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Feb. 23 - 26, 2014, Salt Lake City, UT

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REMEDIATION OF LARGE-SCALE SLOPE INSTABILITY AND IMPACT ON MINE DEVELOPMENT AT THE GOLD QUARRY

MINE R. J. Sheets, Newmont Mining Corp., Carlin, NVS. J. Douglas, Newmont Mining Corp., Carlin,

NVR. M. St. Louis, Newmont Mining Corp., Carlin,

NV

ABSTRACT

In 2009 the Gold Quarry open pit mine, located 11 km northwest of Carlin, Nevada, USA, experienced multiple large-scale slope instabilities othe upper east highwall that reduced gold ore extraction for nearly 18 months. The slope instabilities occurred within a weak, consolidated sedimentarsequence that exhibits strength characteristics that are transitional between soil and rock. Instability initiated as mining exposed the lower clay-ricsub-units of the Carlin Formation (Fm.). This deformation created preferential flow paths that allowed groundwater from the upper sandy sub-units toinfiltrate low permeability clay-rich sub-units, thereby enhancing deformation of the slope toe which in turn destabilized the upper portion of the highwalThe outcome was a 160 m high slope instability that had a lateral run- out of 850 m. The effort to return the pit to ore production required both thegeotechnical and hydrogeological investigations and the preliminary remediation mining activity to be concurrent. This approach required thdevelopment of detailed safety procedures and a requirement to modify the remediation design as new results were obtained. An initial challenge wa

to mitigate a near vertical, 90 m headscarp with blast induced, localized slope failures. Back-analyses applying numerical modeling indicated that thefailure surface did not coincide with initial interpretations based on field observations; drilling results eventually confirmed this alternative failuregeometry. The final remediation design incorporated shallower slope geometries and an approximately 3 Mt buttress along the base of the CarlinFormation and bedrock contact to reinforce the lower clay-rich sub-units. The outcome is a stable highwall within the Carlin Fm. following nearly tenyears of repeated slope instability, and an example of the necessity to conduct appropriate geotechnical and hydrogeological studies during the earlstages of a new layback evaluation or new open pit development.

INTRODUCTION

Modern mining activity began at the Gold Quarry open pit, in the early 1980s. Between 1985 and 2009 numerous slope instabilities haveoccurred within the non-ore bearing Tertiary-age Carlin Fm. that overlies the primary gold deposit (Bates et al., 2006; Sherman and Sheets, 2008Sheets, 2011). The Carlin Fm. exhibits a combination of soil and rock characteristics similar to those recognized in saprolites. Groundwater tends to bdiscontinuous in sand and gravel zones or in flow paths developed within the damage zone along faults; however, the majority of the formationexhibits unsaturated conditions. Slope instability is observed to be soil-like movement along the base and within the slide mass bounded by faults ofractures around the scarp and backplane. The lower sub-units of the Carlin Fm. consist of very weak silts and clays that exhibit strain-softeninbehavior. In 2009, Gold Quarry endured multiple large-scale slope instabilities totaling 12Mt along the east highwall within the Carlin Fm. The impact of these slope instabilities was a significantly reduced gold ore extraction rate for nearly

18 months between December 2009 and May 2011. A focused slope stabilization program and remedial mine design was developed and executed tmitigate the instability from propagating and inducing additional risk. The focus of this paper is on the assessment of failure dynamics and remediastabilization design; mechanisms are discussed in detail in other sources (Yang et al.,2011; Sheets, 2011; Sheets, 2013).

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Remedial stabilization activity commenced following a preliminary slope design based on the existing knowledge and observations of Carlin Fm.behavior. A simultaneous geotechnical and hydrogeological investigation was conducted to determine the cause of the instability and develop, withhigh confidence, a stable final highwall design that would return Gold Quarry to active production. The potential risks of working around the instabilityrequired a dual emphasis on a strong slope monitoring program, and detailed standard operating procedures. These procedures governed everyaspect of work conducted around the instability. Remedial mining activity was complicated by a 90 m near vertical escarpment that could not besafely approached. As a result, an exploration drill rig was used to drill angled blast holes behind the crest to create a shallower, more stable back-scarp that could be safely approached and mined using standard mining methods. The final remediation design, developed from the newlyacquired data collected from the geotechnical and hydrogeological investigations and various back-analyses, incorporated a shallower overall slopegeometry and an approximately3 Mt buttress along the Carlin Fm. and bedrock contact to reinforce the lower clay-rich sub-units. The outcome of this remediation effort is the firststable, manageable highwall within the Carlin Fm. following nearly ten years of repeated slope instability. This greatly improves the capability of

geotechnical and hydrogeological personnel to provide confident recommendations in support of developing reliable open pit slope designs, which inturn improves the ability of mine operations to achieve the mine plan. The result is an indication to the importance of conducting thorough geological,geotechnical, and hydrological investigations during the early stages of a new layback evaluation or open pit development.

GEOLOGICAL AND HYDROGEOLOGICAL SETTING GeologyHistorical slope instability in Gold Quarry open pit primarily has

occurred in the Tertiary Carlin Fm., which overlies the Paleozoic, ore bearing bedrock. The contact between the Carlin Fm. and the underlyingbedrock is an angular unconformity in a portion of the open pit and a fault zone along the remaining portion (Harlan et al., 2002; Regnier, 1960). TheCarlin Fm. is composed of volcanoclastic units deposited within a lacustrine environment grading upward into fluvial silty sands. The formation has athickness of 600 m within structural basins near Gold Quarry; however, the maximum height within the pit is 150 m currently in the upper easternhighwall, most of which is from the lower sub-units. The bedrock-Carlin contact is marked by Basal Clay, and locally by a clay-rich gravel unit, both ofwhich are dominated by montmorillonite. Variably indurated tuffaceous sedimentary sub- units overlie the Basal Clay. The lower tuff units,dominated by the Lower Laminated Tuff, contain significant clay altered tuff. The middle sub-units are characterized by partially indurated, interbeddedsiltstone and sandstone, and unconsolidated sands. The upper most sub-units consist of unevenly cemented sands and debris flow. The currentCarlin Fm. stratigraphic column is shown in Figure 1.

Tectonic activity resulted in structural control on deposition of the Carlin Fm. Major Basin and Range structures developed a stair- stepped

bedrock contact that generally deepens to the east, forming grabens and horsts. Grabens collected thicker sequences of Basal Clays or LowerLaminated Tuff (the weaker sub-units). The varying

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thicknesses of rapid sedimentation resulted in differential loading conditions upon the high plasticity clays. It is conceivable that the differentialoading, growth faulting, and tectonic activity resulted in deformation within the clays during deposition. The reactivation of Paleozoic faults duringTertiary Basin and Range extension is visible within the Carlin Fm. These faults originated in Paleozoic rocks during compressional events, but thesubsequent extensional regime induced reverse movement upon the structures which projected into the Carlin Fm. as normal faults. In isolated casedistinct structures are visible, but more widespread evidence includes a preferential fabricin the northeast sector of Gold Quarry.

Figure 1. Generalized Carlin Fm. stratigraphy based on recent modeling.

