Cerro Corona Relavera

7/25/2019 Cerro Corona Relavera

1/81

Go

S

ld Fields Livenida Via

San

CERR

AGE 4

DE

Prepared fo

ited La Cirincipal Nsidro, Limael: (511) 70

COR

MF CO

IGN R

DRAF

r:

ma S.A. (Gero 155 Int

27 Per-0400

NA MI

STRU

PORT

LCSA). 1401

E

TION


2/81

March 2010 Gold Fields La CimaCerro Corona Mine - Stage 4 TMF Raise 3740 m Page i

TABLE OF CONTENTS

Section No. Page No.

1.0 INTRODUCTION ......................................................................................................... 1

1.1

BACKGROUND ............................................................................ ............................................... 1

1.2

MWH SCOPE OF WORK ...................................................... ....................................................... 4

1.3 EXCLUDED WORK ................................................... ........................................................... ....... 51.4 LIMITATIONS ......................................................... .............................................................. ....... 6

2.0

DESIGN STEWARDSHIP, OBJECTIVES, CRITERIA, AND BASIS ................... 7

2.1 DESIGN STEWARDSHIP ............................................................................................ ................ 72.2

DESIGN OBJECTIVES ........................................................................................................... ..

... 8

2.3 DESIGN CRITERIA ............................................ ................................................................ ......... 92.4

DESIGN BASIS ........................................................... .......................................................... ...... 11

3.0 GENERAL SITE CHARACTERIZATION ............................................................. 12

3.1

PROJECT LOCATION AND TOPOGRAPHY ....................................................... ................... 12

3.2

SITE LAYOUT .......................................................... ............................................................ ...... 123.3 WATER RECOVERY AND TAILING PRODUCTION AND TRANSPORT .......................... 13

3.4 CLIMATE ....................................................... ................................................................ ............. 133.5 SITE HYDROLOGY .................................................... ........................................................... .... 143.6

GEOLOGY ..................................................................................... ............................................. 14

3.7

SEISMICITY ......................................................................................... ...................................... 15

3.8

HYDROGEOLOGY ............................................................ ........................................................ 15

3.9 GEOCHEMISTRY .................................................................................... .................................. 17

4.0 DESCRIPTION OF THE STAGE 4 TMF RAISE ................................................... 18

5.0

PROPOSED FUTURE TMF RAISES ....................................................................... 20

5.1 PROPOSED TMF CONSTRUCTION SCHEDULE ................................................................. .. 205.2

DESCRIPTION OF FUTURE TMF RAISES ............................................................... .............. 21

5.3

DOWNSTREAM EFFECTS OF PROPOSED ULTIMATE DAM DESIGN ............................. 22

6.0 UPDATED MATERIAL PROPERTIES .................................................................. 24

6.1

INTRODUCTION .................................................................................... ................................... 24

6.2

TAILING EVALUATION ............................................................. ............................................. 24

6.3

EVALUATION OF ROCKFILL STRENGTH ..................................... ...................................... 26

6.4 ZONE 1 AND 5 HYDRAULIC PROPERTIES........................................................................... 27

7.0 ENGINEERING ANALYSES .................................................................................... 28

7.1 INTRODUCTION .................................................................................... ................................... 287.2 SEEPAGE ANALYSIS .............................................................................. ................................. 28


3/81

March 2010 Gold Fields La CimaCerro Corona Mine - Stage 4 TMF Raise 3740 m Page ii

8.0 REVIEW OF INDUSTRY PRACTICE .................................................................... 46

9.0

CONSTRUCTION QUANTITY ESTIMATES ........................................................ 48

9.1 INTRODUCTION .................................................................................... ................................... 489.2 ESTIMATED STAGE 4 TMF RAISE CONSTRUCTION QUANTITIES ................................ 489.3

ESTIMATED FUTURE TMF RAISE CONSTRUCTION QUANTITIES ................................. 50

10.0 CONCEPTUAL TAILING DEPOSITION PLANS AND ESTIMATED TMF

STORAGE CAPACITIES .......................................................................................... 51

10.1 INTRODUCTION .................................................................................... ................................... 5110.2

TAILING DEPOSITION PLANS ............................................................. .................................. 52

10.3

LARGE STRAIN CONSOLIDATION MODELING ........................................... ...................... 5310.4

ESTIMATED TMF CAPACITIES ................................................................... ........................... 54

11.0 REFERENCES ............................................................................................................ 56


4/81

March 2010 Gold Fields La CimaCerro Corona Mine - Stage 4 TMF Raise 3740 m Page iii

LIST OF TABLES

Table No. Description

1 Stage 4 TMF Design Criteria2 Stage 4 TMF Design Basis3 Summary of Proposed TMF Construction Schedule4 Estimated Strength of Materials in Tailing Profile5 Summary of Saturated Hydraulic Conductivities Utilized in the Seepage Analysis6 Summary of Estimated Seepage Rates

7 TMF Stages Evaluated in the General Stability Analysis8 Summary of Strength Parameters Utilized in Each Loading Condition9 Summary of Earthwork Material Properties Utilized in the General Stability

Analysis

10 Summary of Geosynthetic Parameters Utilized in the General StabilityAnalysis11 Summary of the Results of the General Stability Analysis12 Summary of Estimated Construction Quantities Stage 4 TMF Raise13 Summary of Estimated Construction Quantities Future TMF Raises

14 Summary of Estimated TMF Capacities15 Contribution of Design Changes to Capacity Increase

LIST OF FIGURES

Figure No. Description

1 Site Location2 Overall Site Plan Stage 4 TMF3 Las Aguilas Dam Section A4 Las Gordas Dam Section B5 Proposed TMF Construction Schedule (Conceptual Level)6 Ultimate TFM Dam Plan (Conceptual Level)7 Ultimate Las Aguilas Dam Section C (Conceptual Level)8 Ultimate Las Gordas Dam Section D (Conceptual Level)9 Ultimate La Hierba Dam Section E (Conceptual Level)10 Ultimate Dam Details (Conceptual Level)11 Original and Revised Ultimate Dam Plan (Conceptual Level)12 Original and Revised Ultimate Dam Sections F and G (Conceptual Level)13 Comparison of Rockfill Strength Envelopes14 Comparison of Estimated PGA and Downstream Slopes for Selected Dams15 Comparison of Height and Downstream Slopes for Selected Dams


5/81

March 2010 Gold Fields La CimaCerro Corona Mine - Stage 4 TMF Raise 3740 m Page iv

LIST OF APPENDICES

Appendix Description

A Tailing Evaluation

B Evaluation of Rockfill Strength

C Seepage Analysis

D Stability Analyses

E Preliminary Seismic Displacement Analysis

F Review of Industry Practice Rockfill Dam Downstream Slopes

G Tailing Deposition Plans and Estimated TMF CapacitiesH SICTA Testing and Large Strain Consolidation Modeling

I Construction Drawings and Specifications

CONSTRUCTION DRAWINGS AND SPECIFICATIONS(Revision 1 Dated March 1, 2010)

Drawing No. Title

1 Cover Sheet and Index of Drawings

2 Overall Site Plan

3 Stage 4 TMF Dam Plan

4 Las Aguilas Dam Section A

5 Las Gordas Dam Section B

6 Dam Transition Zone Profile C

7 Dam Transition Zone Plan Views (Sheet 1 of 2)8 Dam Transition Zone Plan Views (Sheet 2 of 2)

9 Dam Transition Zone Section D

10 Dam Transition Zone Section E

11 Dam Transition Zone Section F

12 Dam Transition Zone Section G

13 Dam Transition Zone Section H

14 Dam Crest Details

15 Las Gordas Upstream Face Reinforcement Detail16 La Hierba Dam Foundation Excavation

17 La Hierba Dam Plan

18 La Hierba Dam Sections J and K

19 La Hierba Dam Sections L and M

20 La Hierba Sections N and P

21 Estimated Construction Quantity Summary


6/81

March 2010 Gold Fields La CimaCerro Corona Mine - Stage 4 TMF Raise 3740 m Page v

LIST OF ACRONYMS

CDA Canadian Dam AssociationCFRD Concrete Faced Rockfill Dam

CPT Cone Penetration Test

CST Cleaner scavenger tailing

GFLCSA Gold Fields La Cima S.A.

EFT Extra Fine Tailing

FHWA Federal Highway Administration

GSHAP Global Seismic Hazard Assessment Program

H:V Horizontal to Vertical Unit Slope RatioHDPE High-density polyethylene

ICOLD International Commission on Large Dams

KP Knight Piesold

LVU Low volume underflow

MCE Maximum Credible Earthquake

MDE Maximum Design Earthquake

MHW MWH Americas, Inc.

