la hierba and rhyolite lvu facilities seepage analyses r3[1]

7/31/2019 La Hierba and Rhyolite LVU Facilities Seepage Analyses R3[1]

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CALCULATIONS

Client: Gold Fields La Cima S.A.A. Sheet: 1 of 32

Project:Cerro Corona MineLa Hierba and East Rhyolite LVU Facilities

Date: 06/22/2011

Description:Seepage Analyses

Job No: 1004923

By: Josh Rogers

CERRO CORONA MINELA HIERBA AND EAST RHYOLITE LVU FACILITIES

SEEPAGE ANALYSES

Revisioning

Rev. Date Description By Checked Date Reviewed Date

0 06/22/11 First Issue Josh RogersJustin

Seavey

06/27/11ChristineWeber

06/28/11

1 06/29/11Revised to addressreview comments

Josh Rogers

ChristineWeber

06/30/11ChristineWeber

06/30/11

Location and Format

Electronic copies of these calculations are located in the project files system at:

J:\Clients_A-H\Gold Fields La Cima\1004923 Cerro Corona\Technical\Calculations\LVUFacilities\La Hierba and Rhyolite LVU Facilities\Analyses

The following calculations were generated using the following software:

Microsoft Excel (Microsoft, 2006)Seep/W (Geoslope, 2009)


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Introduction

Gold Fields La Cima S.A. (GFLC), a subsidiary of Gold Fields Limited, owns the Cerro Corona mine, a coppermine with significant gold content. The mine is located in northern Peru, approximately 760 km north-northwest of

Lima and 80 km by road from the city of Cajamarca. Tailing produced by the Cerro Corona processing plant isstored in the Tailing Storage Facility (TSF), located north of the plant site in the Quebrada Las Aguilas andQuebrada Las Gordas valleys. The TSF is being constructed in stages. MWH was contracted by GFLC to

develop final designs for the TSF. The TSF design scope includes low volume underflow (LVU) facilities. Thesefacilities are designed to mitigate seepage flows from the facility. The purpose of this calculation brief is to detailthe seepage analyses performed in support of the design of the La Hierba and East Rhyolite LVU facilities.

Methodology and Objectives

Steady-state seepage analyses were performed using SEEP/W version 7.15 (GEO-SLOPE 2009), finite elementsoftware for analyzing pore pressure distribution in and flow through porous media. The analyses wereperformed to estimate seepage fluxes through the LVU facilities and facility foundations. These fluxes, which

were estimated based on a two-dimensional section, were then utilized to estimate the total seepage flows fromthe facilities, as discussed in this calculation brief. Additionally, the analyses were performed to estimate porewater pressures in the dam and foundation materials for use as inputs to the limit equilibrium stability analyses

performed in support of the design of the La Hierba and East Rhyolite LVU facilities (MWH, 2011b).

Geometry

Two sections were evaluated as a part of the La Hierba and East Rhyolite LVU facility analyses, one for the La

Hierba LVU facility and one for the East Rhyolite LVU facility. The geometry of the sections utilized in theseepage and stability analyses are based on a combination of the sections presented in the La Hierba and


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Job No: 1004923

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The grout curtain rows were modeled individually in Seep/W as interface elements with a width of 0.5 m.

Table 1 below presents a summary of the key geometric components of each of the sections evaluated, includingthe LVU facility crest elevations, the impounded water elevations used in the analyses, the U/S and downstream

(D/S) embankment slopes, and the grout curtain depths.

Table 1 Summary of Key Geometric Components of the Evaluated Sections

LVUFacility

Crest of LVUEmbankment Liner

ImpoundedWater

Elevation

GroutCurtainDepth

U/SSlope

D/S Slope

(-) (m) (m) (m) (H:V) (H:V)

LaHierba

3638 3636 30 2:1 1.4:1

EastRhyolite

3746 3744 30 2:1 1.4:1

Figures 1 and 2 present the idealized sections utilized in the seepage analyses for the La Hierba and EastRhyolite LVU facilities, respectively. For additional information, refer to the feasibility LVU design drawings

(MWH, 2011a) and the LVU facility design report document (MWH, 2011c).


