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Tailings Storage and Heap Leaching in aCombined Facility A First for the MiningIndustry

Thomas F. Kerr, P.E. Knight Pisold and Co., United States

Michael G. Skurski, P.E. Newmont Mining Company, United States

Peter D. Duryea, Ph.D., P.E. Knight Pisold and Co., United States

ABSTRACT

The La Quinua mill at the Yanacocha mine in Peru has a thickened tailings storage

facility that is contained entirely within an active heap leach pad. In this respect, it isunique and unprecedented in the mining industry and provides the operator, Minera

Yanacocha S.R.L. (MYRSL), with certain cost, land-use and closure efficiencies. The

combined design requires that the leach ore embankments retaining the tailings

provide the same high level of stability and security required of major tailings dams.

Contrary to standard tailings dam construction, the leach ore in these embankments is

placed in thick, uncompacted lifts so that they maintain adequate permeability for

leaching. The resultant loose structure makes static and dynamic liquefaction a critical

issue, and the key design principle for avoiding a liquefaction related failure is to keep

the ore that is saturated or very near saturated when under leach well removed from

the outer faces of the embankments and contained behind large unsaturated structural

shells. Thus, the design and operational effort has become largely one of keeping

these shells adequately drained and below fully saturated. The facility was

successfully commissioned in early 2008 and its performance to date has met or

exceeded all design objectives. The first phase of expansion, Phase 2, has been

designed and will be developed by late 2009.

Introduction

The La Quinua tailings are produced at the back end of the counter current decantation

(CCD) circuit associated with the mill. The tailings are thickened to a solids content of

approximately 67 percent by weight in the last CCD stage before being disposed within

the active La Quinua heap leach pad (HLP). The thickened tailings storage facility

(TTSF) was commissioned in April 2008, and approximately 5.6 million dry metric

tonnes (mdmt) of tailings have been placed in the facility during its first year of

operation. The facility was originally designed to accommodate 45 mdmt of tailings

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produced at a nominal rate of 5.0 mdmt per year over a nine year period but has

flexibility to be expanded to over 50 mdmt. Studies are ongoing to confirm the

necessity of this additional storage capacity.

The TTSF is unique in that it is located within an active HLP and uses leach ore,

strategically placed within specific lines and grades, to form the tailings embankments.

The HLP has been in operation since 2001 and has been expanded in seven stages,

generally in a north to south direction over the natural topography that rises upward to

the south. Through the first quarter of 2009, approximately 390 mdmt of leach ore has

been loaded onto the HLP, and at its final installed capacity it will contain over

530 mdmt. The north to south development presented the opportunity for changing the

loading plan in the latter stages to contain the tailings in the south side of the HLP after

MYSRL made the decision to develop the La Quinua mill. Stages 5 and 6 of the HLP

were re-configured by placing the ore into east and west zones thereby leaving a

central area open that became the TTSF. The east and west zones now form the main

tailings containment embankments. Containment on the north side is provided by the

main body of the HLP developed in Stages 1 through 5, and containment on the south

side is provided by the natural topography. A general plan of the TTSF and HLP is

shown on Figure 1.

Figure 1 - Plan of the La Quinua Operation and Thickened TailingsStorage Facility

The TTSF will be developed in three phases. Phase 1, which is currently in operation,

has leach ore embankments up to elevation 3612 meters (m), or to a maximum height

of approximately 50 m, which provides storage for 10 mdmt of tailings. This amount of

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storage will be used up in January 2010. Phase 2, which is now under development,

will have embankments up to 3644 m and will increase the storage capacity to nearly

29 mdmt. This capacity will be reached in January 2013. Phase 3 will be developed in

2012 up to the final elevation of the TTSF, which is still to be determined depending on

the required final tailings and leach ore capacities. The original design of the TTSF

had a final elevation at 3660 m, but most likely the actual elevation will be between

3665 and 3675 m, which will make the final maximum height of the embankments

between 100 and 115 m.

Overview of Design and Operations to Date

The tailings are thickened in a high rate thickener acting as the final stage of the CCD

circuit and then transported to the TTSF through one of two 1.5 kilometer (km) long, 12-

inch-diameter, unlined, steel pipelines that are fed by one of two centrifugal pump

trains. The tailings are deposited into the TTSF from multiple drop bars comprising 8-

inch diameter high density polyethylene (HDPE) pipes located on approximately 50 m

centers that extend down into the basin from 14-inch diameter HDPE distribution pipes

running around the inside crest of the leach ore embankments. One distribution pipe is

above the east side of the impoundment and a second one is above the north and west

sides. The drop bars convey the slurry down onto the tailings beach to prevent it from

eroding the faces of the embankments. This is important as the upper portions of

these slopes are actively under leach. The drop bars are valve activated from off-takes

on the distribution pipes to allow for controlled placement into the facility. The slurry

discharges out of holes that are drilled by the operators along the top of each drop bar

just ahead of the rising beach, and this leads to some dissipation of energy as the

tailings slurry exits the drop bar onto the tailings beach. The tailings then flow at a

relatively low velocity over the beach, which allows for liquid/solid separation. The

points of active discharge are frequently rotated around the three sides of the TTSF to

form a thin layered, drained and stable tailings deposit against the leach ore

embankments.

