated from direct shear tests ontailings material The values of the constant volume frictionangle were assumed to be the same as the values of the effec-tive friction angle evaluated for the MC model (cf Table 1)The values of the parameter (1198731)60 for tailings were obtainedby using correlations (see [48]) with the results of conepenetration tests which were performed on the Aitik depositThe elastic shear modulus number the plastic shear modulusnumber and the failure ratio were obtained by curve fittingwith the direct shear test results The elastic bulk modulusnumber was related to the elastic shear modulus numberusing Poissonrsquos ratio [19] For geotechnical problems whichinvolve unloading and reloading (eg dynamic loading) alow value of Poissonrsquos ratio is recommended to be used inadvanced soil models [18] Poissonrsquos ratio of 012 was utilizedto evaluate stiffness parameters of the UBCSAND modelThis value of Poissonrsquos ratio is considered to be suitable forthis type of analysis [20 49] For Poissonrsquos ratio of 012the elastic bulk modulus number is approximately equalto the elastic shear modulus number Therefore the samevalue is used for the elastic shear modulus number andthe elastic bulk modulus number The elastic shear modulusindex elastic bulk modulus index and plastic shear modulusindex were assigned as 05 05 and 04 respectively (see[20]) Appropriate values of the densification factor and thepostliquefaction factorwere taken as 03 and 001 respectively(see [19 50]) Table 2 presents the following input parametersthe peak friction angle the elastic shear modulus number

Table 2 Some input parameters of the UBCSANDmodel

Materialzone Material type 1205931015840119901 119870119890119866 119870119901119866 119877119891 (1198731)602 6 Soft sand tailings 186∘ 120 36 095 6

3 8 Layered sandtailings

226∘ 120 36 090 6

5 7 Compacted sandtailings

268∘ 120 72 086 8

Note 1205931015840119901 is the peak friction angle 119870119890119866 is the elastic shear modulus number119870119901119866is the plastic shear modulus number 119877119891 is the failure ratio and (1198731)60

is the corrected standard penetration test value of the soil

the plastic shear modulus number the failure ratio and thecorrected standard penetration test value of the soil

It is to be noted that due to unavailability of cyclic directshear andor cyclic triaxial test results suitable values ofthe damping ratio densification factor and postliquefactionfactor were assumed based on results of a case study of anupstream tailings dam presented in the literature [9 10]It would of course be better if these parameters had beenevaluated from cyclic soil tests on Aitik tailings materialHowever it is believed that the results of the analyses givereasonable estimates of the liquefaction potential and thestability of the dam under seismic loading conditions

34 Damping The amplitude of elastic waves in soils maydecrease with timeThis property of elastic waves is known asmaterial damping (see eg [51]) In dynamic computationsperformed with the finite element program PLAXIS 2Dmaterial damping occurs to some extent due to viscous prop-erties of soil friction and development of plastic strains [18]However the hysteretic damping exhibited by PLAXIS 2Dis generally lower than the actual damping observed in soilsTherefore an extra damping is required to simulate damping

6 Advances in Materials Science and Engineering

VU

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Existing rockfill

Existing rockfill

TW

Figure 7 Placement of the rockfill berms P Q R S T U V and W

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Figure 8 Frequency spectrum for free vibration analysis of the damand two specific earthquakes

effects in soils in a realistic manner [18] This additionaldamping is provided by means of Rayleigh damping whichis a function of mass and stiffness of the soil and is definedas (see [52])

C = 120572M + 120573K (1)

where CM and K are the damping matrix the mass matrixand the stiffnessmatrix respectively120572 and120573 are the Rayleighcoefficients

The Rayleigh damping coefficients 120572 and 120573 can becalculated as a function of two representative frequenciesof the waves which propagate through the soil [18] andan associated damping ratio [53] The two representativefrequencies 03Hz and 18Hz were obtained by taking intoconsideration both the natural frequencies of the dam and theinput frequencies of the seismic waves (Figure 8)The naturalfrequencies were evaluated from a free vibration analysis ofthe Aitik dam It can be seen from the figure that the twodominant frequencies of the input motions are 03Hz and18Hz The average of these two frequencies is close to thedominant natural frequency (ie 12Hz)