OBE Operating Basis Earthquake

PMF Probable Maximum Flood

PMP Probable MaximumPrecipitation

PMP Probable Maximum Precipitation

PVC Polyvinyl Chloride

RST Rougher scavenger tailing

SENAMHI Servicio Nacional de Meteorologa e Hidrologa

TMF Tailing Management Facility

UCB Upstream Containment Blanket

USACE US Army Corps of EngineersUSFS US Forest Service

VST Vane Shear Testing

WMC Water Management Consultants

LIST OF UNITS

ha Hectares

km Kilometer

l/s Liters per second

m Meter

masl Meters above mean sea level

t/d Tonnes per day


7/81

March 2010 Gold Fields La CimaCerro Corona Mine - Stage 4 TMF Raise 3740 m Page 1

1.0 INTRODUCTION

1.1 BACKGROUND

Gold Fields La Cima S.A. (GFLCSA), a subsidiary of Gold Fields Limited, owns the

Cerro Corona mine, a copper mine with significant gold content. The mine is located in

northern Peru, approximately 760 km NNW of Lima and 80 km by road from the city of

Cajamarca, as shown in Figure 1. The mine is currently in operation with further mine

development underway and has an expected life of 14 years. The mine consists of an open

pit, a 20K tonne per day (tpd) concentrator and related ancillary infrastructure. Tailingproduced by the Cerro Corona processing plant will be stored in the tailing management

facility (TMF), located just north of the plant site in the Quebrada Las Aguilas and

Quebrada Las Gordas valleys.

The process engineering for the project was performed by Hatch out of their Santiago,

Chile office. Design of the earth and rock structures, which includes the TMF, was

initially performed by Knight Piesold (KP), who completed a design report in early 2006

(KP, 2006). While GFLCSA views KPs design for the TMF to be technically sound, theyfound it to be challenged with many constructability issues such as the timing for

construction around the wet season, the ability to install a grout curtain at the base of the

starter dams and still provide time for construction of the rockfill, placement and

compaction of a low permeability core for the dams during the wet season, and the

availability of suitable rockfill. Accordingly, in January of 2007, GFLCSA contracted

MWH Americas, Inc. (MWH) to develop construction level designs for a rockfill starter

dam to impound tailing during startup and early operations of the mine.

In November of 2008, subsequent to work performed by MWH and KP, MWH submitted

a design package that included a design report and construction drawings to support

construction of the TMF Starter Dam (MWH, 2008). The design included detailed

designs for the Starter Dam and conceptual level designs for future raises above the

starter dam elevation. Starter dam construction was defined as construction of the TMF to

an elevation of 3720 meters above mean sea level (masl), which would require the

construction of two dams, one in the Quebrada Las Aguilas and one in the Quebrada LasGordas. As noted in the report, detailed engineering, construction drawings, and

specifications were not developed for dam raises above the starter dam level.

The construction concept presented in the Starter Dam design report is as follows:


8/81


Stage 3 through Completion (conceptual) After Stage 2, the tailing dam wasto be raised in a series of annual center-line raises, eventually reaching the

ultimate dam elevation of 3800 masl. The raised dam was to consist of rockfillwith a vertical core. A major component of this method of dam raise is the

presence of a well-drained tailing beach with sufficient strength to serve as part of

the foundation for future dam raises.

Construction of the Las Gordas Starter Dam began in April of 2007 and was completed in

August of 2008 with sub-aqueous tailing deposition into the Quebrada Las Gordas

beginning shortly thereafter. During operations to date, the properties of the produced

tailing have varied from those utilized in the Starter Dam design. This variance is largelyattributed to two factors:

Increased alteration of the mined ore, which has resulted in a higher fines contentof the produced tailing than was expected. This has resulted in lower than

expected rates of flocculation and settlement of the deposited material.

Due to the large volume of initial water in the Las Gordas basin and the very large

rate of rise during the initial filing of the Las Gordas basin, a sub-aerial beach atthe face of the dam has not formed and therefore limited segregation of the tailing

has occurred.

As a result, the tailing densities that are currently being achieved are less than those

estimated during the starter dam design. Both visual observations and investigations of

the beach indicate a material with a relatively low strength and solids content. A

discussion regarding the investigation and evaluation of the tailing properties is included

in Section 6.2.

As a result of the variance in tailing properties, the TMF construction plan laid out in the

Starter Dam design (which assumed the presence of sufficiently strong tailing beaches for

construction of centerline raises for post Stage 2 construction) has been revised. MWH

has developed additional design packages for construction of the TMF that include

revisions to the original Starter Dam design and designs for further raises. These include

design drawings and field instructions detailing the raising of the TMF to elevation 3732

masl (MWH, 2009a and 2009b) and the construction of the Las Flacas dam (MWH,2009c).

The following presents a brief summary of the current construction concept and the

current status of construction for the TMF and the revised stage numbering and sequence:


9/81


Jalca Tailing, a historic tailing pile located near the upstream toe of the Las

Aguilas Starter Dam. Construction of this zone will form a 25 m wide bench at its

maximum elevation of 3732 masl and will include cutting and regrading theexisting La Jalca Tailing. Additionally, Stage 2 includes the Las Aguilas LVU

facility, which is currently under construction in the valley downstream of the

TMF dam. Stage 2 work is currently underway and is based upon the design

drawings developed by MWH (2009a).

Stage 3 (Constructed) Stage 3 construction includes the raising of the LasGordas Dam to an elevation of 3732 masl and construction of the Las Flacas Dam.

Construction of the Stage 3 Las Gordas Dam raise was completed in September of2009. Due to the lack of a suitable tailing beach against the upstream face of the

Las Gordas dam, the dam was raised using the optimized centerline method with a

geosynthetically reinforced upstream face, as presented in the 3732 m raise design

prepared by MWH (2009a) and a field instruction from MWH (2009b). The raise

method is described as an optimized centerline raise as it still relies on the

tailing material for upstream stability but to a lesser extent than the centerline

method.

Stage 3 construction also includes the construction of the Las Flacas dam, whichwas completed in January of 2010. The Las Flacas Dam is a rockfill dam with a

geomembrane lined face located on the Las Flacas ridge to the west of the Las

Gordas Dam. The purpose of this dam is to form a temporary impoundment in the

Quebrada Las Gordas to allow for storage of tailing materials while construction

of the Las Aguilas Starter Dam is underway. It is expected that a channel will be

excavated through the Las Flacas Dam after the Las Aguilas Dam is constructed

to allow for tailing deposition into the Quebrada Las Aguilas.

Stage 4 (Proposed) After completion of Stage 2 and 3 TMF construction, boththe Las Aguilas and Las Gordas Dams will have been constructed to an elevation

of 3732 masl. To continue to increase the capacity of the impoundment, the TMF

will be raised to an elevation of 3740 masl. The selection of the Stage 4 crest

elevation is based upon conceptual level TMF construction scheduling performed

by MWH, as discussed in Section 5.1.

The Stage 4 raise will be constructed using the optimized centerline method forthe Las Gordas portion of the TMF dam with an upstream slope of 0.8 horizontal

units to 1 vertical unit (0.8H:1V). As the La Jalca Tailing bench will provide

upstream support, the Las Aguilas portion of the TMF dam will be raised using

the centerline method, with vertical internal zones and an upstream rockfill face


10/81


The design of the Stage 4 TMF is presented in this report and is discussed furtherin Section 4.0. Seepage and stability analyses and construction quantity estimates

were performed for the Stage 4 TMF raise. The results of these evaluations areincluded in Sections 7 and 9, respectively.

Figure 2 presents an overview of the site with the proposed Stage 4 TMF.Sections through the Stage 4 Dam in the Las Aguilas and Las Gordas valleys are

presented in Figures 3and 4, respectively.

Future Raises (Conceptual) As the design of the Stage 4 TMF raise and futureraises are interdependent, conceptual level designs were developed for future

TMF raises. The proposed design concept assumes the centerline method will not

be suitable for any remaining raises due to the presence of weak tailing throughout

the life of the mine. This design represents a contingency plan as it is likely that

conditions in the impoundment will improve with time due to the addition of

flocculant and the increasing size of the impoundment.

The post-Stage 4 design concept consists of raising the dam using the optimizedcenterline method with an upstream slope of 0.6H:1V and a downstream slope of

1.4H:1V. The upstream slope will be reinforced with geosynthetic and gabions to

achieve global and surficial stability.

A conceptual level schedule was developed to evaluate the required future raisingschedule while maintaining the required freeboard and minimizing the required

wet season construction work. Additionally, seepage and stability analyses and

quantity estimates were performed for the future dam raises. The results of these

evaluations are included in Sections 7 and 9, respectively. The proposed raising

schedule and TMF construction for future raises are described in further detail in

Section 5. Further information regarding the proposed design for future facility

raises is included in Section 5.