The beach is sloped into the south-central area of the facility so that surface water

draining from the tailings and runoff from precipitation is displaced away from the

embankments. A small surface water pond is maintained in the south-central area

directly against a blanket underdrain that covers the lined base of the facility, which

slopes down to a low point in the northwest corner. The underdrain comprises a layer

of free draining gravel within which a network of perforated corrugated polyethylenetubing (CPT) pipes is installed, and this layer is covered with a geotextile to filter out the

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tailings solids. The geotextile is covered with another layer of gravel and cobbles for

erosion and ultraviolet protection of the geotextile.

While the surface water pond is being progressively displaced upwards by the rising

tailings, it is kept in continuous contact with the sloping underdrain, which removes the

water from the facility. A series of decant towers has also been installed in the TTSF

as a backup in the unlikely event that the hydraulic capacity of the underdrain is

exceeded by an extreme event or if a portion of the underdrain becomes blinded off.

To date, the decant towers have not been needed. The water removed by the

underdrain and decant towers is conveyed into a pair of concrete encased outlet pipes

that run under the northwest corner of the HLP to a downstream reclaim pond. From

there, the water is recycled to the mill.

Filling of the tailings basin is at an early stage. Initial deposition commenced from drop

bars in the northwest corner, i.e., the lowest part of the basin, and as the deposit

advanced upward and outward the adjacent drop bars on either side were

progressively brought into use. The objective was to place new tailings over the

existing deposit established from the last set of drop bars in order to avoid tailings

slurry running over the underdrain. The initially placed tailings in the northwest corner

were unable to achieve any appreciable liquid/solid separation or form a beach due to

the small area available. To prevent large amounts of water from decanting into the

leach ore embankments and potentially causing erosion of the ore and dilution of

pregnant solution in the HLP, a geomembrane was installed on the face of the

embankments in this corner. The geomembrane was extended up to elevation 3580

m, and when the tailings deposit reached the top of the geomembrane, a sloping beach

had been developed against the embankments that displaced the surface water pond

away from them.

Photo 2 - View to the west along thenorth side of the TTSF showing thetailings beach developed against theleach ore embankments

Photo 1 - View to the west along thesouth side of the TTSF basin showingthe small surface water pond in thesouth-central area in contact with theunderdrain blanket layer on the linedbase of the facility

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Photo 3 - View looking east showing

that good liquid/solid separation isoccurring over the tailings beach andclear water is present in the surfacewater pond against the slopingunderdrain blanket

Photo 4 - Close up view of clear water

discharging from the outlet pipe from theTTSF underdrain

The program of continuously rotating the active points of tailings deposition around the

beach perimeter has been in use for approximately 6 months, and at the end of the first

quarter of 2009, the top of the beach had reached elevation 3594 m. The average

density of the deposit was approximately 1.53 tonnes per cubic meter (T/m 3), which is

slightly in excess of the design value of 1.50 T/m 3. Modeling indicates the overall dry

density of the deposit will progressively increase to 1.65 T/m 3 over the next few years

as the rate of rise decreases. Figure 2 presents the filling curve for the TTSF, in terms

of elevation versus dry tonnage of tailings deposited, based on the design densities. A

few actual filling points are shown in the lower left hand corner, and as can be seen

they are in good agreement with the design curve. The currently planned phases of

expansion are depicted by the stepped line.

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Figure 2 - TTSF Filling Curve and Staged Development Phases

Vibrating wire piezometers were installed in the lower portion of the tailings deposit at a

height of 2.5 m above the underdrain (on stands placed above the underdrain) and in

the underdrain during Phase 1 construction. Figure 3 shows a timeline plot of the

measured pore pressures in the tailings and in the underdrain.

Figure 3 - TTSF Basin Piezometer Readings

The piezometers in the underdrain (06, 08, 10 and 14) are registering pore pressures

at near zero values, which indicates that the drain is operating at atmospheric pressure

as designed. The piezometers in the lower tailings (07, 09, 11 and 15) are showing

slightly higher pore pressures in the 1 to 3 m (of water) range but these values are wellbelow hydrostatic pressures thereby indicating that the tailings, while saturated, are

well drained and consolidated and passing seepage flow downward to the underdrain.