The UBCSAND model is capable of producing damp-ing However for nonsymmetric cyclic loading conditionsthe UBCSAND model predicts a less amount of dampingcompared to the damping of soils observed in laboratorytests [20] As additional damping 1 of the critical damping(ie damping ratio 001) was assigned to the tailings materialzones whose constitutive behaviour was represented with theUBCSAND model (see [54 55]) On the other hand 3 ofthe critical dampingwas assigned to themoraine rockfill andfilter zones where the MC model was utilized (see [54 55])

35 Input for Ground Motion The seismic behaviour of thedam was evaluated for one somewhat larger than normal andone extreme earthquake in northern Sweden For the normalcase an earthquake of local magnitude 36 was selected Thisearthquake occurred in 2010 outside Skelleftea in northernSweden The input data for this earthquake was obtainedfrom the Swedish National Seismic Network (SNSN) For theextreme case an earthquake of moment magnitude 58 wasselectedThis earthquake occurred in 2011 inVirginia easternUnited States It was chosen because the geology of easternUnited States is similar to that of northern Sweden and norecords of such a largemagnitude earthquake exist in SwedenThe data for the Virginia earthquake was downloaded fromthe US Geological Survey As the Swedish local magnitudescale is moment based a comparison between the magni-tude scales is straightforward and in this case the Virginiaearthquake radiated approximately 14000 timesmore energythan the Skelleftea earthquake From now on the Skellefteaearthquake will be referred to as the normal earthquakecaseand the Virginia earthquake as the extreme earthquakecase

The input data for both the normal earthquake and theextreme earthquake are shown in Figures 9 and 10 respec-tively Both the Skelleftea earthquake and the Virginia earth-quake were recorded on the bedrock The instrument res-ponse has been deconvolved such that Figures 9 and 10 showtrue ground velocity and acceleration for the respective earth-quakes It can be seen that the horizontal ground motionsof the normal earthquake and the extreme earthquake areexpressed in velocity and acceleration respectively The nor-mal earthquake and the extreme earthquake were recorded ata distance of 11 km and 53 km respectively

In this study only the horizontal ground motions (accel-erations andor velocities) were applied and the verticalground motions were not considered The reason is (see[56]) that (i) vertical motions cause much less shear stressescompared to those due to horizontal motions and (ii) pore

Advances in Materials Science and Engineering 7

007006

minus006

005

minus005

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minus004

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minus002

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Figure 9 Input ground motion of Skelleftea earthquake (normal case)

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Figure 10 Input ground motion of Virginia earthquake (extreme case)

pressures that develop due to vertical motions are smallerthan those caused by horizontal motions

Remember in the finite element program PLAXIS aseismic loading can be applied with any of the followingquantities velocity acceleration or displacement In order toreduce computational time low values of acceleration andorvelocity are filtered out It is to be noted that the results of thedynamic analyses are not affected by this filtering process

4 Results

Dynamic analyses were performed for the state directly afterthe 11th raising of the Aitik dam This state was chosenbecause it is considered as critical since excess pore pressureshave developed due to the gradual raising of the dam In thefollowing sections the seismic behaviour of the dam is anal-ysed in terms of excess pore pressures liquefaction earth-quake induced permanent deformations and postseismicstability