Figure 6 presents a plan view of the proposed conceptual level ultimate TMFdam. Ultimate dam sections through the Las Aguilas, Las Gordas, and La Hierba

valleys are presented in Figures 7, 8and 9, respectively.

1.2 MWH SCOPE OF WORK

The lack of a consolidated and well drained beach (to provide a suitable foundation for

future raises) has necessitated a review of the previously completed conceptual design to

identify construction methods for subsequent raises of the dam that will allow for storage


11/81


Review and refinement of the dam material properties utilized in previousanalyses based on newly available information.

Completion of engineering analyses and a review of industry practice to supportany proposed revisions to the dam configuration.

Development of a detailed design complete with construction quantity estimates,construction drawings, and construction specifications for raising the TMF dam to

an elevation of 3740 masl, termed Stage 4 construction.

Compilation of a design report to summarize the Stage 4 TMF raise design and

present the work performed in support of the design.

Development of conceptual level designs to construct the TMF to its ultimateelevation, assuming that the optimized centerline design will be utilized

throughout the construction of the dam.

Development of conceptual level tailing deposition plans and capacity estimatesfor the Stage 4 TMF and future TMF raises to be used in mine planning and

design of pumping and piping systems.

1.3 EXCLUDED WORK

The work performed in this study is limited to the development of a detailed design to

raise the Cerro Corona TMF to an elevation of 3740 masl and the development of

conceptual level designs for future raises. The scope of work does not include the

following:

Detailed Design for Facility Raises Beyond 3740 masl This project does notinclude detailed designs for future facility raises (post-Stage 4). However, in order

to provide a feasible future construction concept and as the design of the Stage 4

TMF and future raises are interdependent, future facility construction was

considered on a conceptual level.

Ancillary Construction The construction of complementary items such as

access roads, expanded borrow facilities, and staging areas were not consideredand are not included as a part of this report.

Hydrology and Water Management The design presented in this report is notthought to deviate significantly from that presented in the starter dam report in

terms of hydrology and water management. However, it is recommended that the


12/81


Facility Closure As the design presented in this report does not deviatesignificantly from that presented in the starter dam report in terms of closure, no

revisions to the closure plan presented in that report were deemed necessary.

1.4 LIMITATIONS

This document contains the expression of the professional opinion of MWH as to the

matters set out herein, using its professional judgment and reasonable care. It is to be read

in the context of the agreement and signed amendments (the Agreement) dated

November 28, 2005 between GFLCSA and MWH, and the methodology, procedures andsampling techniques used, MWHs assumptions, the circumstances and constraints under

which its mandate was performed. This document is meant to be read as a whole, and

sections or parts thereof should thus not be read or relied upon out of context.

Additionally, this report should be read in conjunction with the Starter Dam design report

developed by MWH (2008).

Professional judgments are presented in this report. These are based partly on evaluation

of technical information gathered, partly on our experience with similar projects, andpartly on our understanding of the characteristics of the project. The findings,

interpretations of data, recommendations, professional opinions, and conclusions that are

presented are within the limits prescribed by available information at the time the

analyses and designs were prepared, in accordance with generally accepted professional

engineering practice. In the event that there are any changes in the nature, design, or

characteristics of the project, or if additional data are obtained, the conclusions and

recommendations contained in the report will need to be reevaluated by MWH in light ofthe proposed changes or additional information obtained. Variations from results

presented in the report should be expected due to uncertainties that are inherent in these

types of analyses. Therefore, decisions that are based on these results should consider

these variations as well as limitations of the analyses to predict future performance with a

high degree of accuracy.

Unless expressly stated otherwise, assumptions, data and information supplied by, or

gathered from other sources (including GFLCSA, other consultants, testing laboratoriesand equipment suppliers, etc.) upon which MWHs opinion as set out herein is based has

not been verified by MWH and MWH makes no representation as to its accuracy and

disclaims all liability with respect thereto.

MWH disclaims any liability to any third party in respect of any reliance on this


13/81


2.0 DESIGN STEWARDSHIP, OBJECTIVES, CRITERIA,AND BASIS

2.1 DESIGN STEWARDSHIP

The Cerro Corona TMF is a complex facility consisting of many components which rely

on strict adherence to stringent standards for construction quality control, construction

quality assurance, compliance with safe operating parameters, performance monitoring

and management accountability. The design of this facility assumes that minimum levels

of stewardship will be maintained during the construction, operation, closure and postclosure phases of the project. Compliance with these minimum standards is an integral

component of the design. Failure to comply with these minimum standards shall be

considered to be a material deviation from the design intent and will release MWH from

any responsibility related to the ongoing operation or performance of the facility. The

minimum standards shall include the following:

Compliance with the following standards for construction, operation and care of

tailing impoundments:

o Mining Association of Canada, A Guide to the Management of TailingsFacilities, September 1998

o Australian National Committee on Large Dams, Guidelines on Tailings DamDesign, Construction, and Operation, October 1999

o Mining, Minerals and Sustainable Development, Stewardship of Tailings

Facilities, April 2002

o ICOLD, Increasing Tailings Dam Safety, Critical Aspects of Management,Design, Operation and Closure

An Engineer of Record who is familiar with the design intent, all design drawingsand specifications and relevant standards shall be employed to ensure that all

construction, operation and closure activities, as well as any design modifications

or interpretations are consistent with the original design intent and any applicable

standards and/or regulations.

The owner will prepare an operating strategy, operating manual, operatingprocedures, safe work procedures and emergency response plan which must, in

the opinion of the Engineer of Record, comply with the original design intent.

Failure to build and/or operate the facility in accordance with these documents


14/81


independent review board or the engineer of record, may be considered a

significant departure from the design.

These minimum standards form an integral part of this design.

2.2 DESIGN OBJECTIVES

The main objective of the TMF design presented in this report to develop construction-

level designs and construction documents for raising the Cerro Corona TMF to an

elevation of 3740 masl in an environmentally responsible manner in accordance withPeruvian and International practices.

Additional objectives include:

Satisfy relevant local and international design criteria associated with TMFdesign, construction, and operation

Design and construct the TMF in accordance with internationally recognizedstability criteria

Design the TMF so that it does not impact the operation of the Las Gordas or LasAguilas LVU facilities.

Design a facility that can be constructed with material from onsite borrow sources

Develop conceptual level designs to construct the TMF to its ultimate elevationwhile considering the potential for weak tailing throughout the life of the facility,

allowing for freeboard requirements, and minimizing wet season construction.

The design criteria and basis for the Stage 4 TMF raise are generally based upon those

developed for design work previously performed by MWH. The following sections

summarize the design criteria and basis adopted for the Stage 4 TMF raise. The following

documents were used in the development of the design basis and criteria and are

referenced as used in the design criteria and basis summary tables:

1. Knight Piesold, 2006. Mine Waste and Associated Water Management FacilitiesReport on Design, Final report prepared for Gold Fields La Cima S.A. January

18, 2006.

2. MWH, Review of Existing Data and Engineering Judgment


15/81


6. MWH, 2005. Cerro Corona Frost Depth Analysis, January 10, 2005.

7. Independent Geotechnical and Tailing Review Board (IGTRB), Draft ThirdReport.

8. US Army Corps of Engineers (USACE), 2003. Engineering and Design SlopeStability, Engineering Manual 1110-2-1902. October 31, 2003.

9. Canadian Dam Association (CDA), 2007. Dam Safety Guidelines. 2007.

10.MWH, Results of Cerro Corona pH Monitoring Program (as of January 5th, 2010).

11.MWH, 2010. Cerro Corona Low Volume Underflow Facility Las AguilasValley, Design Drawings, Revision 1. February, 2010.

12.MWH, 2008. Cerro Corona LVU Dam Design, Design Drawings, Revision 3.September, 2008.

2.3 DESIGN CRITERIA

Following previous work performed by MWH, the Stage 4 TMF raise was designed

following the following national standards and other internationally accepted guidelines.

The following Ministry of Energy and Mines (MEM) standards and guidelines were used

to develop this design basis document:

Ministerial Resolution No. 0.5-95-EM-DGAA Governing the Design andReporting of Tailing Facilities (1995).

Guidelines for Preparing Environmental Studies for Mineral Tailing (1995).

Mine Closure Regulations (2003).

MWH also utilized the following international standards to develop the Stage 4 TMF

design:

Canadian Dam Safety Guidelines

Nevada Division of Water Resources Regulating and Permitting GuidelinesPertaining to the Safety of Dams


16/81


The design criteria that have been adopted for the Stage 4 TMF design are presented in

Table 1.