En 2009, la mina a cielo abierto de oro Cantera, situado a 11 km al noroeste de Carlin, Nevada, EE.UU., experiment mltiples inestabilidadespendiente a gran escala de los highwall upper east que redujeron la extraccin de mineral de oro durante casi 18 meses. Las inestabilidades dependiente se produjo dentro de una secuencia sedimentaria dbil, consolidada que presenta caractersticas de resistencia que son de transicin entreel suelo y la roca. Inestabilidad iniciado como la minera expuesto los menores subunidades ricas en arcilla de la Formacin Carlin (Fm.). Estadeformacin crea trayectorias de flujo preferenciales que permitieron agua subterrnea de las sub-unidades superiores de arena para infiltrarse bajapermeabilidad sub-unidades ricos en arcilla, mejorando as la deformacin de la puntera pendiente que a su vez desestabiliz la porcin superior deltalud. El resultado fue una alta inestabilidad de la pendiente 160 metros que tena una Ejecutar- lateral de 850 m. El esfuerzo por devolver el hoyo almineral de produccin requiere tanto de las investigaciones geotcnicas e hidrogeolgicas y la actividad minera preliminar remediacin seaconcurrente. Este enfoque requiere la elaboracin de procedimientos detallados de seguridad y la obligacin de modificar el diseo de remediacincomo se obtuvieron nuevos resultados. Un reto inicial era para mitigar una, 90 m grieta frontal casi vertical con inducida hornos, fallas de pendienteslocalizadas. Volver-anlisis aplicando el modelado numrico indic que la superficie de falla no coincida con las interpretaciones iniciales basadas enlas observaciones de campo; resultados de perforacin finalmente confirmaron esta geometra fracaso alternativa. El diseo final de remediacinincorpora menos profundas geometras de pendiente y una de aproximadamente 3 Mt contrafuerte en la base de la Carlin Formacin y lecho de roca decontactos para reforzar las sub-unidades ricas en arcilla inferiores. El resultado es un talud estable dentro de la Fm Carlin. tras casi diez aos de

inestabilidad de taludes repetido, y un ejemplo de la necesidad de realizar estudios geotcnicos e hidrogeolgicos apropiados durante las primerasetapas de una nueva evaluacin layback o nuevo desarrollo a cielo abierto.

INTRODUCCIN

Actividad minera moderna comenz en el cielo abierto de oro Cantera, en la dcada de 1980. Entre 1985 y 2009 numerosas inestabilidades dependiente se han producido dentro de la no-mineral de rodamiento-Terciario Carlin Fm. que recubre el depsito de oro primario (Bates et al, 2006;.Sherman y Sheets, 2008; Sheets, 2011). La Fm Carlin. exhibe una combinacin de las caractersticas del suelo y de roca similares a los registrados ensaprolitas. El agua subterrnea tiende a ser discontinua en las zonas de arena y grava o en vas de flujo desarrolladas dentro de la zona de avera lolargo de fallas; Sin embargo, la mayora de la formacin exhibe condiciones insaturados. Inestabilidad de la pendiente se observa que es el movimientodel suelo-como a lo largo de la base y dentro de la diapositiva de masa limitada por fallas o fracturas alrededor de la escarpa y el panel posterior. Lassub-unidades inferiores de la Fm Carlin. consistir en limos y arcillas muy dbiles que exhiben comportamiento cepa de ablandamiento. En 2009, GoldQuarry soport mltiples inestabilidades pendiente a gran escala por un total de 12Mt a lo largo del talud oriental dentro de la Fm Carlin. El impacto de estas inestabilidades pendiente era una tasa de extraccin de mineral de ororeducido significativamente durante casi 18 meses entre diciembre de 2009 y mayo de 2011 un programa de estabilizacin de taludes centrado ydiseo de la mina correctivas se desarrollan y ejecutan para mitigar la inestabilidad de propagacin y la induccin de riesgo adicional. El objetivo deeste trabajo es en la evaluacin de la dinmica de fallos y diseo de estabilizacin correctivas; mecanismos se discuten en detalle en otras fuentes

(Yang et al.,2011; Sbanas, 2011; Sheets, 2013).

Actividad de estabilizacin de Remediacin comenz siguiendo un diseo preliminar pendiente basado en el conocimiento y observaciones existentesde Carlin Fm. comportamiento. Se realiz una investigacin geotcnica e hidrogeolgica simultnea para determinar la causa de la inestabilidad y dedesarrollar, con la mxima confianza, un diseo de talud final estable que devolvera Oro Cantera de produccin activa. Los riesgos potenciales detrabajar en torno a la inestabilidad requiere un doble nfasis en un fuerte programa de monitoreo de taludes, y los procedimientos operativos estndardetalladas. Estos procedimientos rigen todos los aspectos de la labor llevada a cabo en torno a la inestabilidad. Actividad minera Remediacin secomplic por una 90 m cerca de escarpe vertical que no podan ser abordados de manera segura. Como resultado de ello, se utiliz un equipo deperforacin de exploracin para perforar barrenos en ngulo detrs de la cresta para crear un back-escarpa ms estable menos profunda que podraser abordado y extrae de forma segura utilizando mtodos de extraccin estndar. El diseo final de remediacin, desarrollado a partir de los datosrecientemente adquiridos recogidos de las investigaciones geotcnicas e hidrogeolgicas y varias copias de los anlisis, incorpora una geometra deinclinacin general menos profunda y un aproximado3 Mt contrafuerte lo largo de la Fm Carlin. y el contacto lecho de roca para reforzar las sub-unidades ricas en arcilla inferiores. El resultado de esteesfuerzo de remediacin es la primera, highwall manejable estable dentro de la Fm Carlin. tras casi diez aos de inestabilidad de taludes repetida. Esto

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mejora en gran medida la capacidad del personal geotcnicos e hidrogeolgicos para proporcionar recomendaciones seguras para apoyar el desarrollode diseos de pendiente del tajo abierto fiables, lo que a su vez mejora la capacidad de las operaciones de la mina para lograr el plan de la mina. Elresultado es una indicacin de la importancia de realizar geolgico profundo, geotcnicos, hidrolgicos e investigaciones durante las primeras etapasde una nueva evaluacin layback o desarrollo a cielo abierto.Geologa marco geolgico e hidrogeolgicoInestabilidad de las laderas Histrico en Gold Quarry cielo abierto tiene principalmenteocurrido en el Carlin Fm Terciario., que se superpone al Paleozoico, lecho de roca mineral de rodamiento. El contacto entre la Fm Carlin. y la rocasubyacente es una discordancia angular en una parte del cielo abierto y una zona de falla a lo largo de la porcin restante (Harlan et al, 2002;. Regnier,1960). La Fm Carlin. se compone de unidades volcanoclsticos depositados dentro de un ambiente lacustre clasificacin hacia arriba en arenas limosasfluviales. La formacin tiene un espesor de 600 m en las cuencas estructurales cerca de Gold Quarry; Sin embargo, la altura mxima dentro de la fosaes de 150 m se encuentran actualmente en el talud oriental superior, la mayora de los cuales proviene de las sub-unidades inferiores. El contacto de

lecho de roca-Carlin est marcada por Basal Clay, y localmente por una unidad de grava rico en arcilla, ambos de los cuales estn dominadas pormontmorillonita. Subunidades sedimentarias tufceas variablemente induradas recubren el Basal Clay. Las unidades de toba inferiores, dominadas porel Lower laminado Tuff, contienen arcilla significativa toba alterada. Las sub-unidades intermedias se caracterizan por parcialmente endurecida,limolitas y areniscas intercaladas, y arenas no consolidadas. Las mayora de los sub-unidades superiores consisten en arenas cementadas de formadesigual y flujo de escombros. La corriente Fm Carlin. columna estratigrfica se muestra en la Figura 1.