41 Excess Pore Pressures Figure 11 shows excess pore pres-sures after 11th raising of the dam prior to the earthquakeshakings Figures 12 and 13 illustrate the development ofexcess pore pressures when the dam is shaken with thenormal and the extreme earthquake respectively It can beobserved that the largest initial excess pore pressures dueto the static loading are of magnitude 95 kPa (Figure 11) Asexpected the largestmagnitudes of excess pore pressures due

to the seismic loadings have increased to (i) about 220 kPafor the normal earthquake (Figure 12) and (ii) about 320 kPafor the extreme earthquake (Figure 13) As anticipated themagnitudes of the excess pore pressures were higher for theextreme earthquake compared to those generated from thenormal earthquake There was an increase in excess porepressures throughout the dam body The maximum increasein excess pore pressures was observed at the lower parts ofthe dam below the embankment dikes (Figures 12 and 13)As expected due to dynamic loading positive excess porewater pressures developed in contractant zones (soft sandtailings and layered sand tailings) and negative excess porewater pressures generated in dilatant zones (moraine dikescompacted sand tailings and rockfill) of the dam

42 Liquefaction Liquefied zones in the dam were identifiedby evaluating the excess pore pressure ratio 119903119906 which isdefined as [20]

119903119906 = 119891(1199061198901199060) = 1 minus1205901015840V1205901015840V0 (2)

where 119891 stands for function 119906119890 is the excess pore pressureand 1205901015840V is the vertical effective stress in the dam after seismicshaking 1199060 is the initial excess pore pressure and 1205901015840V0 is theinitial vertical effective stress in the dam prior to earthquakeloading

8 Advances in Materials Science and Engineering

340

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minus500

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Figure 11 Excess pore pressures in the dam after 11th raising Negative sign indicates suctionminus4000

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Figure 12 Excess pore pressures after the dam is shaken with normal earthquake Negative sign indicates suction

It is assumed that liquefaction is likely to occur in the damat such zones where the values of the excess pore pressureratio are equal to or greater than 07 [54] For the normalearthquake no liquefaction was observed in the dam withthe selected input parameters For the extreme earthquakepotential liquefied zones (119903119906 ge 07) are illustrated in Figure 14It can be seen that liquefaction is likely to occur in thedeposited tailings below the ground surface The potentialliquefied zone is located about 4m below the ground surfacenear the embankment dikes It can be observed (Figure 14)that the extent of the liquefied zone is limited It is thereforeinterpreted that this liquefied zone could not affect thestability of the dam The occurrence of a liquefied zonebelow the surface is consistent with general observationsthat liquefaction initiates at the surface and then extendsdownwards if the seismic shakings generate high excess porepressures in the deep layers (see eg [57])

43 Earthquake Induced Permanent Deformations The hor-izontal displacements in the dam induced due to the nor-mal earthquake and the extreme earthquake are shown inFigures 15 and 16 respectively The maximum horizontaldisplacements of about 03m and 06m are predicted in thevicinity of the downstream slope for the normal earthquakeand the extreme earthquake respectively As expected thehorizontal displacements are controlled by the rockfill berms(see Figures 15 and 16)

Figures 17 and 18 show the predicted vertical displace-ments after the dam is shaken with the normal earthquakeand the extreme earthquake respectively It can be seen thatthe maximum vertical displacements of about 03m and04m mainly occurred on the crest of the embankment

for the normal earthquake and the extreme earthquakerespectively The vertical displacements are tolerable becausethey are much smaller than the freeboard (vertical distancebetween the surface of tailings and crest) which is 2m Ingeneral total displacements of these magnitudes in a dam(see [58]) are considered to be within acceptable limits

44 Slope Stability Slope stability of the dam under staticand seismic loading conditions was assessed on the basisof a safety factor In the finite element program PLAXIS astrength reduction technique is employed to compute safetyfactors [18] In this technique the tangent of the frictionangle and the cohesion of the soil are gradually reduced inthe same proportion until the geotechnical structure fails[18] In the strength reduction method the safety factor isdefined in the same way as described in the limit equilibriummethods [59] It is to be noted that the strength reductiontechnique is applicable to the MC model only Thereforethe MC model was utilized for computation of postseismicsafety factors after performing dynamic analysis with theUBCSANDmodel