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Material Properties

The material properties utilized in the seepage analyses presented in this document are a combination of those inprevious analyses performed at the Cerro Corona site, properties estimated based on the results of the La Hierbaand East Rhyolite LVU facility subsurface investigation programs, and properties estimated based on experience

at the site and engineering judgment.

The saturated hydraulic conductivities utilized in the seepage analyses are summarized in Table 2 below.

Table 2: Saturated Hydraulic Conductivity of Materials Utilized in the Seepage Analyses

Material

SaturatedHydraulic

Conductivity(m/s)

Basis

Zone 2B(Rockfill)

5x10-2

Estimated (MWH, 2010b)

Zone 3(Filter)

2x10-3

Estimated (MWH, 2011d)

Zone 4(Filter)

4x10-2

Estimated (MWH, 2011d)

Topsoil 1x10-7

Assumed

Glacial Till 3x10-8

Laboratory1

Weathered Bedrock 1x10-6

Estimated2

Bedrock 1x10-7

Estimated2

Formatted Table


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Where:

Geomembrane defects were incorporated into the geomembrane saturated hydraulic conductivity value using anarea weighted average, as follows:

Where:

Finally, Darcys law was used to convert the calculated composite saturated hydraulic conductivity values toequivalent saturated hydraulic conductivity values considering the 0.1 m composite liner thickness used in theanalyses:


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assumption of one defect hole (with a size of 0.005 in2

(0.03 cm2)) per 4000 m

2. Nosko and Touze-Foltz

performed a statistical analysis of the results from electrical damage detection systems installed at over300 sites, covering more than 3.2 million m

2of lined area. The results of their analysis indicate defect

sizes ranging from less than 0.5 to greater than 10 cm2. The frequency of the defects was found to be

approximately one every 750 m2

(based on 4194 defects noted for a total evaluated area of 3,250,000m

2). An approximate average defect size of 5 cm

2was calculated from the defect number and size data

presented by Nosko and Touze-Foltz, assuming a minimum defect size of 0.5 cm2

and a maximum defectsize of 10 cm

2. Based on the greater number of lined facilities included in the Nosko and Touze-Foltz

data set and the more conservative data presented (i.e. greater defect size and number of defects) , theresults of their work was selected as a basis to estimate the defects in the liner at the LVU facilities.Accordingly, a defect rate of one 5 cm

2(0.0005 m

2) defect per 750 m

2was assumed.

Based on the assumed defect size and frequency, the equivalent geomembrane saturated hydraulic

conductivity incorporating defects was calculated using Eqn. 2 above , as presented below:.

The composite liner saturated hydraulic conductivity was then estimated using Eqn. 1 above, consideringthe equivalent geomembrane saturated hydraulic conductivity calculated considering defects, the GCLsaturated hydraulic conductivity presented in Table 2 and thicknesses of 2 mm and 6 8 mm for the

geomembrane and the GCL, respectively.


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model, as shown below:

2. Scenario 3 2 GCL OnlyThis analysis was performed as a bounding analysis to evaluate thesensitivity of the calculated flow rates to the liner defect assumptions made in Scenario 2 above byproviding an upper bound estimate of seepage flows with no liner. In this analysis, the GCL saturatedhydraulic conductivity presented in Table 2 above was converted to an equivalent saturated hydraulicconductivity value for use in the seepage model using Eqn. 3 above, considering a GCL thickness of 6 8

mm and a model liner thickness of 0.1 m.

This value was then used to represent the normal and tangential saturated hydraulic conductivity valuesfor the liner. This value is presented in Table 3 below.

1.3. Scenario 3Composite Liner with GCL and Geomembrane Damage In this analysis it is assumed

that the composite liner has been damaged and a portion of the geomembrane and GCL have beenremoved from the face of the dam. Potential causes for such an event include either operational damageor vandalism.