La actividad tectnica result en control estructural sobre la deposicin de la Fm Carlin. Cuenca Mayor y estructuras Rango desarrollaron un 'stair- pis'contacto lecho de roca que por lo general se profundiza hacia el este, formando fosas tectnicas y horsts. Grabens recogen secuencias ms gruesasde Basal Arcillas o inferior laminado Tuff (las sub-unidades ms dbiles). los distintos

espesores de rpida sedimentacin resultaron en condiciones de carga diferenciales sobre las arcillas de alta plasticidad. Es concebible que la cargadiferencial, fallamiento de crecimiento, y la actividad tectnica dieron lugar a la deformacin dentro de las arcillas durante la deposicin. La reactivacinde las fallas del Paleozoico durante Cuenca Terciaria y Range extensin es visible dentro de la Fm Carlin. Estas fallas se originaron en rocas delPaleozoico durante los eventos de compresin, pero el rgimen extensional posterior inducidos movimiento inverso a las estructuras que seproyectaban hacia la Fm Carlin. como fallas normales. En casos aislados distintas estructuras son visibles, pero la evidencia ms extendido incluye una'tela' preferencial en el sector noreste de Oro Cantera.

HydrogeologyThere are three aquifers at Gold Quarry. Bedrock groundwater within the Paleozoic formations has been actively dewatered since the early 1990

in order to provide dry mining conditions within the ore body and depressurize bedrock slopes. A confined, shallow bedrock aquifer was identifieto the southeast of Gold Quarry in recent years. This shallow bedrock aquifer is perched upon the Roberts Mountain Thrust Fault. The third aquifer ipresent within the Carlin Fm. Overallthe hydraulic conductivities in the Carlin Fm. are quite low (10 -6 to 10-9 m/s) due to the fine-grained material (silts and clays) within the sub- units, busubstantially higher permeability occurs within localized sand or gravel zones. Low permeability across faults resulted in variably- saturated zonesseparated from zones that contain dry sub-units. Deformation-induced alteration in clay zones, facies changes, and reworking result in laterahydraulic discontinuity and verticalanisotropy.

NINE POINTS SLOPE INSTABILITIES

Slope deformation that progressed into the Nine Points slope instabilities originated during summer 2008. Bench-scale failures developewithin the lower Carlin Fm. sub-units as mining progressed into a graben bound by the pitward dipping Challenger Fault, and the highwall dipping GrayTuff Fault. The Carlin Fm./bedrock contact at the Nine Points slope toe is along the Gray-Tuff Fault, shown in cross- section in Figure 2. The slopeinstability propagated up the highwall towards the Nine Points intersection located behind the pit crest. This progressive development was interpreteas displacement of the slope toe within the graben, which over-steepened the upper highwall, allowing it to displace along a weak contact into the

continually displacing toe. The interramp slope within the lower sub-units wasmined at a 35o slope angle; the overall Carlin Fm. slope height was150 m. Mining activity at the bedrock contact was suspended to allow a five bench, 680 kt unweighting cut to be completed while the lower portion othe Carlin Fm. was sloped to between 20 and 25o. The remedial measures were temporarily successful in mitigating slope movement; however, sincremediation did not address reinforcing the

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already weakened failure surface and slope to, overall slope movement reactivated in February 2009

HidrogeologaHay tres acuferos de Gold Quarry. Aguas subterrneas de Bedrock dentro de las formaciones del Paleozoico se ha deshidratado activamente desdeprincipios de 1990 con el fin de proporcionar condiciones de la minera en seco dentro del cuerpo de mineral y despresurizar laderas de roca madre.Un confinado, lecho de roca acufero superficial se identific al sureste de Gold Quarry en los ltimos aos. Esta roca acufero superficial se alzasobre la falla de cabalgamiento Roberts Mountain. La tercera acufero est presente dentro de la Fm Carlin. generallas conductividades hidrulicas en la Fm Carlin. son bastante bajos (10-6 a 10-9 m / s) debido a los materiales de grano fino (limos y arcillas) dentrode las subunidades, pero sustancialmente mayor permeabilidad se produce dentro de las zonas de arena o grava localizadas. Baja permeabilidad atravs de fallos result en variably- zonas separadas de las zonas saturadas que contienen sub-unidades secas. Alteracin inducida por deformacinen zonas de barro, facies cambios y resultado reelaboracin en discontinuidad hidrulica lateral y vertical

anisotropa.

NUEVE PUNTOS Taludes inestable

Pendiente deformacin que avanz en los nueve puntos inestabilidades pendiente originaron durante 2008 fracasos a escala de banco de verano sedesarrollan en el Carlin Fm inferior. subunidades como la minera progresaron en un graben obligado por la pitward inmersin Challenger Falla, y eltalud inmersin Gray-Tuff Falla. El contacto Carlin Fm. / Lecho de roca en el dedo del pie Nueve Puntos pendiente es a lo largo de la Falla de Gray-Tuff, se muestra en la seccin transversal en la Figura 2 La inestabilidad de las laderas propagado hasta el talud hacia los nueve puntos deinterseccin se encuentra detrs de la cresta de boxes. Este desarrollo progresivo se interpret como el desplazamiento del dedo del pie dentro de lapendiente graben, que sobre-empinamiento el talud superior, permitiendo que se desplace a lo largo de un contacto dbil en el dedo del piedesplazando continuamente. La pendiente interramp dentro de las sub-unidades inferiores eraminado en un ngulo de inclinacin de 35 ; la Fm general Carlin. altura de la pendiente era

150 m. La actividad minera en el contacto de la roca madre fue suspendido para permitir que un banco de cinco, 680 kt descarga de peso corte paraser completado mientras que la parte inferior de la Fm Carlin. estaba inclinado a entre 20 y 25o. Las medidas correctivas fueron un xito temporal enla mitigacin de movimiento pendiente; Sin embargo, desde la remediacin no abord el refuerzo de la

superficie de falla ya debilitada y la pendiente a, el movimiento global pendiente reactivado en febrero de 2009

Figure 2. Southwest-northeast cross-section, location identified in Figure 6, viewed to the northwest, of the Nine Pointstopography and geological model at the time of the Nine Points slope failure.

April 2009 Slope InstabilityThe Nine Points slope toe continued to displace, with resultant deformation within the upper slope. A rainy early April was followed by acceleration

in the middle of the month, and interpretation of inverse velocity data from highwall prism surveys indicated a potential failure event in late April or earlyMay. Overall slope failure occurred April 26thas deformation rates approached 4.5 m/day. The Nine Points slide encompassed the entire vertical exposure of the Carlin Fm. at 160 m with a width of450 m. The toe displaced approximately 45 m with the primary event. The continuing toe displacement resulted in slide material covering an 85 mwide haulage intersection and spilling onto the active mining area below the intersection. The overall slope failure was estimated at 7.25 Mt.

Up to this time, the Nine Points slide exhibited behavior similar to historic Carlin Fm. instabilities. The progressive failure development overseveral months was consistent with the strain-softening strength behavior of the lower sub-units. Laboratory tests indicated that with increasing shearstrain, the cohesion and friction angle for the clay-rich sub-units deteriorate to a cohesionless state with an average residual frictional strength of 8.However, these lower sub-units characteristically are observed in core samples and pit slope exposures to be sheared and slickensided in-situ which suggests that the pre-mining shear strength is at or near the residual strength.

Continual deformation of the slide mass created a vertical headscarp approximately 60 m high. The inherent hazards associated with the slide

geometry limited the remediation until either a safe method to mine the headscarp could be developed, or the scarp attained a more stablegeometry. The slope remediation plan consisted of developing a 15 slope through the slide mass, cleaning the material from the intersection that wasfounded on bedrock, and constructing a buttress to provide resistance along the Carlin Fm./bedrock contact.