These results indicate that the seepage gradient into the underdrain is approximately

1.5, and based on a coefficient of permeability of the lower tailings of 2.910 -5

centimeters per second (cm/sec) from laboratory tests conducted during the design, the

calculated rate of vertical drainage from the tailings into the underdrain is about 0.09

cubic meters per second (m 3/sec) over the approximately 205,000 square meters (m 2)

inundated by tailings at the time these measurements were taken. This represents a

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fraction of the total flow discharging from the underdrain, the remainder being decanted

water from the small surface water pond.

Leach Ore Characteristics and Dynamic Stability of

Embankments

Leach Ore EmbankmentsCombining heap leach and tailings storage into a single facility changed the operating

dynamics of what had previously been only a heap leach operation. Two key design

challenges for the TTSF were that:

The leach ore embankments would continue to be loaded and leached but this

would now happen concurrently with tailings deposition, which provides certain

interfacing, logistical and solution management issues, and

The leach ore would be placed into the embankments in thick, uncompacted

lifts (to enhance leaching), which is contrary to the fill placement in typical

tailings dams, but despite this the embankments would still need to meet

rigorous safety standards set by Newmont, Knight Piesold and the international

mining community, particularly under earthquake loading.

These challenges were met by: (1) configuring the embankments to have wide cross

sections (to support ore loading and leaching) thus creating large structural shells that

would remain unsaturated, (2) placing coarser and finer ores in selected locations to

create zones in the embankments adapted from the heap design, (3) thickening the

tailings to reduce the amount of water entering the TTSF, and (4) using a rotational

tailings deposition method to build well drained and stable beaches against the

embankments.

Zones in the embankments that comprise coarser and finer ores are described below.

Zones A and C with up to 15 percent fines that includes higher quality run-of-

mine (ROM) ore is well drained and unsaturated even when under leach. Zone

A is a coarse grained, 50-meter wide zone placed on the exposed ultimate

downstream face of the embankment with minimum 50 percent gravel content;

Zone C is placed a minimum of two 16-meter lifts thick at the base of the

embankment over the leach pad.

Zone B with up to 20 percent fines forms the majority of the ore in theembankments in their middle elevations.

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Zone D with up to 25 percent fines may be produced by mixing (on a truck by

truck basis) higher fines content ore with lower fines content ore. This zone

generally comprises the last two lifts placed on the heap and, while not part of

the original design for the embankments, is being considered in current studies.

Seepage and Stability AnalysesThe key design principle for stability is to keep saturated or near saturated areas of

leach ore in the embankments well removed from the outer faces and contained behind

the large structural shells. By keeping the leach ore in the shells well drained and less

than fully saturated at all times, including when the embankments are under leach,

liquefaction by static or dynamic loading will be prevented. Thus, the design effort

became one of keeping these shells unsaturated. Finite element seepage modeling

using the computer program SEEP/W (GEO-SLOPE, 2004a) was conducted to define

the zones of potential saturation in the embankments.

Available data with respect to the saturated hydraulic conductivity of the La Quinua ore

were largely derived from testing a large number of samples in a previous study with

samples selected to represent a substantial range of fines contents. During later

analyses, conductivity versus depth relationships were developed from those

laboratory data for the subset of samples that were considered to be representative of

the ore types currently of interest (e.g., Zones A/C, Zone B, and Zone D). These

relationships were further refined by calibrating the hydraulic/seepage model results

against the observed field performance of the Stage 1 heap. In the seepage modeling

presented later in this report, the TTSF embankments were subdivided into depth

regions on 16-meter increments in accordance with the predominant lift thickness.

Appropriate saturated hydraulic conductivities were then assigned to each depth range.

Once a saturated conductivity was assigned to each portion of the relevant cross

sections, it was also necessary to define the relationship between hydraulic

conductivity (saturated and unsaturated) and soil suction for input into the variably

saturated seepage model. The van Genuchten-Mualem formulation was adopted for

analyses of the La Quinua facility, which requires definition of five curve fit parameters

including: s (theta-s), r (theta-r), (alpha), n and m, where m=1-1/n. The previous

laboratory testing program that generated the data on saturated hydraulic conductivity

of the leach ore also included capillary moisture retention testing on most of the

samples. Analyses of these data, once they had been corrected for the removal of

oversize material, yielded the unsaturated flow parameters at two different test

densities for the samples with a range of fines contents. Further evaluation of the data

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resulted in the recommendations for modeling the Zones A/C, B and D materials. The

resultant combination of saturated hydraulic conductivity and van Genuchten

parameters define the required conductivity versus suction relationship for each

material type and depth increment.