For this analysis, it was assumed that a 4 m by 4 m wide (along the dam face) square of liner materialwas removed from the U/S face of the dam It is assumed that the top of the damaged portion of the liner


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Table 3: Key Parameters Utilized in the Composite Liner Sensitivity Analyses

Analysis

Equivalent Saturated HydraulicConductivity Utilized in the Seepage

Analysis(m/s)

1 Composite Liner withGeomembrane Defects

kn, = 5.01x10-13

kt = 86.25.3x10

-170

2 GCL Only kn, kt = 61.725x10-10

3 Composite Liner with GCLand Geomembrane Damage

Liner Area:kn ,=5.01x10

-13

kt = 6.25x10-108.3x10-17

Damaged Area: No Liner

kn = Normal Saturated Hydraulic Conductivitykt = Transverse Saturated Hydraulic Conductivity

Boundary Conditions

Upstream Boundary ConditionsConstant head boundary conditions representing the impounded water level elevations were applied to each LVUfacility impoundment. The elevations utilized for the sections evaluated are presented in Table 1 above

Downstream Boundary ConditionsThe downstream boundary conditions consisted of a potential seepage face boundary condition. A potentialseepage face boundary acts as a no-flow boundary until it becomes saturated, after which flow will occur across


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Results

The calculated phreatic surfaces and gradients for each of the sections evaluated are provided in Attachment C.

The estimated seepage fluxes (per meter width of dam) and the estimated total seepage rates for flow throughthe LVU facilities and foundation materials are summarized in Table 4 below. The total seepage rates wereestimated by multiplying the unit seepage flux (m3/s/m) by an assumed effective width for each of the LVUfacilities. The calculations to develop these total seepage rate estimates are included in Attachment B.

Table 4: Estimated Seepage Rates

LVU Facility Sensitivity Analysis Seepage FluxRate(m3/s/m) Total Seepage*(m

3/s)

Total

Seepage*(L/s)

La Hierba

1. Composite Liner withGeomembrane Defects

1.14x10-6

9.69x10-5

0.09697

2 GCL Only 21.1745x10-6

1.8423x10-4

0.1842


Liner Area: 1.14x10-

Damaged Area: 0.0509

0.204 200204

East Rhyolite

1. Composite Liner withGeomembrane Defects

2.59x10-6

3.12x10-4

0.312

2 GCL Only 4.023.01x10-6

3.634.84x10-4

0.36484


Liner Area: 2.59x10-6


0.22216 21620

*Total seepage rates were estimated approximated based on valley and dam geometry (see Attachment B forcalculations)


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Conclusions

For both the La Hierba and East Rhyolite LVU facilities, the estimated total seepage rates are relatively similar.This result appears reasonable, as the geometry and foundation of the two LVU facilities are relatively similar.However, as noted in Table 4, the estimated seepage fluxes for the East Rhyolite LVU facility are slightly higher.It is surmised that these higher fluxes are due to the higher water depth in the East Rhyolite LVU facility and thefact that the base of the facility is founded in weathered bedrock, while the base of the La Hierba facility isfounded in glacial till, which was modeled with a higher saturated hydraulic conductivity.

Considering liner Scenarios 1 and 2, the estimated seepage rates for both the La Hierba and East Rhyolite LVUfacilities are less than 1 L/s. Additionally, the estimated seepage rates for sScenario 2 are relatively close tothose estimated for scenario Scenario 1, indicating that the seepage rates calculated in sScenario 1 would not bevery sensitive to increases in the assumed geomembrane defect frequency and defect size.

Considering liner Scenario 3, it is clear that the significant damage to the geomembrane liner and GCL modeledin the analyses (the removal of a 4 m by 4 m section of geomembrane and GCL from the dam face) has asignificant impact on the seepage from the LVU facilities, with seepage rates of 200 to 220 L/s estimated. Basedon the estimated seepage rates for this scenario, it is clear that any damage of this type would require repair toallow for continued use of the LVU facilities.