December 2009 Slope InstabilityOn December 24th, at approximately 7:00 AM, a second large- scale instability developed within the previous Nine Points slide. A 1.25

Mt block, extending nearly 40 m behind the original failure scarp crest, rapidly accelerated toward failure. The sudden change in loading condition atthe head of the slide mass caused the mass to mobilizeand run-out up to 850 m into the pit floor. This extensive run-out distance has been attributed primarily to air-entrainment of the slide mass. Thenewly-formed headscarp was 90 m high. Although the instability had developed within the 160 m Carlin Fm. exposure, the effect of the run-out resultedin 425 m of highwall height impacted by the slide (Fig. 3). The pit bottom was filled with nearly 40 m of slide debris. The original estimated size ofthis instability was 25 Mt; however, it was reduced to 12 Mt following a thorough investigation.

The extensive, sudden run-out behavior of this instability was unlike all historic slope failures in the Carlin Fm. at Gold Quarry. Previous slopeinstabilities exhibited observable and measurable signs of deformation over a period of weeks or months prior to failure events,

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but this slide began exhibiting movement 15 hours prior to failure. Initially, the April 2009 slide mass provided support for the near vertical scarp; buthere would have been a high stress condition at the toe of the scarp at the intersection with the failure surface underneath the slide mass. Thisstress imbalance, compounded by removing the slide material through the on-going slope remediation caused development of a slip surface in thescarp and rapid displacement along this surface. The instability culminated in a rapid failure of the April 2009 headscarp into the base of the slide masand along the previous slip surface.

Seccin transversal Figura 2. suroeste-noreste, lugar identificado en la figura 6, se ve hacia el noroeste, de los nuevepuntos topografa y modelo geolgico en el momento de los nueve puntos de falla de la pendiente.

04 2009 Inestabilidad PendienteLos nueve puntos pendiente del dedo del pie continu para desplazar, con la deformacin resultante dentro de la partesuperior del talud. A principios de abril de lluvias fue seguido por la aceleracin en la mitad del mes, y la interpretacin delos datos de velocidad inversas de las encuestas de prisma Highwall indica un potencial evento de fallo a finales de abrilo principios de mayo. Fracaso total pendiente se produjo el 26 de abrilcomo las tasas de deformacin acercaron 4,5 m / da. Los nueve puntos de diapositivas abarcaba toda la exposicinvertical de la Fm Carlin. a 160 m con una anchura de 450 m. El dedo del pie desplazada aproximadamente 45 m con elevento principal. El desplazamiento del dedo del pie continua result en el material deslizante que cubre una ampliainterseccin transporte 85 m y derramar en la zona minera activa por debajo de la interseccin. La falla de la pendientegeneral se estim en 7,25 Mt.

Hasta este momento, los nueve puntos de diapositivas mostr un comportamiento similar a la histrica Carlin Fm.inestabilidades. El desarrollo de la insuficiencia progresiva durante varios meses era consistente con el comportamientode resistencia cepa de ablandamiento de las sub-unidades inferiores. Las pruebas de laboratorio indican que al aumentarla deformacin de corte, el ngulo de la cohesin y la friccin de las sub-unidades ricas en arcilla deteriore a un estadosin cohesin, con una fuerza de friccin residual media de 8 . Sin embargo, estas sub-unidades inferiorescaractersticamente se observan en muestras de ncleo y las exposiciones de pendiente fosa para ser cortado yslickensided in situ lo que sugiere que la resistencia al cizallamiento pre-minera est en o cerca de la resistenciaresidual.

Deformacin continua de la masa deslizante cre una grieta frontal vertical de aproximadamente 60 m de altura. Losriesgos inherentes asociados con la geometra de deslizamiento limitado la remediacin hasta que sea un mtodo seguropara la explotacin del grieta frontal se podra desarrollar, o la escarpa alcanzaron una geometra ms estable. El plan deremediacin pendiente consisti en el desarrollo de una pendiente de 15 a travs de la masa deslizante, la limpieza delmaterial de la interseccin que se fund sobre la roca, y la construccin de un contrafuerte para proporcionar resistenciaa lo largo de la Fm Carlin. / Contacto lecho de roca.

12 2009 Inestabilidad Pendiente

El 24 de diciembre, aproximadamente a las 7:00 de la maana, un segundo inestabilidad a gran escala desarrollada en eanterior Nueve Puntos de diapositivas. A 1,25Bloque Mt, que se extiende casi 40 metros por detrs de la cresta escarpa fallo original, se aceler rpidamente hacia elfracaso. El cambio repentino en la condicin de carga a la cabeza de la masa deslizante caus la masa de movilizary run-out hasta 850 m en el fondo del foso. Esta extensa distancia run-out se ha atribuido principalmente a la inclusin deaire de la masa deslizante. La grieta frontal recin formada era de 90 m de altura. Aunque la inestabilidad se habadesarrollado dentro de los 160 m Carlin Fm. la exposicin, el efecto de la carrera de salida result en 425 m de alturahighwall impactado por la corredera (. Fig 3). El fondo del foso se llen con casi 40 metros de escombros de diapositivas.El tamao original estimado de esta inestabilidad fue de 25 Mt; sin embargo, se redujo a 12 Mt a raz de unainvestigacin a fondo.

La extensa comportamiento, run-out repentina de esta inestabilidad era a diferencia de todos los fallos pendienteshistricos en la Fm Carlin. en el Gold Quarry. Inestabilidades pendiente anteriores exhibidas signos observables ymedibles de la deformacin en un periodo de semanas o meses antes de los eventos de fallo,

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pero esta diapositiva comenz a exhibir movimiento 15 horas antes de la falla. Inicialmente, la masa deslizante apoyadoabril de 2009 para la escarpa casi vertical; pero no habra habido una condicin de alto estrs en el dedo del pie de laescarpa en la interseccin con la superficie de falla por debajo de la masa deslizante. Este desequilibrio estrs, agravadapor la eliminacin del material de deslizamiento a travs de la remediacin pendiente en curso causada desarrollo de unasuperficie de deslizamiento en la escarpa y el desplazamiento rpido a lo largo de esta superficie. La inestabilidadculmin en un rpido fracaso de April de 2009, grieta frontal en la base de la masa deslizante y a lo largo de la superficiede deslizamiento anterior.

Figure 3. The Nine Points slope instability following the 24December 2009 failure.

Safe Working Procedures for the Slide AreaSpecial procedures were implemented to govern all activities related to the remediation. These procedures were designed to ensure the safety o

both Newmont and contractor personnel, and equipment, working within and adjacent to the slide area. Prior to work commencing or whechanges to the procedures were necessary, the new procedures would be discussed and communicated to the work force. The area(s) for which thesprocedures were applicable was visible on a display monitor installed on the shovels, front-end loaders, and dozers. For all other personnel andequipment, the boundary was marked in the field with stakes and signage. The boundary was also included on a daily plan map that would bedistributed to the entire workforce with the special work procedure document.

The most basic requirement to work within the slide area was that the area had been evaluated and determined to be safe bgeotechnical staff and the mine operations foremen. A qualified spotter was stationed in an old dispatch tower that had a view of the entire slidearea. Any personnel that needed to enter the area were required to contact the spotter to confirm that it was safe to enter; personnel also wererequired to communicate to the spotter when they were exiting the area. Additional spotters would be strategically located based on mininconditions.

All spotters were trained by geotechnical staff to understand visible signs of movement. The spotters stationed in the old dispatch towereceived training on the slope monitoring software. This training was not intended to enable spotters to interpret the deformation data, but to simplyrecognize if or when conditions had changed and to contact the shift foremen and geotechnical staff. The slope deformation monitoring system was

required so that real time data was available to the spotters in the old dispatch tower, and in Mine Control, so that if alarms were triggered personneworking in the hazard zone as well as geotechnical staff could be notified and immediately evacuated. When conditions visually changed, or the slopmonitoring system indicated a movement alarm, the mining area would be evacuated until the proper inspections could be conducted and thpotentially hazardous condition no longer existed.