A typical result of the seepage modeling is illustrated on Figure 4. The saturated zone

is shown to be within the center of the embankment contained behind the wedge

shaped structural shells. It is largely contained within the Zone B material while the

Zone C material at the base of the embankment and the Zones A and B materials

forming the outer shells are freely drained and unsaturated. This is consistent with the

design objective. Figure 4 also shows flow vectors and contours of total head that

define the flow regime through the maximum cross section of the west embankment

(crest elevation 3660 m representing the original design) for the case where the full

crest width of the embankment is under leach at the average maximum design solution

application rate of 10 liters per hour per square meter (L/m 2/hr). The flow regime is

shown to be is gravity dominated with flow lines largely vertically downward and little

lateral spreading.

Figure 4 - Seepage Analysis Resul ts - Crest Elevation 3660

Limit equilibrium static and post-earthquake stability analyses of the embankments

using SLOPE/W (GEO-SLOPE, 2004b) were then completed by importing the pore

pressure data from the seepage files. The pore pressure data included the limits of the

saturated and unsaturated ore together with the pore pressure values at each finiteelement node. Following this, dynamic stability analyses under the influence of the

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maximum credible earthquake for the site, as established by a seismic risk

assessment, were completed using the geomechanical finite difference-modeling

program FLAC (Itasca, 2001). The initial step was to input density, modulus, strength

and pore pressure values to determine the in-situ states of stress in the section under

static loading conditions (self weight loads due to gravity alone). Material properties

and a soil-liner interface strength relationship at the base were input from the results of

extensive test work over several years on the HLP.

In the second stage of the dynamic analysis, the earthquake was applied by entering

an appropriate acceleration time-history and incorporating a liquefaction triggering

subroutine developed for use with FLAC at the University of British Columbia (Beatty

and Byrne, 1999). In that subroutine, a real-time analysis is performed where

triggering of liquefaction is evaluated in each finite difference zone by counting the

number of cycles of shear stress experienced by that zone until a threshold number is

reached. Strength and stiffness values are then re-assigned to the appropriate post-

liquefied value at the moment of liquefaction. A conservative undrained shear strength

(S u/p) ratio of 0.10 was selected to represent the strength of the liquefied leach ore.

Experience dictates that this is a conservative (low) value for a material that contains

significant gravel and sand percentages. Additionally, it was conservatively assumed

that the unsaturated leach ore would be subject to cyclic softening under the design

earthquake event, meaning that cyclic loading could generate positive pore pressures

less than those necessary to fully liquefy the material but which still could result in a

decrease in its shear strength. This was accounted for by reducing the friction angle of

the softened material to two-thirds of its original value.

Results of Dynamic AnalysesResults of the model showed the development of several shear bands that closely

parallel the potential critical slip surfaces defined by limit equilibrium slope stability

analyses (not reported in this paper). This close comparison between the two different

models added confidence to the analyses. The shear bands are shown on Figure 5.

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Figure 5 - Static Stress State of Dynamic Model Zones - Crest Elevation 3660

Additional results from the FLAC analyses, consisting of contours of total deformation,are shown on Figure 6.

Figure 6 - Post -Earthquake Deformations - Crest Elevation 3660

The predicted permanent deformations at critical locations on the embankment,

including along the underlying liner system and the crest, are small. Maximum

permanent deformations along the liner are predicted to be on the order of 15

centimeters, which should not result in significant damage to the liner or any loss of

containment of process fluids or impounded tailings. Maximum vertical deformations

on the crest of the facility are expected to be less that 25 centimeters, and the facility

will have sufficient freeboard at all times to accommodate well in excess of this. The

model shows that the unsaturated outer shells of the leach ore embankments provide

sufficient strength and stiffness, even after strain softening, to confine the inner

liquefied materials and keep the overall deformations to a minimum.

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Post-earthquake static stability analyses were completed to estimate the limit

equilibrium factor of safety of the embankment after the earthquake shaking ceases but

with strain softened or liquefied zones remaining (i.e., before the excess pore

pressures generated by the earthquake are dissipated). The results gave factors of

safety in excess of 1.1, a generally accepted minimum, for all but two cases, which

were in an area that will be buttressed by a downstream stage of the HLP (termed

Stage 7). This stage was not modeled. The conclusion was that this stage needed to

be raised to elevation 3548 meters before the leach ore embankments can be loaded

to their ultimate height. Alternatively, this constraint can be described as a maximum

seven-lift-differential between the crest of the TTSF embankment and the crest of the

Stage 7 buttress during incremental loading of the HLP. Stage 7 of the HLP is

currently under construction.