A safety factor of about 145 was achieved directly afterthe 11th raising (static loading) Postseismic safety factors ofabout 132 and 122 were obtained from the slope stabilityanalyses for the normal earthquake and the extreme earth-quake respectively As expected these postseismic safetyfactors are less than the safety factor obtained under staticloading This is because shear strength is reduced due to anincrease in excess pore pressures caused by the earthquakes

Postseismic stability of the dam was evaluated with thesafety factor criterion suggested by ANCOLD [60] whichstates that a safety factor of at least 12 is required for a

Advances in Materials Science and Engineering 9

minus4000

000

4000

8000

12000

24000

16000

20000

28000

32000

( kN

m2)

340

365

390

415

200 300 400 500 600 7000 100

Figure 13 Excess pore pressures after the dam is shaken with extreme earthquake Negative sign indicates suction

340

365

390

415

200 300 400 500 600 7000 100

Liquefied zones

Figure 14 Potential liquefied zones after shaking with extreme earthquake Dimensions are in meters For visibility scale of vertical axis islarger than the horizontal axis

minus2000

2000

6000

10000

14000

18000

22000

26000

30000

34000

340

365

390

415

200 300 400 500 600 7000 100

(lowast10minus3

m)

Figure 15 Horizontal displacements in the dam after shaking with normal earthquake The positive direction is to the right

000

8000

12000

24000

16000

20000

40000

28000

32000

36000

44000

48000

52000

56000

340

365

390

415

200 300 400 500 600 7000 100

minus4000

4000

minus8000

(lowast10minus3

m)

Figure 16 Horizontal displacements in the dam after shaking with extreme earthquake The positive direction is to the right

340

365

390

415

200 300 400 500 600 7000 100

000

8000

minus4000

4000

minus8000

minus12000

minus16000

minus20000

minus24000

minus28000

(lowast10minus3

m)

Figure 17 Vertical displacements in the dam after shaking with normal earthquake The positive direction is upward

10 Advances in Materials Science and Engineering

340

365

390

415

200 300 400 500 600 7000 100

000

4000

8000

12000

16000

20000

minus36000

minus40000

minus4000

minus8000

minus12000

minus16000

minus20000

minus24000

minus28000

minus32000

(lowast10minus3

m)

Figure 18 Vertical displacements in the dam after shaking with extreme earthquake The positive direction is upward

Figure 19 Failure zone of the dam after shaking with normal earthquake

Figure 20 Failure zone of the dam after shaking with extreme earthquake

tailings dam to be safe enough against seismic shakingsThe stability of the dam is considered to be satisfactory forboth the earthquakes according to the guidelines provided byANCOLD [60] It is to be noted for the extreme earthquakethe value of the postseismic safety factor of the dam is juston the limit recommended by ANCOLD [60] This impliesthat the dammay not be safe enough for earthquakes strongerthan the Virginia earthquake presented in this paper

In this type of analysis in addition to the value of thesafety factor it is important to identify possible failure zonesthat would have developed if the dam had failed Figures 19and 20 show such failure zones that were obtained from thepostseismic slope stability analyses for the normal earthquakeand the extreme earthquake respectively It can be seen thatthe failure zone caused by the extreme earthquake is less wideand deep compared to the one that developed due to thenormal earthquake This is because the extreme earthquakehas led to larger negative pore pressures and higher strengthin the moraine dikes and the rockfill berms (cf Figure 3)therefore the failure zone did not extend to these zones

5 Concluding Remarks

In this paper seismic behaviour of the Aitik tailings damwas evaluated based on results of dynamic finite elementanalyses in terms of liquefaction potential permanent defor-mations and postseismic stability The seismic behaviourof the dam was analysed for two cases (i) a normal case(an earthquake of Swedish local magnitude 36) and (ii) anextreme case (an earthquake of moment magnitude 58) Theresults for the extreme case indicate that liquefaction couldoccur in a limited zone located below the surface near theembankments Because of the limited extent of this liquefiedzone it is interpreted that the stability of the dam could not

be affectedThe predicted displacements induced by both theearthquakes are considered to be within acceptable limitsPostearthquake slope stability analyses suggest that the damcould resist the abovementioned earthquakes However forthe Virginia earthquake the postseismic safety factor is juston the limit suggested by the available guidelines Since novertical ground accelerations were used in the numericalmodelling the resultsmay slightly differ if the vertical groundaccelerations are applied in addition to the horizontal groundaccelerations