Headscarp Drill and Blast ProgramThe geometry of the headscarp and potential for brittle failure of partially cemented sands exposed in the scarp prohibited the use of

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standard mining methods and equipment near the crest. Since safety restrictions precluded access to an area within 25 m along the crest, it wasdetermined that a drill and blast approach was a potential solution. The concept was to use angle-drilled blast holes that were sequentially timed toinitiate small, controlled slope failures around the headscarp crest to develop a shallower slope that could be remediated using typical miningmethods (Acorn, 2011). A key blast design criterion was to minimize damage to the wall behind each blast.

Surveying of the headscarp was completed using a commercial laser scanner. Detailed surface topography was necessary to design each blasthole to be drilled perpendicular to the scarp with the appropriate burden. A reverse circulation drill was used to drill blast holes that varied in length from45 to 70 m. Blast hole inclination was originally designed between -60 to -65 below horizontal to ensure that the blast product could be placed downan inclined borehole that waslined with PVC casing. Over the course of the project, the inclination was successfully decreased to -45 . Borehole surveys were conducted as drillingadvanced to ensure that each borehole was not significantly deviating, which allowed the current and subsequent drill holes to be redesigned as

necessary to maintain the appropriate drill hole spacing.Each blast hole was loaded with two charge decks. These were not cast blasts. Instead, the intent was to create a failure plane and force

material to fail. The bottom charge generated the largest displacement by pushing out the toe, destabilizing the upper scarp in the process. The topcharge created a crack between the holes, similar to a pre-split, which acted as a failure plane for the material to displace down onto the slide mass.The actual blast initiation and performance was evaluated from review of high speed and normal speed video recordings. A post blast survey wasconducted with a laser scanner to assess the new slope geometry with respect to the designed post blast geometry.

Figura 3 El Nueve Puntos inestabilidad de laderas despus de la 2412 2009 fracaso.

Procedimientos de trabajo seguro para el Area de diapositivasProcedimientos especiales se llevaron a cabo para gobernar todas las actividades relacionadas con la remediacin. Estos procedimientos fueron

diseados para garantizar la seguridad tanto Newmont y el personal del contratista, y el equipo, trabajando dentro y adyacente a la zona deldeslizamiento. Antes de comenzar el trabajo o cuando los cambios en los procedimientos eran necesarios, los nuevos procedimientos se discutirn yse comunicarn a la fuerza de trabajo. El rea (s) para los que estos procedimientos eran aplicables era visible en un monitor de pantalla instalado enlas palas, cargadores frontales y bulldozers. Para el resto de personal y equipo, la frontera fue marcada en el campo con estacas y sealizacin. El

lmite tambin se incluy en un mapa plan diario que se distribuye a toda la plantilla con el documento especial procedimiento de trabajo.

El requisito ms bsico para trabajar en el rea de la diapositiva era que la zona haba sido evaluado y determinado que es seguro por elpersonal geotcnico y los capataces de operaciones de la mina. Un observador cualificado estaba estacionado en una antigua torre de despacho quetena una vista de toda la zona del deslizamiento. Se necesita para ello cualquier personal que necesitan para entrar en la zona de contacto con elobservador de tiro para confirmar que era seguro entrar; personal tambin estaban obligados a comunicar al observador cuando estaban saliendo delrea. Observadores adicionales seran ubicados estratgicamente sobre la base de condiciones de la minera.

Todos los observadores fueron capacitados por personal geotcnico para entender los signos visibles del movimiento. Los observadoresapostados en la antigua torre de despacho recibieron capacitacin sobre el software de monitoreo de taludes. Esta formacin no se debe permitir aobservadores para interpretar los datos de deformacin, pero al reconocer simplemente si o cuando las condiciones haban cambiado y ponerse encontacto con los capataces de turno y personal geotcnico. El sistema de control de deformaciones pendiente se requiere para que los datos entiempo real estaba a disposicin de los observadores en la antigua torre de la expedicin, y en el control de la mina, de manera que si las alarmas seactivaron el personal que trabaja en la zona de peligro, as como personal geotcnico podra ser notificado y de inmediato evacuado. Cuando lascondiciones cambian visualmente, o el sistema de monitoreo de taludes indican una alarma de movimiento, la zona minera se evacu hasta que lasinspecciones adecuadas, podran llevarse a cabo y la condicin potencialmente peligrosa ya no existan.

Explosin Programa de Perforacin grieta frontal yLa geometra de la grieta frontal y el potencial de rotura frgil de arenas cementadas parcialmente expuestas en la escarpa prohibido el uso de

mtodos de extraccin estndar y el equipo cerca de la cima. Desde las restricciones de seguridad impedan el acceso a un rea dentro de 25 ma lo largo de la cresta, se determin que un enfoque de perforacin y explosin fue una solucin potencial. El concepto era utilizar barrenos angularesperforados que fueron secuencial para iniciar pequeos fracasos, controlados pendiente alrededor de la cresta grieta frontal para desarrollar unamenor pendiente que podra ser remediada mediante mtodos mineros tpicos (bellota, 2011). Un criterio clave de diseo explosin fue para minimizarel dao a la pared detrs de cada explosin.

Topografa de la grieta frontal se complet mediante un escner lser comercial. Topografa de la superficie detallada fue necesario disear cadabarreno para taladrar perpendicular a la escarpa con la carga adecuada. Un taladro de circulacin inversa se utiliza para perforar barrenos quevariaban en longitud desde 45 hasta 70 m. Inclinacin explosiva hoyos fue diseado originalmente entre -60 a -65 debajo de la horizontal paraasegurar que el producto explosin podra ser colocado hacia abajo un pozo inclinado que era

forrado con revestimiento de PVC. En el curso del proyecto, la inclinacin se redujo exitosamente a -45 . Se llevaron a cabo encuestas de pozocomo la perforacin avanz a asegurar que cada pozo no se desve de manera significativa, lo que permiti a los pozos de perforacin actuales yposteriores a redisear si es necesario para mantener la distancia entre los agujeros de taladro apropiada.

Cada barreno se carg con dos cubiertas de carga. Estos no fueron emitidos explosiones. En cambio, la intencin era crear un plano de falla yforzar el material a fallar. La carga de fondo generado el mayor desplazamiento empujando hacia fuera el dedo del pie, desestabilizando la escarpasuperior en el proceso. La carga de la parte superior crea una grieta entre los agujeros, similar a un pre-split, que actu como un plano de falla para elmaterial para desplazar hacia abajo en la masa de diapositivas. El inicio explosin real y el rendimiento se evalu a partir de la revisin de altavelocidad y grabaciones normales de vdeo de velocidad. Una encuesta de post rfaga se llev a cabo con un escner lser para evaluar la nuevageometra de la pendiente con respecto a la geometra de post rfaga diseado.

a. b.

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c.

Figure 4. Progressive images of the first headscarp blast.

A series of photographs in Figure 4 depict the first headscarp blast. In Figure 4a the lower portion of the blast area is being displaced outward;stemming material from three blast holes is visible beingrifledfrom the collar. The second image, Figure 4b, shows the upper portion dropping into the slide mass.