Table 1 Stage 4 TMF Design Criteria

ITEM CRITERIADATA

SOURCE(1)

General

Facility Hazard CategoryVery High Canadian Dam Association

1High Nevada Division of Water Resources

Seismicity

Design EarthquakeArequipa, Peru 197, Bokajan, Burma, 1988, and Panchimilco,

El Salvador.7

Operating Basis Earthquake (OBE) 1/475 yr event 5

Maximum Design Earthquake (MDE)Maximum Credible Earthquake (MCE)

M = 8.07

Hydrology

Facility Design Storm PMP, 24 hr event 3

PMF Flood volume resulting from occurrence of the 24-hr PMP event 3

Facility Configuration and Construction

FreeboardVertical distance from maximum estimated level of reclaim

pond and the PMF to the dam crest2

TMF CapacityStore tailing, maximum estimated reclaim pond volume, and

PMF2

Stability Requirements

Static Dam Stability USACE Stability Recommendations 8

Post-Cyclic Dam Stability CDA Dam Safety Guidelines 9


Dam Construction Material Source On-site Borrow 2

Ultimate TMF Dam MaximumDownstream Toe Location

Will not impact the performance of the Las Gordas Low VolumeUnderflow (LVU) facility or the Las Aguilas LVU facility

(currently under construction)2

1. Data source numbers refer to the list of reference documents listed in Section 2.2.


17/81


2.4 DESIGN BASIS

The design basis that has been adopted for the Stage 4 Cerro Corona TMF design ispresented in Table 2.

Table 2 Stage 4 TMF Design Basis

ITEM BASISDATA

SOURCE(1)

General Site Characteristics

Location 900 km NNW of Lima, Peru 1

Elevation 3500 to 4000 masl 1

SeasonsWet Season - October through March 2

Dry Season - April through September 2

Minimum Design Temperature -1.0oC 1

Maximum Design Temperature 20.0oC 1

Frost Depth 0.0 m 6

Minimum Design Humidity 32% 1

Maximum Design Humidity 100% 1Minimum Annual Design Precipitation 398 mm 1

Maximum Annual Design Precipitation 3,104 mm 1

Maximum Design Wind Speed 40 km/hr 4

Prevailing Wind Direction East/East-Northeast 4

TMF pH Range 10.5 - 12 10

Seismic Conditions

Operating Basis Earthquake (OBE) 0.24g (1/500 yr) 5 and 7

Maximum Design Earthquake (MDE) 0.50g 5 and 7

Minimum Stability Requirements

Minimum Static Factor of Safety (End of Construction) 1.3 8

Minimum Static Factor of Safety (Long-Term) 1.5 8

Minimum Pseudo-Static Factor of Safety None Based on Deformations

Minimum Post Cyclic Factor of Safety 1.2 9

Hydrologic Conditions

PMF501,291 m3(Las Aguilas Valley) 3

485,709 m3(Las Gordas Valley) 3


Freeboard 2 m 2

Minimum Downstream Dam Slope 1.4H:1V 2


18/81


3.0 GENERAL SITE CHARACTERIZATION

The following subsections present a brief summary of the project setting and are basedupon more detailed information presented in the TMF Starter Dam design report

developed by MWH (2008), unless noted otherwise.

3.1 PROJECT LOCATION AND TOPOGRAPHY

The Cerro Corona mine is located in northern Peru, approximately 760 km NNW of Lima

and 80 km by road from the city of Cajamarca. The mine site is in the Department ofCajamarca, approximately 1.5 km west-northwest of the village of Hualgayoc, as shown

in Figure 1. The project site is typical of mine sites in the Peruvian Andes with elevations

ranging from 3,500 to 4,000 masl. The site topography ranges from shallow valley floors

sloping at 5 degrees or less to steep rock bluffs sloping at angles up to 70 degrees.

Overall, the local topography slopes at an average of 10 to 35 degrees.

3.2 SITE LAYOUT

The Cerro Corona site is relatively compact, covering an area of approximately 200

hectares (ha). The site contains two major valleys, the Quebrada Las Gordas and

Quebrada Las Aguilas, which are separated by a north-south trending ridge, known as the

Las Flacas Ridge. The Las Flacas Dam was constructed along the Las Flacas ridge to

separate the Las Gordas and Las Aguilas impoundments. A rhyolite quarry is in operation

along the northern edge of the Las Gordas impoundment and the Corona open-pit mine is

located in the eastern portion of the site.

The westernmost valley, the Quebrada Las Aguilas, contains a historic tailing deposit

from previous mining activities at the site, known as the La Jalca Tailing. The Quebrada

Las Aguilas generally flows south to north with drainage exiting into the Tingo River.

The Las Aguilas Starter Dam is currently under construction to an elevation of 3732 masl,

with La Jalca tailing being placed and compacted against the upstream face of the dam.

Construction of this dam will serve to form the Las Aguilas portion of the Cerro Corona

TMF.

To the east of the Quebrada Las Aguilas is the Quebrada Las Gordas. This valley flows

southeast to northwest, with drainage exiting into the Tingo River. The Las Gordas

Starter Dam has been constructed and subsequently raised to an elevation of 3732 masl in

th d t ti f thi ll th Ti Ri T ili i tl b i


19/81


Figure 2presents an overview of the site, including the major site components along with

the proposed Stage 4 TMF.

3.3 WATER RECOVERY AND TAILING PRODUCTION AND TRANSPORT

The TMF stores both rougher scavenger tailing (RST) and cleaner scavenger tailing

(CST). The RST is produced from the rougher flotation cells after a scavenging stage and

makes up the majority of the tailing produced in the project (approximately 85% to 95%

of the total tailing stream). The CST is produced from the cleaner flotation cells after a

scavenging stage and has a finer gradation as it is passed through a regrind stage. The

RST has a lower sulfide content as most of the sulfide mineralization is floated off in the

rougher circuit. At deposition into the TMF, the pH of the tailing streams is elevated due

to the addition of lime to the process.

The potential for acid generation from the CST is high while the potential for acid

generation from the RST is considered negligible. Currently, the CST is being co-mingled

with the RST material and is being deposited sub-aerially. It is understood that the co-

mingling of the RST and CST tailing will only be allowed during the early years ofoperations. Later in the mine life, the tailing streams will be separated and the CTS tailing

will be deposited sub-aqueously to prevent ARD generation.

Currently, the combined RST and CST tailing streams are being thickened to a solids

content of approximately 55% by weight prior to discharge into the TMF. The RST

generated at the concentrator plant is delivered to the tailing thickener area by means of a

gravity line. After thickening, the material is conveyed to the RST feeder box where it is

combined with the CST and then transported via a high-density polyethylene (HDPE)

gravity pipeline to its final discharge at the TMF. The RST delivery system includes a

series of drop pipes that enable the dissipation of excess energy.

Water is removed from the surface water pond and reclaimed to the mine process circuit

by floating decant pumps located in the TMF. As noted previously, slow rates of

flocculation and settlement of the tailing materials have reduced the pumping rates from

the facility.

LVU collection facilities are located downstream of both the Las Gordas and Las Aguilas

dams (the Las Aguilas LVU facility is currently under construction). The purpose of these

facilities is to collect seepage water from the tailing impoundments. Water collected in

the Las Aguilas LVU facility will be pumped to the Las Gordas LVU facility From this


20/81


Based on an adjustment for elevation of the data from the Hualgayoc meteorological

station, average annual precipitation is reported as 1,398 mm with the wet and dry year

totals being 2,423 and 907 mm, respectively. An average annual catchment evaporationof 507 mm has been calculated based on a 65% average annual pan evaporation estimate.

A new meteorological station has been installed at the site by GFLCSA and has been

taking site-specific data readings for over 2 years. This station will operate over the full

life of the mine and its data will be used to refine current data sets used for calculating

future water balances for the operation as well as for planning and designing the staged

expansions of the TMF, general project improvements and closure planning.

3.5 SITE HYDROLOGY

In the Cerro Corona project area, six sub-catchments can be defined: Quebrada Las

Aguilas, Chorro Blanco, Las Gordas, Las Flacas, Mesa de Plata, and Corona. The

Hualgayoc River, located south of the site, originates at an elevation of over 3,700 masl at

Cerro Coyomolache. The river flows from the southwest to the northeast. Both the

Quebrada Mesa de Plata and the Quebrada Corona sub-catchments flow to this river. TheCerro Corona pit is located on the watershed between the Quebrada Mesa de Plata sub-

catchment and the Quebrada Corona sub-catchment.

The Quebrada Las Gordas, Las Aguilas, and Las Flacas drain to the Tingo River. The

Tingo River is the predominant river in this area. It originates at an elevation of 3,900

masl on the flanks of the Cerros de Tantahuatay. The river, which flows from east to

west, has a catchment area of 9 square kilometers (km2) above its confluence with the

Quebrada Las Aguilas. The river has an average annual flow of 241 liters per second (l/s)

downstream of its intersection with the Quebradas Las Aguilas and Las Gordas. The TMF

is located in the Las Gordas and Animas sub-catchments. The Hualgoyoc River originates

above 3,700 masl at Cerro Coyomolache and flows from the southwest to the northeast.