The stability of the Aitik tailings dam has been studieddue to the occurrence of one earthquake at a time Thepossibility of subsequent weakening of the structure bymultiple earthquakes of various magnitudes has not beenexamined

Due to unavailability of cyclic laboratory test results onthe Aitik tailings material dynamic material properties wereevaluated with engineering judgement based on results of acase study relevant to this dam If cyclic laboratory tests areconducted in the future they could provide valuable infor-mation in further understanding of the seismic behaviour ofthe dam

Competing Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

Theauthors would like to express their sincere thanks to Boli-den AB Sweden for giving them an opportunity to carry outthe presented study regarding the tailings dams at the Aitikmine and its disposal facilities and for giving access to site

Advances in Materials Science and Engineering 11

information Kerstin Pousette at Lulea University of Technol-ogy Sweden and Fredrik Jonasson at Sweco InfrastructureAB Sweden are to be acknowledged for performing labo-ratory tests and providing information from field tests andfor evaluation of material parameters The first author wouldlike to thank Lars Erik Lundbergs Foundation Sweden forgranting him a scholarship to conduct some part of his PhDresearch The ldquoIRISrdquo project within the EU Fp7 frameworktogether with Lulea University of Technology Boliden ABand ldquoSwedish Hydropower Centre-SVCrdquo are acknowledgedfor financial support which made the work possible SVChas been established by the Swedish Energy Agency Elforskand Svenska Kraftnat together with Lulea University ofTechnology The Royal Institute of Technology ChalmersUniversity of Technology and Uppsala University Partic-ipating hydropower companies are Andritz Hydro IneparSweden AndritzWaplans EONVattenkraft Sverige FortumGeneration Holmen Energi Jamtkraft Karlstads EnergiLinde Energi Malarenergi Skelleftea Kraft SollefteaforsensStatkraft Sverige Statoil Lubricants Sweco InfrastructureSweco Energuide SveMin Umea Energi Vattenfall Researchand Development Vattenfall Vattenkraft VG Power andWSP

References

[1] SNSN ldquoSvenska nationella seismiska natetrdquo 2013 httpsnsngeofysuuse

[2] R Bodvarsson B Lund R Roberts and R Slunga ldquoEarthquakeactivity in Sweden study in connectionwith a proposed nuclearwaste repository in Forsmark or Oskarshamnrdquo SKB RapportR-06-67 2006 httpwwwskbseuploadpublicationspdfR-06-67pdf

[3] GruvRIDAS Gruvindustrins Riktlinjer for DammsakerhetSvensk Energi ABSveMin Stockholm Sweden 2007

[4] S G Vick Planning Design and Analysis of Tailings DamsBiTech Publishers Richmond Canada 1990

[5] P K Robertson and C EWride ldquoEvaluating cyclic liquefactionpotential using the cone penetration testrdquo Canadian Geotechni-cal Journal vol 35 no 3 pp 442ndash459 1998

[6] WISE ldquoChronology of major tailings dam failuresrdquo 2013httpwwwwise-uraniumorgmdafhtml

[7] M Seid-Karbasi H Hawson and U Atukorala ldquoSeismic sta-bility of a Peruvian tailings earth-rockfill dam with liquefiablefoundationrdquo in Proceedings of International Symposium onDams and Reservoirs under Changing Challengesmdash79 AnnualMeeting of ICOLD Swiss Committee on Dams Lucerne Switzer-land 1 June 2011 pp 613ndash620 CRC PressBalkema LeidenTheNetherlands 2011