The final picture in Figure 4c shows the area following the blast. The eight light colored vertical traces are the remnants of the blast holeswhich visually indicate that the upper charge was able to create the necessary crack upon which the material could slide. A visual inspection ofthe scarp following the blast did not locate signs of back- break. The drill and blast approach was very successful in developing a stable scarpconfiguration that could be mined using standard methods. Between February and July 2010, seven blasts were carried out; the number of blast holesin a shot varied from a minimum of 8 holes to a maximum of 43 holes.

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Copyright 2014 by SME

Preliminary Slope Remediation DesignIn January 2010 a major drilling campaign was initiated to improve the resolution of the Carlin Fm. geological model. However, a preliminar

remediation design was necessary to direct a near-term de-weighting effort while the investigation progressed. The preliminary design analyseincorporated material strengths obtained from testing of core samples collected during a prior small-scale drilling program and back-analyseusing limit equilibrium modeling software. The model assumption was based on interpretations that the instability had been located within the BasaClay at the bedrock contact, as shown in Figure 5 with respect to the modified remediation design topography discussed in the next paragraph. Thelocation of the cross-section is shown in Figure 6.

Figure 5. Southwest-northeas t cross-section, location identified in Figure 6, looking northwest, through the centerline of the Nine Point

modified preliminary slope remediation design topography that depicts the basic Carlin Fm. geologic model.

Figure 6. Modified preliminary Nine Points slope remediation design which depicts the preliminary design limit, the inner cut limit, thtransition to a 12 slope, and the buttress foundation located on bedrock.

The preliminary remediation design developed in January 2010 required an overall 12o slope angle through the Carlin Fm. around the slide areaThis design was based on attaining an overall FOS of 1.0 with the slope cut. In order to raise the overall FOS to a minimum acceptable value of 1.20the design incorporated a wide bedrock bench on the contact upon which a buttress could be constructed. InJune 2010, the preliminary design was modified to split the upper portion of the layback into two potential phases to limit the extent of thunweighting cut through the more competent upper sand sub-units. After mining the first three benches (12 Mt, 37 m height) to the overaremediation design limits, indicated by the green line in Figure 6, the unweighting cut would step-in to develop a 20 interramp slope through the uppesands and silts, between the orange and red lines. Theremediation design would resume the 12o slope through the LowerLaminated Tuff, Basal Clay, and slide mass; the slope to the west of

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the red line in Figure 6. The modified remediation design maintained the approach of developing a slope that had an initial FOS of 1.0 (Figure5 and indicated by the section line in Figure 6) while incorporating a buttress that would raise the FOS to 1.20 (Call & Nicholas Inc., 2010).

NINE POINTS SLOPE REMEDIATION DESIGN

The preliminary slope remediation plan outlined above was followed while details for the final design were developed during the on-goinginvestigation and characterization work. Back-analyses slope stability models were regularly updated as new interpretations or results wereobtained. The analyses utilized limit equilibrium software packages (RocScience Inc. SLIDE and GEO-SLOPE Inc. Slope/W) similar to studies ofprevious Carlin Fm. instabilities. However, it was recognized that these tools did not readily lend themselves to the time- based, progressive nature ofthe Nine Points instabilities. Therefore, numerical methods were used to provide for more representative modeling of the instability.

Numerical Model Back-Analyses

Two external consulting groups were tasked with developing numerical models to analyze the Nine Points highwall through various stages ofprogressive instability, one using RocScience Inc. Phase2 and the other Itasca s FLAC2D. The preliminary numerical models were based on theobservational data. Observations indicated that the toe was within the Basal Clay sub-unit along the bedrock contact, so initial limit equilibrium modelingfocused on that scenario. However, it was soon recognized that the numerical models had difficulty re- producing a failure surface within the Basal Clayor along the bedrock contact (Yang et al., 2011 and Itasca Denver Inc., 2010).

Both parties found that their numerical models generated shallower failure surfaces that were not within the Basal Clay. In both cases ahypothetical, shallower weak sub-unit or surface was incorporated into the models. Examples of these results are illustrated in Figure 7 (same cross-section location identified in Figure 6); the model simulations were able to reproduce the actual headscarp and toe locations with the shallowerfailure surface. Due to the lack of a detailed geological model at the time, there was no definitive evidence to confirm the modeling results, specificallywith respect to the slip surface above the Basal Clay. Thus, the analyses moved forward considering both the deep- and shallower-seated scenariosuntil the slide mass could safely be drilled to locate the failure surface.

Figure 7. Southwest-northeast cross-sections, looking northwest, location shown in Figure 6, of numerical modeling results developed

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from two independent analyses that more readily explained the occurrence of the Nine Points instability by assuming a shallower slip surface.

The failure surface was identified in October 2010 through a sonic drilling program that was conducted to drill through the slide mass. The sonicdrill core showed the failure surface along the Lower Laminated Tuff and Basal Clay contact, rather than within the Basal Clay at the bedrock contactThe identification of a shallower failure surface with drilling, confirmed the alternate failure surface identified in numerical modeling results, and meant adecrease in the overall size of the slope failure from initial estimates of 25 Mt to approximately 12 Mt.

Final Slope Remediation DesignThe location of the failure surface was incorporated into the new, detailed Carlin Fm. geological model to develop the final remediation design as

shown in Figure 8; the section location is identified in Figure

6. There is a significant increase in geological detail in Figure 8 when compared to the geological model depicted in Figure 5. Due to the presence ostrain-softened, high plasticity clay, and low-strength preferential slip surfaces within the Carlin Fm., a slope design with an acceptable FOS based onthe slope angle would be problematic because of the necessary stripping, the proximity of infrastructure, and an abandoned tailing storage facilitlocated along the eastern crest of Gold Quarry. However, previous experience had demonstrated that when the Carlin Fm. slope was mined at ageometry that would remain temporarily stable, then either a timely layback to reduce driving force or a rock buttress constructed along the toe toincrease the resisting force could be used to control deformations. Based on the schedule for the next pit layback, the decision was to pursue aremediation design that incorporated a buttress.

Figure 8. Southwest-northeast cross-section, location identified in Figure 6, viewed to the northwest, of t he Nine Point

remediation final slope design with the toe buttress and the updated Carlin Fm. geological model. The red lines indicate faults.The design integrated empirical observations and understanding of stand-up time for previous Carlin Fm. slope failures that exhibited strain

softening behavior (Call & Nicholas Inc., 2011). The slope design required a FOS of 1.0 based on modeling the material strength of key sub-unitscaled at a Skempton R-value between 0.8 and 0.9 (Skempton, 1964; Mesri and Shahien, 2002). The stand-up time provided by designing the slopenear residual strength condition allows for construction of a rock buttress that increases the minimum FOS to1.20 if water levels remained approximately 1530 m.r.s.l. This was considered feasible, because of an improved understanding of geological andhydrogeological conditions, an enhanced understanding of residual shear strength behavior of key sub-units, the ability to determine thSkempton R-value and stand-up time for previous slopeinstabilities, and previous operational experience with buttress construction to mitigate slope instabilities.

The final design analyses considered whether the preliminary design geometry that had been the basis for nearly 10 months of mininactivity could be incorporated into the final design. If this was not feasible, continued mining of the exterior cut (stopped after three benches) wouldbe required in order to achieve the necessary slope geometry through the Carlin Fm. Conceptual designs were analyzed with numerical and limiequilibrium models for comparison to historic instability studies. The numerical models incorporated pore pressure conditions obtained from vibratingwire piezometers into the stability analysis, and the limit equilibrium models used the measured water levels while adjusting the Hu or Ru in the stabilitanalysis.