Site hydrology studies were completed by both KP and Water Management Consultants

(WMC). As WMC completed the most recent assessments, their studies were utilized.

WMC completed two hydrology studies. The first study (WMC, 2005) calculated theProbable Maximum Flood (PMF) runoff volume upstream of the TMF dam. The second

study (WMC, 2006) evaluated the precipitation depth for the 100 yr, 1000 yr, and PMF

events. These studies can be found as appendices to the starter dam design report

developed by MHW (2008).


21/81


intrusive forms a near vertical tube, which is exposed over an area approximately 800 m

wide and 1000 m long, elongated in a north-south direction. The main body of the

intrusive is a porphyry diorite, with two smaller bodies (a diorite and a quartz diorite)intruded in the southern part of the body. The two small bodies are located along a

northeast-southwest axis coincident with the main fault trend in the area, and suggesting

that the intrusions are partially fault controlled. The centre of the intrusion is extensively

silicified with numerous quartz veins, while outside this central core the intrusion is

weathered, as indicated by the development of clay minerals and iron oxides.

Another intrusive body outcrops on both sides of the Tingo River, approximately 6 km

west northwest of Cerro Corona. This intrusive is associated with the rhyolitic flowswhich outcrop on the flanks of Cerro Las Gordas and Cerro Aguilas. The limestone host

rock shows little deformation related to the intrusion of the Cerro Corona body, with

some cleavage development or faulting along the contacts. Bedding within the limestones

is disturbed within a 10m zone around the intrusive contact. Skarn is developed within a

zone extending approximately 30 m from the intrusive contact. Rafts of limestone host

rock, up to 70 m in thickness are present within the intrusive body.

The surface deposits in the area include colluvial, morranic and fluvioglacial deposits.

Numerous landslips are seen associated with saturated superficial deposits, while a larger

scale landslide, affecting both superficial deposits and underlying bedrock is observed in

the Tingo River valley, below the confluence with the Las Gordas valley. The saturated

superficial deposits range from 0.5-4 m in thickness, while the colluvial and scree

deposits present in the Las Gordas and Aguilas valleys have thicknesses in the range of 5-

20 m. These superficial deposits can form a shallow aquifer, which can transmit water to

the underlying basement rocks or feed local shallow spring flows.

3.7 SEISMICITY

The northern part of Peru in the Cerro Corona project area is characterized by significant

seismicity. The regional geologic structure is dominated by northeast-southwest trending

faults and related folds. In the Cajamarca area these are intersected by a series of east-

west trending structures.

A number of historic site seismic hazard documents were reviewed as a part of the Starter

Dam design. Based on recommendations from the Independent Geotechnical and Tailing

Review Board (IGTRB), MWH contracted URS to develop a new hazard study for the

project URS was selected by MWH due to their familiarity with Peruvian seismicity as


22/81


the WMC study, as presented in the Starter Dam design report (MWH, 2008) is as

follows:

The Cerro Corona intrusive body has a variable, but moderate permeability withvariations likely related to fracture development and variations in alteration

Zones of increased fracturing generally correlate with fault zones and with thelimestone intrusive contact areas.

The Cerro Corona intrusive body intruded into a sequence of carbonate rocks.Groundwater flow in the carbonates is enhanced along faults, fractures, and local

karst features.

Groundwater elevation contours indicate that Cerro Corona represents agroundwater high. Groundwater flow away from Cerro Corona follows

topography with flow occurring towards the north and east into the Mesa de Plata

basin and to the south-southeast into the Hualgayoc basin.

A hydrogeologic flow model was developed by WMC for GFLCSA (WMC, 2008). The

model was developed using the USGS three-dimensional groundwater flow codeMODFLOW. The results of hydrogeologic modeling are as follows:

The development of the TMF would affect the surface drainage and recharge ofthe Las Gordas and Aguilas sub-catchments, potentially reducing the natural

baseflow of the Tingo River by 33.6 l/s or 19.5%

Three pit dewatering scenarios were evaluated. Modeling indicates that the

dewatering operations in the open pit for the base case scenario (pit dewatering of30 L/s for 15 years) may reduce the baseflow at the confluence of the Hualgayoc

River with the Quebrada Mesa de Plata between 2% and 4% (approximately 3

L/s).

The cone of depression resulting from the base case pit dewatering may extend tothe topographic level of 3,620 masl, which may result in a reduction in certain

observed spring flows in the Quebrada Corona region.

However, the hydrogeologic model developed by WMC represents a preliminary model

and does not account for the karst formations present at the site. Accordingly,

GWI was contracted by GFLCSA to conduct a detailed karst hydrogeological assessment

of the Gordas Valley tailing and waste rock facilities. This study included field mapping,

drilling and installation of piezometers hydraulic testing speleological and dye tracer


23/81


Pumping from the drain behind the UCB will result in significant reductions of therisk of off-site seepage.

With the UCB and the drain installed and functioning correctly, there will be alow risk of offsite seepage associated with raising the tailing level in the

impoundment to an elevation of 3760 masl.

It may be possible to raise the tailing level above elevation 3760 masl ifmonitoring of the UCB and the Hualgayoc seep collection systems indicate that

they are working correctly.

The risks of off-site water quality impacts are much higher from the mine wastedumps as they are acid-generating and are within a karstic limestone rechargearea. It is understood that water from any impacted springs will be collected and

treated by GFLCSA.

3.9 GEOCHEMISTRY

Various geochemical characterization studies have been performed at the site. A brief

summary of the results of the geochemical characterization studies is presented below:

Material from the rhyolite quarry in the Gordas valley is not expected to generateacidic drainage or leach elevated concentrations of metals.

A portion of the alluvial/colluvial material from the Aguilas valley containsminimal net neutralization potential.

The La Jalca tailing materials are not suitable for construction considering theirgeochemical properties. However, their use in Stage 3 construction against the

upstream face of the Las Aguilas Dam is considered acceptable as the placement

location is upstream of the Zone 1 core material.

The TMF will store two types of tailing, CST and RST. The CST contains agreater concentration of sulfide-sulfur and a smaller net neutralizing potential than

the RST fraction of the tailing. Humidity cell testing indicates that the CST

fraction quickly turns acidic and produces leachate with increasing concentrationsof metals and decreasing pH values during the test period. The RST portion,

however, had stable concentrations of metals and circumneutral pH after 25

weeks. Accordingly, it was recommended that the CST be deposited sub-

aqueously in the later stages of the mine life to mitigate acid generation while the

RST can be deposited in a subaerial manner


24/81


4.0 DESCRIPTION OF THE STAGE 4 TMF RAISE

At the onset of Stage 4 construction, it is expected that the construction of stages 1through 3 of the TMF will be completed. Stage 4 construction will consist of raising both

the Las Aguilas and Las Gordas Dams to an elevation of 3740 masl and constructing a

relatively small saddle dam across the La Hierba valley. At this elevation, both the Las

Gordas and Las Aguilas dams will be at a sufficient elevation to form a single connected

dam spanning both the Quebrada Las Aguilas and the Quebrada Las Gordas. A plan view

of the site, including the Stage 4 TMF, is presented in Figure 2.

Stage 4 TMF construction will be performed utilizing two raise methods, the optimizedcenterline raise for the Las Gordas portion of the Stage 4 TMF raise and a vertical

centerline raise for the La Hierba Dam and the Las Aguilas portion of the Stage 4 TMF

raise. The Las Aguilas and La Hierba portions of the raise will be constructed with a

sloping rockfill zone (placed at a slope of 1.4H:1V) placed along the upstream face of the

raise to support the vertically placed adjacent material zones. For the Las Aguilas portion

of the raise, this upstream rockfill zone will be supported by the La Jalca tailing bench

while for the La Hierba dam, this upstream rockfill zone will be supported by the existingground.

The Las Gordas portion of the raise is described as an optimized centerline raise as it

still relies on the tailing material for upstream stability but to a lesser extent than the

centerline method. The upstream face and the internal material zones of the Las Gordas

portion of the Stage 4 TMF raise will be constructed at a slope of 0.8H:1V. Due to the

steepness of the upstream slope of the optimized centerline portion of the raise, gabion

facing with geosynthetic tails has been specified to provide lateral support for placementand compaction of rockfill at the upstream face and to mitigate raveling during the service

life of the Stage 4 TMF. Further information regarding the gabion facing is presented in

Section 7.5.2 of this report and in the Construction Drawings included as Appendix I.

Near the middle of the main Stage 4 TMF dam (the Las Aguilas and Las Gordas portions

of the dam), the two raise methods intersect. The intersection of the two raise types will

be performed by sloping the optimized centerline raise method from its Stage 4 elevationof 3740 masl down to the previous TMF dam elevation of 3732 masl. In this transition

zone, a vertical centerline raise method will be used above the sloped optimized

centerline method to bring the dam to its full Stage 4 elevation. Additional information

regarding the transition zone can be found in the construction drawings, located in

Appendix I.