[8] M Seid-Karbasi and U Atukorala ldquoDeformations of a zonedrockfill dam from a liquefiable thin foundation layer subjectedto earthquake shakingrdquo in Proceedings of the 31st AnnualUSSD Conference on 21st Century Dam Design-Advances andAdaptations pp 1351ndash1367 United States Society on Dams SanDiego Calif USA April 2011

[9] M Seid-Karbasi and P M Byrne ldquoEmbankment dams andearthquakesrdquo International Journal on Hydropower and Damsvol 11 no 2 pp 96ndash102 2004

[10] PM Byrne andM Seid-Karbasi ldquoSeismic stability of impound-mentsrdquo inProceedings of the 17thAnnual Symposium VancouverGeotechnical Society Vancouver Canada 2003

[11] G Elia and M Rouainia ldquoSeismic performance of earth em-bankment using simple and advanced numerical approachesrdquoJournal of Geotechnical and Geoenvironmental Engineering vol139 no 7 pp 1115ndash1129 2013

[12] G Elia A Amorosi A H C Chan and M J Kavvadas ldquoFullycoupled dynamic analysis of an Earth damrdquo Geotechnique vol61 no 7 pp 549ndash563 2011

[13] MAlyamiMRouainia and SMWilkinson ldquoNumerical anal-ysis of deformation behaviour of quay walls under earthquakeloadingrdquo Soil Dynamics and Earthquake Engineering vol 29 no3 pp 525ndash536 2009

[14] S Sica L Pagano and A Modaressi ldquoInfluence of past loadinghistory on the seismic response of earth damsrdquo Computers andGeotechnics vol 35 no 1 pp 61ndash85 2008

[15] K K Muraleetharan S Deshpande and K Adalier ldquoDynamicdeformations in sand embankments centrifuge modeling andblind fully coupled analysesrdquo Canadian Geotechnical Journalvol 41 no 1 pp 48ndash69 2004

[16] O Aydingun and K Adalier ldquoNumerical analysis of seismicallyinduced liquefaction in earth embankment foundations Part IBenchmark modelrdquo Canadian Geotechnical Journal vol 40 no4 pp 753ndash765 2003

[17] MM Dewoolkar H-Y Ko R Y S Pak and A Engrg ldquoSeismicbehavior of cantilever retaining walls with liquefiable backfillsrdquoJournal of Geotechnical and Geoenvironmental Engineering vol127 no 5 pp 424ndash435 2001

[18] R B J Brinkgreve W M Swolfs and E Engine PLAXIS UserrsquosManual PLAXIS Delft The Netherlands 2011

[19] A Petalas and V Galavi ldquoPlaxis liquefaction model UBC3D-PLM documentation reportrdquo January 2013 httpswwwplaxiscomcontentuploadsimportkbkb-downloadsUBC3D-PLM2028REPORT29Jan2013pdf

[20] M H Beaty and P M Byrne UBCSAND Constitutive ModelVersion 904aR Documentation Report 2011 httpwwwitasca-udmcommediadownloadUBCSandUBCSAND UDM Doc-umentationpdf

[21] P England and J Jackson ldquoUncharted seismic riskrdquo NatureGeoscience vol 4 no 6 pp 348ndash349 2011

[22] E Lindblom Microearthquake study of end-glacial faults innorthern Sweden [Licentiate thesis] Uppsala University Upp-sala Sweden 2011

[23] E Lindblom and B Lund ldquoFocal mechanisms and the stateof stress along the Parvie end-glacial fault Northern Swedenrdquoin Microearthquake Study of End-Glacial Faults in NorthernSweden Licentiate thesis E Lindblom Ed Uppsala UniversityUppsala Sweden 2011

[24] E Lindblom B Lund A Tryggvason et al ldquoMicroearthquakeactivity on the Parvie end-glacial fault system northern Swe-denrdquo in Microearthquake Study of End-Glacial Faults in North-ern Sweden E Lindblom Ed Licentiate thesis Uppsala Uni-versity Uppsala Sweden 2011