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Stability analyses results indicated that it would be possible to attain an overall slope with a FOS of 1.0 based on the existing slope remediation,i.e. for the most part the preliminary design could be adopted as the final design. However, a modification was necessary in order to allow theentire lower lift of the buttress to be constructed on a bedrock foundation. Stability analyses results indicated that a minimum buttress height of 36 mwith a total weight of nearly 2 Mt would be required to cover the Lower Laminated Tuff/Basal Clay contact to provide sufficient reinforcement for theclay-rich sub-units in order to achieve a minimum FOS of 1.20. Examples of the numerical modeling results are shown in Figure 9, section locationidentified in Figure 6, (Yang et al., 2011 and Itasca Inc., 2011). The upper figure displays the sensitivity of the FOS results with respect to the averagegroundwater elevation to show the potential benefit of successful dewatering. It also indicated that by increasing the buttress height an additional 6 m,the FOS improved by more than 0.1.

Figura 4. imgenes progresivas de la primera explosin grieta frontal.

Una serie de fotografas en la Figura 4 representan la primera explosin grieta frontal. En la Figura 4a la parte inferiordel rea de la explosin est siendo desplazado hacia fuera; derivada de material de tres agujeros de explosin es servisible'Estriado' del collar. La segunda imagen, la Figura 4b, muestra la porcin superior de caer en la masa deslizante.

La imagen final en la figura 4c muestra el rea tras la explosin. Los ocho trazas verticales de color claro son los restosde los barrenos que indican visualmente que la carga superior fue capaz de crear la grieta necesaria sobre la cual elmaterial podra deslizarse. Una inspeccin visual de la escarpa tras la explosin no localizar signos de ruptura fondo. Elenfoque de perforacin y voladura tuvo mucho xito en el desarrollo de una configuracin escarpa estable que podra

ser extrado usando mtodos estndar. Entre febrero y julio de 2010, siete explosiones se llevaron a cabo; el nmero deagujeros de explosin en un tiro vari desde un mnimo de 8 agujeros a un mximo de 43 agujeros.

Pendiente de diseo preliminar RemediacinEn enero de 2010 se inici una importante campaa de perforacin para mejorar la resolucin de la Fm Carlin. modelogeolgico. Sin embargo, un diseo preliminar remediacin fue necesario dirigir un esfuerzo de ponderacin a cortoplazo, mientras que la investigacin avanzaba. Los anlisis de diseo preliminar incorpora resistencias de los materialesobtenidos a partir de anlisis de muestras bsicas recogidas durante un programa de perforacin pequea escala antesy back-anlisis utilizando software de modelado de equilibrio lmite. El modelo suposicin se basa en interpretacionesque la inestabilidad haba sido localizado dentro de la basal de la arcilla en el lecho de roca de contacto, como semuestra en la Figura 5 con respecto a la topografa de remediacin diseo modificado discutido en el prrafo siguiente.La ubicacin de la seccin transversal se muestra en la Figura 6.

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Seccin transversal 5. suroeste-noreste figura, la ubicacin identificada en la figura 6, mirando al noroeste, a travs dela lnea de centro de los nueve puntos modificados preliminar remediacin pendiente topografa diseo que representael Carlin Fm bsica. modelo geolgico.

Figura 6. modificacin de diseo preliminar Nueve Puntos pendiente remediacin que representa el lmite de diseopreliminar, el lmite de corte interno, la transicin a una pendiente de 12 , y la fundacin contrafuerte situado en la roca

madre.

El diseo preliminar de remediacin elaborado en enero de 2010 requiere un ngulo global 12o pendiente a travs de laFm Carlin. alrededor del rea de la diapositiva. Este diseo se basa en la consecucin de un FOS generales de 1,0 conel corte pendiente. Con el fin de elevar los FOS en general a un valor mnimo aceptable de 1,20, el diseo incorpora unaamplia banco de lecho de roca en el contacto sobre la que un contrafuerte podra construirse. enJunio de 2010, el diseo preliminar se modific para dividir la porcin superior de la layback en dos fases potencialespara limitar la extensin del corte descarga de peso a travs de las sub-unidades de arena superior ms competentes.Despus de la minera de los tres primeros bancos (12 Mt, 37 m de altura) a los lmites globales de diseo deremediacin, indicadas por la lnea verde en la figura 6, el corte descarga de peso sera un paso en el desarrollo de unapendiente interramp 20 a travs de las arenas superiores y limos , entre las lneas de color naranja y rojo. Eldiseo de remediacin se reanudara la pendiente 12o a travs de la BajaLaminado Tuff, Basal Clay, y la masa de diapositivas; la pendiente hacia el oeste de

la lnea roja en la Figura 6 El diseo de remediacin modificado mantiene el enfoque de desarrollar una pendiente quetena una cantidad inicial de FOS de 1,0 (Figura 5 y se indica mediante la lnea de corte en la figura 6), mientras que laincorporacin de un contrafuerte que elevara los FOS a 1.20 (Call & Nicholas Inc., 2010).

NUEVE PUNTOS DE DISEO REHABILITACIN PENDIENTE

Fue seguido mientras se desarrollaron los detalles para el diseo final durante el trabajo de investigacin ycaracterizacin en curso El plan preliminar de remediacin pendiente descrito anteriormente. Back-anlisis de losmodelos de estabilidad de taludes se han actualizado regularmente como se obtuvieron nuevas interpretaciones oresultados. Los anlisis utilizados paquetes de software de equilibrio lmite (Rocscience Inc. diapositiva y GEO-SLOPEInc. Slope / W) similares a los estudios de anterior Carlin Fm. inestabilidades. Sin embargo, se reconoci que estasherramientas no se prestan fcilmente a la naturaleza progresiva basada en el tiempo de los nueve puntos

inestabilidades. Por lo tanto, se utilizaron mtodos numricos para proporcionar ms de modelado representante de lainestabilidad.

Numrico Modelo Back-anlisisDos grupos de consultora externa se encargaron de desarrollar modelos numricos para analizar los nueve puntosHighwall travs de varias etapas de inestabilidad progresiva, uno usando Rocscience Inc. Phase2 y el otro FLAC2D deItasca. Los modelos numricos preliminares se basan en los datos de observacin. Las observaciones indican que eldedo estaba dentro de la sub-unidad basal de la arcilla a lo largo del lecho de roca de contactos, por lo que el modeladoinicial de equilibrio lmite se centr en ese escenario. Sin embargo, pronto se reconoci que los modelos numricos seestaban re producir una superficie de falla dentro de la arcilla basal oa lo largo del lecho de roca de contacto (Yang etal., 2011 y Itasca Denver Inc., 2010).

Ambas partes se encontraron con que sus modelos numricos generan superficies de falla menos profundas que noestaban dentro de la Basal Clay. En ambos casos, una, ms superficial dbil sub-unidad o superficie hipottica fueincorporado en los modelos. Ejemplos de estos resultados se ilustran en la Figura 7 (misma ubicacin de la seccintransversal identificada en la figura 6); las simulaciones de los modelos fueron capaces de reproducir las ubicacionesreales grieta frontal y de los pies con la superficie de falla ms superficial. Debido a la falta de un modelo geolgicodetallado en el momento, no haba ninguna prueba definitiva para confirmar los resultados de los modelos, en concretocon respecto a la superficie de deslizamiento por encima de la basal Clay. Por lo tanto, los anlisis se adelantaronteniendo en cuenta tanto los escenarios profundas y menos profundas-sentados hasta que la masa deslizante conseguridad se podra perforar para localizar la superficie de falla.

Figura 7. suroeste-noreste secciones transversales, que buscan al noroeste, lugar que se indica en la figura 6, de losresultados de modelos numricos desarrollados

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a partir de dos anlisis independientes que explicaron ms fcilmente la aparicin de la inestabilidad Nueve Puntossuponiendo una superficie de deslizamiento superficial.