25/81


Further information regarding the selection of the Zone 5 hydraulic parameters can be

found in Section 6.4. Seepage analyses were performed to evaluate the effect of the Zone

5 width and parameter revision on the estimated hydraulic gradients and seepage ratesthrough the dam. Further information regarding the seepage and stability analyses

performed in support of the Stage 4 design is included in Section 7.

For the entire TMF dam, the Stage 4 downstream rockfill slope will be steepened from

the design value of 1.5H:1V, as presented in the Starter Dam design (MWH, 2008) to a

slope of 1.4H:1V. This steepening is based on newly available information that was used

to revise the rockfill strength envelope and necessitated by the need to minimize any

downstream movement of the downstream embankment toe. Section 8 discusses theindustry precedent for this revision while Section 7 discusses the analyses performed to

evaluate the suitability of this revised slope.

Sections of the Las Aguilas and Las Gordas portions of the Stage 4TMF dam raise are

presented in Figures 3and 4, respectively. Additional information regarding the Stage 4

TMF dam design is included in the Construction Drawings.


26/81


5.0 PROPOSED FUTURE TMF RAISES

To evaluate the suitability of the Stage 4 TMF raise design and to provide a contingencyframework for future TMF construction, a conceptual level design was developed for

future TMF dam raises above the Stage 4 TMF crest elevation of 3740 masl. As a part of

this design, a conceptual level construction schedule and quantity estimates were

developed and seepage and stability analyses were performed. The proposed future TMF

raise design (for post-Stage 4 raises) represents a contingency plan, assuming that weak

tailing will be impounded within the TMF throughout the life of the facility. Accordingly,

the proposed design for future raises employs the optimized centerline method. The

following sections discuss the proposed TMF staging schedule and the proposedconfiguration for future TMF raises.

5.1 PROPOSED TMF CONSTRUCTION SCHEDULE

A conceptual level TMF construction schedule was developed by MWH to estimate the

optimal crest elevations for the Stage 4 and other future TMF raises to the ultimate crest

elevation of 3800 masl. The major schedule objectives include the following:

Minimize wet season construction to the extent practical

Minimize the required number of raises to reduce the requirements forconstruction fleet mobilization

Maintain a minimum of 2 m of freeboard at all times

The following assumptions and conditions were used to develop the schedule:

A uniform tailing deposition rate of 17,000 tpd

Historic construction rates from dam construction to date were used to estimatethe construction durations. To be conservative, a factor of safety of two was

applied to the duration times used in the schedule.

The impoundment volume is based upon stage-storage data developed fromsurveys of the existing ground performed by GFLCSA in July and August of 2009

and assumes that the TMF dam will be raised using the centerline method for all

post Stage 3 raises.

The impoundment capacity is based upon the stage storage data noted above and


27/81


TMF construction can be performed largely in the dry season (April throughSeptember), with three of the eleven stages being constructed entirely in the dry

season and eleven partially in the wet season. However, given the fact that a factorof safety of two was applied to all duration times, it is likely that a majority of the

partial wet season construction campaigns could be performed entirely during the

dry season.

No construction is required in years 2016, 2019, 2021, and 2023.

A minimum freeboard of 2 m is maintained at all times, given the assumptionslisted above. The maximum freeboard during the construction of the TMF is 14 m

during the construction of the Stage 4 raise and 8.5 m for all subsequent raises.

Table 3 presents the conceptual level TMF schedule for Stages 4 through 14 (TMF

construction from elevation 3732 masl to elevation 3800 masl). Figure 5 presents the

results graphically, showing the crest elevation of the TMF dam, the UCB, and the TMF

pond with time.

Table 3 Summary of Proposed TMF Construction Schedule

YEAR

FINAL DAMCREST

ELEVATION(M)

TMFSTAGE

ALLOWEDCONSTRUCTION

TIME(MONTHS)

STARTMONTH

ENDMONTH

MAX.FREEBOARD

(M)

MIN.FREEBOARD

(M)

2010 3740 4 7 January July 14 3.77

2011 3746 5 4 April July 6.56 2.96

2012 3752 6 4 January April 6 3.72

2012 3758 7 4 September December 7.44 4

2013 3764 8 4 July October 8 42014 3770 9 4 July October 8 4.22

2015 3776 10 4 August November 8.54 N/A

2016 No construction N/A N/A N/A N/A 2.4

2017 3782 11 4 April July 6.96 3.1

2018 3788 12 4 July October 7.9 N/A

2019 No construction N/A N/A N/A N/A 2.5

2020 3794 13 4 May August 7.4 N/A

2021 No construction N/A N/A N/A N/A 2

2022 3800 14 4 March June 6.8 N/A

2023 No construction N/A N/A N/A N/A 2

5 2 DESCRIPTION OF FUTURE TMF RAISES

M h 2010 G ld Fi ld L Ci C C Mi S 4 TMF R i 3740 P 22


28/81


4 design. However, to reduce the reduction in the capacity of the Las Aguilas LVU

facility, a portion of the downstream slope of the Las Aguilas portion of the ultimate dam

will be steepened, as discussed in Section 5.3 below.

Due to stability requirements, the upstream Zone 2B and Zone 5 material will be

maintained at horizontal widths of 6 m each, as specified for the Stage 4 raise. Due to the

steepness of the upstream slope proposed for the future raises, gabion facing and

geosynthetic reinforcement have been specified for placement within the upstream Zone

2B material to enhance slope stability and mitigate surficial raveling of the slope, as

presented in Figure 10. Further information regarding the slope reinforcement is included

in Section 7.5. Further information regarding the seepage and stability analysesperformed in support of the proposed design is included in Section 7.

A plan view of the site, including the proposed ultimate TMF, is presented in Figure 6.

Sections through the Las Aguilas, Las Gordas and La Hierba portions of the ultimate dam

are presented in Figures 7, 8, and 9.

5.3 DOWNSTREAM EFFECTS OF PROPOSED ULTIMATE DAM DESIGN

As a result of the revision in the ultimate dam design from the centerline concept

proposed in the Starter Dam design (MWH, 2008) to the optimized centerline concept,

the downstream toe of the ultimate dam has shifted downstream. Within the Las Gordas

valley, this shift is not large enough to encroach upon the Tingo River or the Las Gordas

LVU facility (as presented in Figures 6 and12). However, in the Las Aguilas Valley this

shift does encroach upon the Las Aguilas LVU facility (as presented in Figures 6 and

12).

This revised design, with a uniform downstream slope of 1.4H:1V, results in a reduction

in the capacity of the Las Aguilas LVU facility from its maximum capacity of 17,300

cubic meters (m3) to approximately 11,800 m3 (neglecting storage capacity within the

rockfill voids and accounting for a 1 m freeboard for both cases). This is greater than the

minimum required Las Aguilas LVU capacity of 11,300 m3, accounting for the estimated

seepage from the impoundment, groundwater flows, and surface water flows. However,this revised design results in rockfill overlying a substantial portion of the lined area,

increasing the risk of damage to the liner. This impacted area also includes the sump area

of the LVU facility, where a floating barge is expected to be installed to pump impounded

water to the Las Gordas LVU facility. In addition, placement of rockfill in this area would

prevent the use of a weir which is intended to be installed in the upstream portion of the

March 2010 Gold Fields La Cima Cerro Corona Mine Stage 4 TMF Raise 3740 m Page 23


29/81


accounting for a 1 m freeboard). Slope stability analyses indicate that this proposed

steepening provides factors of safety in excess of the required minimum values, as

discussed in Section 7.3.

It should be noted that a variety of options exist for mitigating the effects of the

downstream ultimate embankment toe on the Las Aguilas LVU facility. Other options

include the construction of a retaining wall near the LVU facility or the use of a lower

height steepened section with a steeper slope. Further work to optimize this design feature

should be performed at a later date when additional information is known regarding the

ultimate TMF design.

March 2010 Gold Fields La Cima Cerro Corona Mine - Stage 4 TMF Raise 3740 m Page 24


30/81


6.0 UPDATED MATERIAL PROPERTIES

6.1 INTRODUCTION

Seepage and slope stability analyses were performed in support of the design of the Stage

4 TMF and the conceptual level future TMF raises, as described in Section 7. These

analyses utilized material properties from previously performed designs completed by

MWH and more recent information gained since their completion. Newly gained

information includes the results of investigations of the insitu tailing properties, a revised

evaluation of the rockfill strength envelope, and the results of hydraulic testing of the

Zone 1 and 5 materials, as discussed below.