[25] R Lagerback and M Sundh ldquoEarly Holocene faulting andpaleoseismicity in northern Swedenrdquo Research Paper C836Geological Survey of Sweden 2008 httpresourcesguseproduktercc836-rapportpdf

[26] R Arvidsson ldquoFennoscandian earthquakes whole crustal rup-turing related to postglacial reboundrdquo Science vol 274 no 5288pp 744ndash746 1996

12 Advances in Materials Science and Engineering

[27] R Muir Wood ldquoExtraordinary deglaciation reverse faulting innorthern Fennoscandiardquo in Earthquakes at North-Atlantic Pas-siveMargins Neotectonics and Postglacial Rebound S Gregersenand P W Basham Eds pp 141ndash173 Kluwer Academic Publish-ers Dordrecht The Netherlands 1989

[28] B Lund P Schmidt and C Hieronymus ldquoStress evolution andfault instability during theWeichselian glacial cyclerdquo Tech RepTR-09-15 Swedish Nuclear Fuel andWaste Management 2009httpwwwskbseuploadpublicationspdfTR-09-15pdf

[29] W Y Kim O Kulhanek T van Eck and R Wahlstrom ldquoTheSolberg Sweden Earthquake of September 29 1983rdquo ReportNo 1-85 Seismological Department University of UppsalaUppsala Sweden 1985

[30] FENCAT Fennoscandian Earthquake Catalog 2011 httpwwwseismohelsinkifibulletinlistcatalog formathtml

[31] P Mantyniemi ldquoEarthquake of 4 November 1898 in northernEurope new insightsrdquo Journal of Geophysical Research SolidEarth vol 113 no 11 Article ID B11303 2008

[32] P Mantyniemi ldquoIntraplate seismicity and seismic hazardthe Gulf of Bothnia Area in Northern Europe Revisitedrdquo inEarthquake Research and Analysis-New Frontiers in SeismologyS DrsquoAmico Ed pp 283ndash298 InTech Rijeka Croatia 2012httpwwwintechopencombookshowtoreferenceearthquake-research-and-analysis-new-frontiers-in-seismologyintraplate-seismicity-and-seismic-hazard-the-gulf-of-bothnia-in-northern-europe-revisited

[33] H Bungum and O Olesen ldquoThe 31st of August 1819 Luroslashyearthquake revisitedrdquoNorwegian Journal of Geology vol 85 no3 pp 245ndash252 2005

[34] European Standard ldquoGeotechnical investigation and testingmdashidentification and classification of soilmdashpart 1 identificationand descriptionrdquo EN ISO 14688-1 2002

[35] ASTM ldquoStandard practice for classification of soils for engi-neering purposesrdquo ASTMD2487-10 ASTM InternationalWestConshohocken Pa USA 2010

[36] Google Maps ldquoSatellite view of Aitik tailings dam nearGallivare Swedenrdquo 2011 httpmapsgooglecom

[37] L Ormann M A Zardari H Mattsson A Bjelkevik and SKnutsson ldquoNumerical analysis of strengthening by rockfill em-bankments on an upstream tailings damrdquo Canadian Geotechni-cal Journal vol 50 no 4 pp 391ndash399 2013

[38] L Ormann M A Zardari H Mattsson A Bjelkevik and SKnutsson ldquoNumerical analysis of curved embankment of anupstream tailings damrdquo Electronic Journal of Geotechnical Engi-neering vol 16 pp 931ndash944 2011

[39] A M Britto and M J Gunn Critical State Soil Mechanics viaFinite Elements Ellis Horwood West Sussex UK 1987

[40] D M Potts and L Zdravkovic Finite Element Analysis in Geo-technical EngineeringTheoryThomasTelford Ltd LondonUK1999