The analyses that were performed do not account for any degradation of the GCL due to the chemistry of theGCL permeant fluid (i.e. the fluid impounded in the LVU facilities). GCLs are known to be sensitive todegradation when in contact with a permeant that has extremely low or high pH values or contains highconcentrations of polyvalent cations, such as calcium (Thiel et al., 2001). It is recommended that the final designof these facilities include testing and analyses to evaluateimpact the effects of the permeant fluid on the GCL

performance.

As noted previously, the spatial extents of the foundation materials and a number of the material propertiesutilized in this analysis are based on assumptions and the information available and the time of the developmentof the feasibility level design for the La Hierba and East Rhyolite LVU facilities. These limitations should beconsidered when reviewing the results of these analyses and any analyses based on these results Additional


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References

Daniel, D.E. (editor), 1993. Geotechnical Practice for WasteDisposal, Chapman and Hall, London, UK. 1993.

Forget, B., Rollin, A.L., Jacquelin, T., 2005. Impacts and limitations of quality assurance on geomembraneintegrity, North American Geosynthetics Society GRI 19, Las Vegas, Nevada, December 14 16, 2005.

GEOSLOPE International, Ltd 2009. Program SEEP/W. Version 7.15.

Giroud, J.P., Badu-Tweneboah, K., Soderman, K.L., 1997. Comparison of Leachate Flow Through CompactedClay Liners and Geosynthetic Clay Liners in Landfill Liner Systems, Technical Paper, GeosyntheticsInternational, Vol. 4, Nos. 3-4, 1997.

Giroud, J.P., and Bonaparte, R., 1989. Leakage through Liners Constructed with Geomembranes, Geotextilesand Geomembranes, Vol. 8, pp. 71-111, Elsevier. 1989.

Microsoft, 2006. Microsoft Office Excel 2007, Part of Microsoft Office Professional Plus 2007, Version12.0.6545,5000, 2006.

MWH, 2010a. Cerro Corona Mine Stage 4 TMF Construction Design Report, Final Design Report, April2010.

MWH, 2010b. Cerro Corona Stage 4 TMF Design Las Gordas and Las Aguilas Dams Seepage Analysis,Revision B, Calculation brief, Revised 2/16/2010.

MWH, 2010c. "Cerro Corona Tailing Storage Facility - Offsite Laboratory Testing Summary," Report Developedby MWH for GFLC. August 2010.

MWH, 2011a. Feasibility Design Drawings Cerro Corona Low Volume Underflow Facility La Hierba &Rhyolite LVU Systems Design Drawings Prepared for GFLC by MWH Revision 0 Revised May 2011


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Client: Gold Fields La Cima S.A. Sheet: 13 of 32

Project: Cerro Corona MineStage 5 TSF Design

Date: 06/08/2011


Job No: 1004923

By: Josh Rogers

Document1J:\Clients_A-H\Gold Fields La Cima\1004923 Cerro Corona\Technical\Calculations\LVU Facilities\La Hierba and Rhyolite LVU Facil ities\Calculation Brief\La Hierba andRhyolite LVU Facilities Seepage Analyses R3.docx

Figure 1 La Hierba LVU Facility Section


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Figure 2 Rhyolite East LVU Facility Section


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Figure 3 La Hierba LVU Facility Section Scenario 3 Composite Liner with GCL and Geomembrane Damage


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Figure 4 East Rhyolite LVU Facility Section Scenario 3 Composite Liner with GCL and Geomembrane Damage


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Project:Cerro Corona MineStage 5 TSF Design

Date: 06/08/2011


Job No: 1004923

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Zone 2Zone 4

Zone 3

Glacial Till

Weathered

Bedrock

Bedrock

Topsoil

X-Conductivity(m/sec)


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Zone 2, 2A,

and 2B

Zone 3 and 4

Weathered

Bedrock

Glacial Till

Bedrock

Topsoil

Vol.WaterConten

t(m/m)

0.1

0.2

0.3

0.4


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Attachment B Total Seepage Rate Calculations


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Total Seepage Rate Calculations

To develop estimates of the total seepage from the La Hierba and East Rhyolite LVU Facilities, the seepage fluxesestimated in the seepage analyses were multiplied by assumed effective widths for the modeled sections. Forreference, the seepage fluxes calculated in the seepage analysis are presented in Table B.1 below.