Actual Leach Ore Placement

In 2006, a sonic drill was purchased by MYSRL to aid in definition of fines content in

the leach ore in the La Quinua pit and on the HLP. The purpose of this is to reduce the

risk of placement of out of spec material within specific zones within the HLP and TTSF

embankments. The drilling method was selected based on the requirement for

continuous and maximum sample recovery. Previously this type of rig had been used

successfully in geotechnical and process investigations of the original Stage 1 heap.

Leach ore placed into the early stages of the HLP before implementation of the

zonation described previously would have been classed as Zone B, comprising typical

La Quinua leach ore. Later, Zones A and C, and Zone D represented selective mining,

blending and placement of cleaner and dirtier material, respectively. Review of

leach ore placement data from the sonic rig indicated that there was not adequate

differentiation in the fines contents of the various zones in the TTSF embankments,

e.g., much of the material placed resembled Zone B. It is expected that more intensive

use of the sonic drill as described above, coupled with improvements in ore control

protocols, should improve compliance with the zone characteristics during future ore

placement.

It must be noted that the stability of the HLP and TTSF embankments are strongly

influenced by the zones of saturation and pore pressures within the leach ore.

Monitoring of water levels and positive and negative pressure heads within the

embankments during operations will provide critical feedback with respect to theperformance of these structures relative to that which was predicted (i.e., maintenance

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of an adequate factor of safety against slope failure). While some analyses of the HLP

and/or TTSF indicated that a reduced leach application rate may be required when fully

loaded, actual operational control will be based on the observed pore pressures and

saturated zones within the embankments. Higher leach application rates and/or the

inadvertent inclusion of higher fines content materials may be acceptable as long as

the threshold levels are not exceeded. Because of the importance of controlling the

process solution level and maintaining structural shells at a sufficiently low degree of

saturation, future efforts will include an emphasis on expansion of the number and

types of instruments within the HLP and TTSF embankments. These may comprise

instruments capable of measuring soil suction as well as pressure head to assess

moisture content (and hence the degree of saturation) via the moisture characteristic

curves already defined for the various leach ore types/embankment zones.

Conclusions

The La Quinua TTSF is unique in that it is located within the La Quinua HLP and uses

heap leach ore, strategically placed within specific lines and grades, to form the

containing embankments. Contrary to standard tailings dam practices these

embankments have been built with leach ore placed in thick, uncompacted lifts and are

subject to leach solution being applied to, and percolating through, them. The loose

structure makes static and dynamic liquefaction potentially a critical issue, and the key

design principle for avoiding a liquefaction related failure is to keep the saturated or

near saturated zones well contained in the embankments behind large structural shells

that are unsaturated at all times. Dynamic analyses have shown that while the

potential for liquefaction or strain softening of some saturated or near saturated zones

in the embankments is predicted under to occur under the maximum credible

earthquake, the predicted deformations would be minimal. Post-earthquake stability

analyses using cyclic strain softened and liquefied strengths also showed that

adequate stability will be maintained. These findings will be confirmed with a

comprehensive geotechnical investigation and follow-up slope stability and dynamic

deformation analyses of the actual materials placed in the TTSF and in the unlikely

event that this indicates conditions markedly different from those discussed herein, the

achievement of non-liquefiable conditions in the structural shell zones of the leach ore

embankments will be established by controlling the leach solution application rate and

verifying with pore pressure and soil suction monitoring.

The advantages of this combined tailings and heap leach facility for its owner were: (1)reduced capital construction costs, (2) reduced projected operating cost, and (3) ability

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to meet a 2008 schedule commissioning. While other tailings depositional techniques

and locations were studied, designing and constructing the tailings impoundment within

the HLP proved to be a truly innovative step, which in part defined MYSRLs La Quinua

mill projects success.

ReferencesBeatty, M.H. and P.M. Byrne, 1999, A Synthesized Approach for Modeling Liquefaction

and Displacements, Proceedings of the International FLAC Symposium on

Numerical Modeling in Geomechanics , A.A. Balkema Publishers.

GEO-SLOPE, 2004a, SEEP/W Version 5.20 , GEO-SLOPE International Ltd., Calgary,

Alberta.

GEO-SLOPE, 2004b, SLOPE/W Version 5.20 , GEO-SLOPE International Ltd., Calgary,

Alberta.

Itasca, 2001, FLAC Version 4.00 - Users Manual , Itasca Consulting Group Inc.

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