[41] R L Kuhlemeyer and J Lysmer ldquoFinite element method accu-racy for wave propagation problemsrdquo Journal of Soil Mechanicsand Foundations Division vol 99 pp 421ndash427 1973

[42] T Stal P Wedel and S Aven EdsHandboken Bygg GeoteknikLiber Stockholm Sweden 1984 (Swedish)

[43] Vattenfall Jord-och Stenfyllningsdammar Vattenfall Stock-holm Sweden 1988

[44] U Bergdahl E Ottosson and B S Malmborg Plattgrund-laggning AB Svensk Byggtjanst och Statens Geotekniska Insti-tut 1993 (Swedish)

[45] Bro Vagverket Vagverkets Allmanna Tekniska Beskrivningfor Nybyggande och Forbattring av Broar Borlange 2004(Swedish)

[46] H Puebla P M Byrne and R Phillips ldquoAnalysis of CANLEXliquefaction embankments prototype and centrifuge modelsrdquoCanadian Geotechnical Journal vol 34 no 5 pp 641ndash657 1997

[47] M H Beaty and P M Byrne ldquoAn effective stress model forpredicting liquefaction behaviour of sandrdquo Geotechnical SpecialPublication vol 75 pp 766ndash777 1998

[48] P K Robertson and K L Cabal Guide to Cone PenetrationTesting for Geotechnical Engineering Gregg Drilling amp TestingInc Signal Hill Calif USA 2010

[49] B O Hardin ldquoThe nature of stress strain behavior of soilsrdquo inProceedings of the ASCE Geotechnical Engineering Division Spe-cialty Conference Earthquake Engineering and Soil Dynamicspp 3ndash90 American Society of Civil Engineers Pasadena CalifUSA June 1978

[50] E Naesgaard A hybrid effective stressmdashtotal stress procedure foranalyzing soil embankments subjected to potential liquefactionand flow [PhD thesis] The University of British Coulombia2011

[51] B M Das and G V Ramana Principles of Soil DynamicsCengage Learning Stamford Conn USA 2011

[52] O C Zienkiewicz R L Taylor and J Z Zhu Finite ElementMethodmdashIts Basis and Fundamentals Elsevier Butterworth-Heinemann 2005

[53] ITASCA Fast Lagrangian Analysis of Continua (FLAC) UserrsquosGuide Itasca Consulting Group Inc Minneapolis Minn USA2011

[54] MH Beaty andVG Perlea ldquoSeveral observations on advancedanalyses with liquefiable materialsrdquo in Proceedings of the 31stAnnual USSD Conference and 21st Conference on Century DamDesign-Advances and Adaptations pp 1369ndash1397 US Society onDams San Diego Calif USA April 2011

[55] T D StarkMH Beaty PM Byrne et al ldquoSeismic deformationanalysis of Tuttle Creek Damrdquo Canadian Geotechnical Journalvol 49 no 3 pp 323ndash343 2012

[56] H B Seed K L Lee I M Idriss and F Makdisi ldquoAnalysis ofthe slides in the San Fernando dams during the earthquake ofFeb 9 1971rdquo Report no EERC 73-2 University of CaliforniaBerkeley Calif USA 1971

[57] J A Knappett andR F CraigCraigrsquos SoilMechanics SponPressLondon UK 2012

[58] R N Kostaschuk J M TWilkinson and PM Byrne ldquoPredict-ing the deformation of tailings dams from earthquake lique-factionrdquo in Proceedings of the 6th International Conference onTailings and Mine Waste rsquo99 pp 237ndash248 AA Balkema FortCollins Colo USA January 1999

[59] R B J Brinkgreve and H L Bakker ldquoNon-linear finite elementanalysis of safety factorsrdquo in Proceedings of the 7th InternationalConference on Computer Methods and Advances in Geomechan-ics pp 1117ndash1122 AA Balkema Cairns Australia May 1991

[60] ANCOLD Guidelines on Tailings Dams Planning Design Con-struction Operation and Closure 2012

Submit your manuscripts athttpswwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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