Table B.1: Calculated Seepage Fluxes

LVUFacility

Sensitivity AnalysisSeepage RateFlux

(m3/s/m)

La Hierba

1. Composite Linerwith Geomembrane

Defects1.14x10

-6

2 GCL Only 1.452.17x10-6

3 Composite Linerwith GCL and

GeomembraneDamage

Liner Area: 1.14x10-6


EastRhyolite

1. Composite Linerwith Geomembrane

Defects2.59x10

-6

2 GCL Only 3.014.02x10-6

3 Composite Linerwith GCL and

GeomembraneDamage

Liner Area: 2.59x10-6Damaged Area: 0.0538

The effective widths utilized are based upon engineering judgment and include considerations of the shape of the


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Figure B.1 La Hierba LVU Facility Embankment Longitudinal Section


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Figure B.2 East Rhyolite LVU Facility Embankment Longitudinal Section


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Considering the reach widths presented on Figures B.1 and B.2 and the methodology described above, thecalculations to estimate the effective widths for the La Hierba and East Rhyolite LVU Facilities are as follows:

La Hierba

( ) (

)

East Rhyolite LVU

(

) (

)

For sensitivity analysis 3, the total seepage rate was estimated as the sum of the flow rate through the damagedportion of the liner plus the flow rate through the remaining portions of the liner. This rate was calculated bymultiplying the seepage flux calculated for the damaged area case by the assumed longitudinal damage width of4 m and then adding this calculated seepage flow rate to seepage flow rate calculated by multiplying the liner areaseepage flux by the remaining portion of the assumed effective section width (i.e. the total assumed effective widthminus 4 m). The calculations to estimate the total seepage rates are presented below for each of the 3 sensitivityanalyses performed for the La Hierba and East Rhyolite LVU Facility embankment sections evaluated.

La Hierba LVU Facility

Sensitivity Analysis 1: Composite Liner with Geomembrane Defects


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East Rhyolite LVU Facility

Sensitivity Analysis 1: Composite Liner with Geomembrane Defects

Sensitivity Analysis 2: GCL Only

Sensitivity Analysis 3: Composite Liner with GCL and Geomembrane Damage


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Attachment C Results of Seepage Analyses


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Client: Gold Fields La Cima S.A. Sheet: 27 of 32Project:

Cerro Corona MineStage 5 TSF Design

Date: 06/08/2011


Job No: 1004923

By: Josh Rogers

Document1J:\Clients_A-H\Gold Fields La Cima\1004923 Cerro Corona\Technical\Calculations\LVU Facilities\La Hierba and Rhyolite LVU Facilities\Calculation Brief\La Hierba andRhyolite LVU Facilities Seepage Analyses R3.docx

La Hierba LVU Facility Embankment Section Seepage Analysis ResultsLiner Scenario 1 Composite Liner with Geomembrane Defects

1.1371e-006m

/sec

Comment [M1]: THIS IS A NEW OUTPUTFIGURE UPDATE THE ORIGINAL FILEWITH THIS


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La Hierba LVU Facility Embankment Section Seepage Analysis ResultsLiner Scenario 2 GCL Only

2. 1

744e-006m/s

ec



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La Hierba LVU Facility Embankment Section Seepage Analysis ResultsLiner Scenario 3 Composite Liner with GCL and Geomembrane Damage

0.0509m/sec



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East Rhyolite LVU Facility Embankment Section Seepage Analysis ResultsLiner Scenario 1 Composite Liner with Geomembrane Defects

2.59

37

e -006

m/s

ec



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East Rhyolite LVU Facility Embankment Section Seepage Analysis ResultsLiner Scenario 2 GCL Only

4.0

236e

-

006

m/s

ec



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East Rhyolite LVU Facility Embankment Section Seepage Analysis ResultsLiner Scenario 3 Composite Liner with GCL and Geomembrane Damage

0.053

7

65m/s

ec


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