6.2 TAILING EVALUATION

In the Starter Dam design report submitted by MWH in November of 2008, geotechnical

strength properties were estimated for the tailing materials for use in stability modeling of

the TMF dam. The estimated properties were based on tests conducted by Knight Piesold

(KP) in January of 2006. In the Starter Dam design report, it was noted that the tailingsamples tested by KP did not accurately reflect the tailing materials that would be

impounded in the TMF. Accordingly, it was noted that additional tailing characterization

testing should be conducted with actual production samples when they are available.

In 2009, investigations were performed under the supervision of MWH on tailing

deposited in the Las Gordas impoundment to estimate the shear strength of the tailing

materials and to evaluate the variation of the tailing profile with depth. The purpose of

testing was to develop data for use in stability analyses and to further understand the

tailing profile being developed from the current deposition method. Testing was

performed using a floating platform near the upstream face of the Las Gordas Dam

utilizing cone penetration and vane shear methods in a grid pattern with each gridline

aligned parallel to the Starter Dam. In addition, limited sampling was performed with

Shelby tubes to allow for index testing.

A total of nineteen cone penetration tests with pore pressure measurements (CPTU),seven cone penetration tests (CPT), and ten vane shear tests (VST) were performed as a

part of the investigation program. CPTU testing was only performed to depths ranging

from 8 to 14 m due to risk of exceeding the maximum capacity of the floating platform

and buckling of the CPTU rods. CPTU tests provided measurements of tip resistance and

id f i ti t t ti f th d t f th

March 2010 Gold Fields La Cima Cerro Corona Mine - Stage 4 TMF Raise 3740 m Page 25


31/81

March 2010 Gold Fields La CimaCerro Corona Mine Stage 4 TMF Raise 3740 m Page 25

The general conclusions from the tailing investigation program are as follows:

The thickness of very soft and soft soils generally increase as distance from thedam increases

Lenses of weak soils were found underlying stiffer soils, as typically seen intailing materials deposited subaqueously.

The fluids in the impoundment were found to be separated into two layers, with aclear layer of water overlying a layer of turbid water containing extra fine tailing

(EFT) in suspension.

Based on the tailing investigation program, a tailing profile was developed. A summary

of this profile is as follows:

Clear water was encountered from the surface to depths of 0.4 to 2.6 m

Turbid water, with soils in suspension (EFT), was encountered at depths of 1 to4.1 m

Soft and Very soft soils (with a CPT tip resistance value, qc, less than 0.4 MPa)were encountered from depths of 3 to 10.5 m

Medium and Stiff soils (with qc > 0.4 MPa) were encountered at depths of 5 mand greater

As there is a relatively high amount of spatial variation and interbedding of the four

material types described above, the tailing profile described above was simplified for the

purposes of analysis into three zones, water, weak tailing, and tailing. Strength

parameters were selected to characterize these three zones based on the results of the

insitu tailing investigation, and were modeled as a ratio of the effective overburden stress,

/'v, as presented in Table 4.

Table 4 Estimated Strength of Materials in Tailing Profile

STRENGTH

'v

Water N/A

Weak Tailing 0.12

Tailing 0.30

Residual Tailing Strength (Post-Earthquake) 0.07

Th t ili t th t li t d b tili d i b k l i f d t



32/81

g g

6.3 EVALUATION OF ROCKFILL STRENGTH

The rockfill strength envelope used in the stability analyses performed in support of the

Starter Dam design (MWH, 2008) was based on relationships developed by Leps (1970)

and used preliminary information regarding the rockfill properties as inputs to the

evaluation. The relationships developed by Leps are based on an empirical database and

can be used to relate the friction angle to the normal stress for a rockfill. The assigned

rockfill strength and dam configuration (with a downstream slope of 1.5H:1V) utilized in

the starter dam design resulted in a downstream factor of safety of about 1.5 for the

ultimate configuration of the Gordas dam under long-term static conditions (MWH 2008).

Based on a preliminary evaluation of the dam using the rockfill strength envelope fromthe Starter Dam analyses, it was estimated that steepening the slope to 1.4H:1V would

result in a long-term factor of safety less than 1.5, which would be lower than the

minimum allowable factor of safety for this loading condition (see Table 2).

Since the time that the initial rockfill strength estimate was developed, the site quarry has

been commissioned and new evaluations of the rockfill properties have been performed.

These evaluations include unconfined compressive strength, point load testing and

gradations of the rockfill. The results of rockfill strength testing indicate average

unconfined compressive strengths between 62 and 67 MPa. Based upon gradations

performed on Zone 2B material produced at the site, the rockfill is generally well graded,

with a limited amount of fines (MWH, 2009d). Additionally, construction using the

produced rockfill has been based on good practice with the rockfill material being placed

in maximum loose lifts of 1 m; moisture conditioned, and then compacted with 8 passes

of a 19 tonne vibratory smooth drum roller.

Based on the information from site, the rockfill is considered to be well-graded and well

compacted. Additionally, the quarried rhyolite and limestone encountered at the site has a

UCS slightly less than the strong particles defined by Leps. Accordingly, a

representative shear strength envelope for the rockfill at Cerro Corona would fall between

the Average Rockfill shear strength envelope and the High-density, Well-graded

Strong Particles envelope. This revised rockfill envelope, selected to fall between these

two categories, represents an increase in the shear strength envelope from that estimated

during the Starter Dam design. Figure 13presents a plot of the rockfill strength envelopeutilized in the Starter Dam design versus the revised strength envelope estimated for the

Stage 4 TMF raise.

It is important to note that the revised rockfill strength evaluation is based on properties



33/81

6.4 ZONE 1 AND 5 HYDRAULIC PROPERTIES

During the Starter Dam design, saturated hydraulic conductivity values of 1 10-4and

1 10-6cm/s were used to characterize the Zone 5 and 1 materials, respectively. These

same values were specified as minimum values for the Zone 5 and 1 materials in the

construction specifications accompanying the Starter Dam design. The specified Zone 5

saturated hydraulic conductivity value was later reduced to 1 10 -5cm/s in the Stage 3

TMF construction specifications. Testing performed during construction of the TMF to

date has indicated average saturated hydraulic conductivity values of less than 1 10-6

cm/s for the Zone 1 material and approximately 10-5cm/s for the Zone 5 material (MWH,

2009d).

As noted previously, to counteract the reductionin the Zone 5 width associated with the

proposed TMF design, the specification for the Zone 5 material placed above elevation

3732 masl (for Stage 4 construction and beyond) has been revised to require the

placement of Zone 5 material with a reduced saturated hydraulic conductivity of 10-6

cm/s. It is expected that complying with this revised specification can be accomplished by

limited additional moisture conditioning and compactive effort during construction which

should have minimal effects on the overall construction of the TMF dam. Accordingly,

for the purposes of analysis, two values were selected to represent the saturated hydraulic

conductivity of the material. Up to an elevation of 3732 masl, a saturated hydraulic

conductivity of 1 10-5cm/s was used to characterize the Zone 5 material. Above that

elevation, to the ultimate crest elevation of 3800 masl, a saturated hydraulic conductivity

of 1 10-6cm/s was used, matching the value used for the Zone 1 material.



34/81

7.0 ENGINEERING ANALYSES

7.1 INTRODUCTION

Seepage and slope stability analyses were performed in support of the design of the Stage

4 TMF and the conceptual level future TMF raises. Sections within the Quebrada Las

Gordas and Quebrada Las Aguilas were evaluated for both the Stage 4 Dam raise and for

future raises up to the ultimate TMF elevation of 3800 masl. The sections analyzed were

based on a compilation of record surveys and proposed designs for Stage 2 and 3 TMF

construction that is currently underway. Reinforcement analyses were performed to

evaluate and design upstream face reinforcement for the future dam raises using acombination of gabions and geosynthetic reinforcement. Additionally, a preliminary

seismic deformation analysis was performed (a final deformation analysis is currently

being performed) to estimate the displacement of various locations of the ultimate TMF

dam due to seismic loading

The following sections discuss the analyses performed and summarize the results of the

analyses.

7.2 SEEPAGE ANALYSIS

7.2.1 Introduction

A steady-state seepage analysis was performed using SEEP/W version 7.15 (GEO-

SLOPE, 2009a), a finite element software for analyzing pore pressure distribution in and

flow through porous media. The model was used to estimate seepage through the TMFdams and foundation materials and to estimate gradients through the low permeability

dam materials.

7.2.2 Geometry and Evaluated Cases

The seepage analysis was performed for the TMF Dam using sections of the dam through

the Quebrada Las Gordas and the Quebrada Las Aguilas. The sections considered aresimplified versions of those presented in the Stage 4 TMF Construction Drawings

developed by MWH with future raises modeled upon it at a upstream slope of 0.6H:1V

and a downstream slope remaining at 1.4H:1V, as described in Section 5.2. The existing

surface presented in the sections is based upon a combination of survey data (from a

survey performed by GFLCSA in November of 2009) and designs for elements that have



35/81

2. Stage 4 TMF The TMF dam constructed to elevation 3740 masl and themaximum tailing and impounded water elevation at 3738 masl, allowing for 2 m

of freeboard.

3. Stage 11 TMF The TMF dam constructed to elevation 3782 masl and themaximum tailing and impounded water elevation at 3780 masl, allowing for 2 m

of freeboard.

4. Stage 14 TMF The TMF dam constructed to the ultimate dam elevation of 3800masl and the maximum tailing and impounded water elevation at 3798 masl,

allowing for 2 m of freeboard.

The impounded tailing material was modeled as a homogeneous material with a beach

slope of 1.5%, as estimated in previous studies (KP, 2006). For each case considered, the

impounded water level was conservatively assumed to be equivalent to the maximum

tailing elevation. This represents a conservative assumption as this would be an extreme

case, particularly for the later raises where the impoundment surface area is large. More

information, including figures presenting the cases and geometry utilized in the seepage

analysis are included in Appendix C.

7.2.3 Material Propert ies

The saturated hydraulic conductivities utilized in the seepage analysis are summarized in

Table 5. Generally, the saturated hydraulic conductivities were obtained from the seepage

analysis performed in support of the Starter Dam Design (MWH, 2008). However, as

noted in Section 6.4, the saturated hydraulic conductivity of the Zone 5 material wasrevised to reflect the results of testing of placed Zone 5 material and alterations to the

construction specifications. Accordingly, for the purposes of analysis, a saturated

hydraulic conductivity of 1x10-5 cm/s was utilized for Zone 5 material below elevation

3732 masl, while a saturated hydraulic conductivity of 1x10-6 cm/s was utilized for the

Zone 5 material above elevation 3732 masl, reflecting the revised construction

specifications and placement methodology for future raises.

Table 5 Summary of Saturated Hydraulic ConductivitiesUtilized in the Seepage Analysis

MATERIALSATURATED HYDRAULIC CONDUCTIVITY

(cm/s)

Zone 1 1 10-6



36/81

All materials in the analysis, with exception of the bedrock, were characterized using the

Saturated/Unsaturated model in SEEP/W. The unsaturated material properties used in the

seepage analysis were developed using the built-in estimation option for the volumetric

water content function. The estimated water content functions are selected based on

material type (e.g., silty sand, clay). Hydraulic conductivity functions were estimated

using the water content functions and either the van Genuchten model (rockfill) or the

Fredlund-Xing model (all materials except the rockfill and bedrock). The volumetric

water content curves and the hydraulic conductivity curves used to characterize the

unsaturated properties of the embankment and impoundment materials are included in

Appendix C.

7.2.4 Boundary Conditions

Constant head boundary conditions representingtailing pond water level elevation were

applied to the top of the tailing impoundment. The downstream boundary conditions

consisted of a potential seepage face boundary condition. A potential seepage face

boundary acts as a no-flow boundary until it becomes saturated, after which flow will

occur across the boundary with no pressure at the surface.

Numerical issues developed in the seepage model due to the presence of materials with

several orders of magnitude difference in the hydraulic conductivities (i.e., core material

adjacent to filter/transition/rockfill material). The numerical issues were resolved by

removing the downstream materials from the model (Zones 2, 2A, 3, 4 and downstream

Zone 2B) and applying the potential seepage face boundary to the downstream side of the

Zone 1 material and along the ground surface beneath the downstream shell. The removal

of coarse-grained materials in the model is the procedure recommended by GEO-SLOPE(2009a) for resolving numerical issues stemming from steep hydraulic conductivity

functions and large differences in material permeabilities. This methodology is valid as

long as the filter, transition and rockfill zones are free-draining (negligible head losses)

and that the zones have the capacity to convey downstream any water coming through the

cores materials (Zone 1 and Zone 5). This was verified by comparing the flow

characteristics of the adjacent Zone 1 and 3 materials, which represent the critical flow

interface (MWH, 2010a).

7.2.5 Seepage Analysis Results

The estimated phreatic surfaces and hydraulic gradient for each analyzed case are

included in Appendix C. Estimated seepage rates (per meter width of dam) and the



37/81

Table 6 Summary of Estimated Seepage Rates

SEEPAGEANALYSIS

GORDAS AGUILAS

SEEPAGERATE

(m3/s/m)

TOTAL SEEPAGE*(m

3/s)

TOTALSEEPAGE*

(L/s)

SEEPAGERATE

(m3/s/m)

TOTALSEEPAGE*

(m3/s)

TOTALSEEPAGE*

(L/s)

Stage 4 TMF10 m Freeboard

(Case 1)3.18 10

-5 0.0068 6.8 3.52 10

-5 0.0081 8.1


(Case 2)3.54 10

-5 0.0076 7.6 5.35 10

-5 0.0012 12

Stage 11 TMF

2 m Freeboard(Case 3) 5.52 10

-5

0.014 14 8.12 10

-5

0.029 29


(Case 4)6.64 10

-5 0.017 17 9.36 10

-5 0.036 36

*Total seepage rates were approximated based on valley and dam geometry (see Appendix C).

Note that the analysis assumes that water infiltration into the system from other sources

such as groundwater or surface water is negligible. These sources should be considered

when sizing infrastructure.

7.2.6 Conclusions

For both the Las Aguilas and Las Gordas dams, the estimated total seepage rates are 12

L/s or less for Stage 4 conditions. The current seepage rates measured in the field during

operation of the TMF in the Las Gordas valley are approximately 1 L/s. This variation is

considered reasonable as conservative hydraulic properties were adopted for the bedrockand Zone 1 and 5 materials.

For both the Las Aguilas and Las Gordas dams, the estimated seepage rates for the

ultimate (Stage 14) conditions are less than 40 L/s. In the seepage analysis performed in

support of the Starter Dam design (MWH, 2008), the estimated total seepage rates for the

ultimate configuration of the Las Aguilas and Las Gordas dams were approximately 18

L/s and 16 L/s, respectively. This variation is considered due to differences in the

methods of calculation used to extrapolate the seepage flux obtained from SEEP/W.

Decisions based on the seepage rates estimated above should consider the variability in

the hydraulic conductivity along with the variability induced by extrapolating two-

dimensional models to estimate three-dimensional seepage volumes. The properties



38/81

7.3 STABILITY ANALYSES

7.3.1 Introduction

Slope stability analyses were performed in support of the Stage 4 and post-Stage 4 TMF

dam design. These analyses include performing a general stability analysis to evaluate

both upstream and downstream stability of the TMF dam at various elevations as well as

five additional analyses to estimate material properties and to evaluate the sensitivity of

the estimated factors of safety to variations in specific material parameters. The five

additional analyses performed are as follows:

1. Back Analysis of Zone 1 and 5 Shear Strength This analysis was performed toestimate the minimum allowable Zone 1 and 5 undrained shear strength based on

the stability of the Las Gordas dam during construction of the TMF to date.

2. Evaluation of Freeboard Sensitivity This analysis was performed to evaluatethe sensitivity of the factors of safety estimated in the general stability analysis to

the maximum tailing elevation.

3. Evaluation of Minimum Required Geosynthetic Reinforcement Strength This analysis was performed to evaluate the sensitivity of the factors of safety

estimated in the general stability analysis to the tensile strength of the upstream

face reinforcement. The reinforcement parameters utilized in the general stability

analysis are based upon those developed to describe the reinforcement placed in

the Stage 3 TMF and may not be directly applicable to future construction.

4. Evaluation of Minimum Required Tailing Strength for Use of CenterlineRaise Method This analysis was performed to estimate the minimum required

tailing strength to allow for the use of the centerline raise method in the future.

5. Sensitivity of Optimized Centerline Stability to Tailing Strength Thisanalysis was performed to evaluate the sensitivity of the stability of the upstream

slope of the dam to variations in the tailing strength parameters, assuming the use

of an optimized centerline raise method. This analysis was utilized in conjunctionwith the results of the tailing evaluation to conservatively estimate the long-term

and short-term tailing strength parameters for use in this study.

Stability analyses were performed using SLOPE/W version 7.15 (GEO-SLOPE 2009b).



39/81

7.3.2 General Stabili ty Analysis

The general stability analysis was performed for critical TMF dam sections in the

Quebrada Las Gordas and the Quebrada Las Aguilas to evaluate the upstream and

downstream stability of the Stage 4 TMF raise and future conceptual level TMF dam

raises to the ultimate Stage 14 TMF elevation of 3800 masl. A minimum failure surface

depth of 0.3 m was utilized for the downstream cases analyzed and a minimum failure

surface depth of 3 m was utilized for the upstream cases evaluated. This was done to

account for slope reinforcement that will be placed along the upstream face of future

raises, per the design concept presented in this doc

Documents

Cerro Corona Relavera