La superficie de falla se identific en octubre de 2010 a travs de un programa de perforacin snica que se realiz paraperforar a travs de la masa deslizante. El ncleo de la perforacin snica mostr la superficie de falla a lo largo delcontacto inferior laminado Tuff y Basal Clay, en lugar de dentro de la arcilla basal en el contacto de la roca madre. Laidentificacin de una superficie de rotura superficial con la perforacin, confirm la superficie de falla alternativoidentificado en resultados de modelos numricos, y significaba una disminucin en el tamao total de la falla de la

pendiente de las estimaciones iniciales de 25 Mt a aproximadamente 12 Mt.

Diseo Final Remediacin PendienteLa ubicacin de la superficie de falla se incorpor en la nueva, detallada Carlin Fm. modelo geolgico para desarrollar eldiseo final de remediacin como se muestra en la Figura 8; seccin de la ubicacin se identifica en la figura6. Hay un aumento significativo en detalle geolgica en la Figura 8 en comparacin con el modelo geolgicorepresentado en la Figura 5. Debido a la presencia de la cepa-suavizado, arcilla de alta plasticidad y baja resistenciasuperficies de deslizamiento preferencial dentro de la Fm Carlin., un diseo pendiente con un FOS aceptables basadosen el ngulo de la pendiente sera problemtico debido al despojo es necesario, la proximidad de la infraestructura, yuna instalacin de almacenamiento de relaves abandonados situados a lo largo de la cresta oriental de Oro Cantera. Sinembargo, la experiencia previa ha demostrado que cuando la Fm Carlin. pendiente se extraa en una geometra quepermanecer temporalmente estable, entonces o bien un layback oportuna para reducir la fuerza de conduccin o de uncontrafuerte de roca construido a lo largo de la punta para aumentar la fuerza de resistencia se podra utilizar para

controlar deformaciones. Con base en el calendario de la prxima layback hoyo, la decisin fue seguir un diseo deremediacin que incorpor un contrafuerte.

Seccin transversal Figura 8. suroeste-noreste, lugar identificado en la figura 6, se ve hacia el noroeste, del diseo de lapendiente final, nueve puntos remediacin con el contrafuerte dedo del pie y el Carlin Fm actualizado. modelogeolgico. Las lneas rojas indican los fallos.

El diseo integra observaciones y comprensin del tiempo de espera para la anterior Carlin Fm empricos. fallas dependientes que exhibieron comportamiento cepa de ablandamiento (Call & Nicholas Inc., 2011). El diseo de pendienterequiere un FOS de 1.0 basado en el modelado de la resistencia del material de sub-unidades clave escalados a unaSkempton valor R entre 0.8 y 0.9 (Skempton, 1964; Mesri y Shahien, 2002). El tiempo de stand-up proporcionado por eldiseo de la pendiente el estado casi resistencia residual permite para la construccin de un contrafuerte de roca queaumenta la mnima para FOS1,20 si el nivel del agua se mantuvo aproximadamente 1.530 MRSL Esto se consider factible, debido a una mejorcomprensin de las condiciones geolgicas e hidrogeolgicas, una mejor comprensin del comportamiento deresistencia al corte residual de subunidades principales, la capacidad de determinar el valor R Skempton y stand-up detiempo para la pendiente anteriorinestabilidades y experiencia operativa anterior con la construccin contrafuerte para mitigar la inestabilidad dependientes.

Los anlisis de diseo final consider si la geometra del diseo preliminar que haba sido la base de casi 10 meses deactividad minera podra incorporarse en el diseo final. Si esto no fue posible, seguido de minera del corte exterior(detenido despus de tres bancos) se requerira para alcanzar la geometra de la pendiente necesaria a travs de la FmCarlin. Diseos conceptuales se analizaron con modelos numricos y equilibrio lmite para la comparacin de losestudios histricos de inestabilidad. Los modelos numricos incorporados condiciones de presin de poros obtenidos apartir de piezmetros de cuerda vibrante en el anlisis de estabilidad, y los modelos de equilibrio lmite utilizan losniveles de agua medidos mientras se ajusta el Hu o Ru en el anlisis de estabilidad.

Estabilidad analiza los resultados indican que sera posible alcanzar una inclinacin general con un FOS de 1.0 basadoen la remediacin pendiente existente, es decir, en su mayor parte el diseo preliminar podra ser adoptado como eldiseo final. Sin embargo, era necesaria una modificacin con el fin de permitir que toda la elevacin inferior del

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contrafuerte a ser construida sobre una base de lecho de roca. Estabilidad analiza los resultados indicaron que unaaltura contrafuerte mnimo de 36 m con un peso total de casi 2 Mt estara obligado a cubrir el contacto inferior laminadoTuff / Basal arcilla para proporcionar refuerzo suficiente para las sub-unidades ricas en arcilla con el fin de lograr unFOS mnimos de 1,20. Ejemplos de los resultados del modelado numricos se muestran en la Figura 9, la ubicacinseccin identificada en la Figura 6, (Yang et al., 2011 y Itasca Inc., 2011). La figura superior muestra la sensibilidad delos resultados FOS con respecto a la elevacin media de las aguas subterrneas para mostrar el beneficio potencial dedeshidratacin xito. Tambin indic que al aumentar la altura contrafuerte un adicional de 6 m, el FOS mejor en msde 0,1. Figura 9. resultados de modelos numricos para el diseo pendiente Nueve Puntos remediacin con elcontrafuerte del dedo del pie que indican un FOS generales aceptables de al menos 1,20 se puede lograr, la ubicacin

de seccin transversal es la misma que la mostrada en la Figura 6.

Estabilidad analiza los resultados indican que sera posible alcanzar una inclinacin general con un FOS de 1.0 basadoen la remediacin pendiente existente, es decir, en su mayor parte el diseo preliminar podra ser adoptado como eldiseo final. Sin embargo, era necesaria una modificacin con el fin de permitir que toda la elevacin inferior delcontrafuerte a ser construida sobre una base de lecho de roca

Figure 9. Numerical modeling results for the Nine Points remediation slope design with the toe buttress that indicate anacceptable overall FOS of at least 1.20 can be achieved, the cross-section location is the same as that shown in Figure 6.

Stability analyses results indicated that it would be possible to attain an overall slope with a FOS of 1.0 based on the existing slope remediation,i.e. for the most part the preliminary design could be adopted as the final design. However, a modification was necessary in order to allow theentire lower lift of the buttress to be constructed on a bedrock foundation. Stability analyses results indicated that a minimum buttress height of 36 mwith a total weight of nearly 2 Mt would be required to cover the Lower Laminated Tuff/Basal Clay contact to provide sufficient reinforcement for theclay-rich sub-units in order to achieve a minimum FOS of 1.20. Examples of the numerical modeling results are shown in Figure 9, section location

identified in Figure 6, (Yang et al., 2011 and Itasca Inc., 2011). The upper figure displays the sensitivity of the FOS results with respect to the averagegroundwater elevation to show the potential benefit of successful dewatering. It also indicated that by increasing the buttress height an additional 6 m,the FOS improved by more than 0.1.

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NINE POINTS BUTTRESS CONSTRUCTION

Excavation of the last four benches (49 m height) of the remediation cut for the Nine Points slide was temporarily suspended to focus theremediation effort on the Challenger/Gray-Tuff Graben slide. This was a concurrent, separate Carlin Fm. instability, not discussed in this case historywhich was similarly remediated with a 1 Mt buttress along the bedrock contact. Remediation mining had been at a pace to concurrently expose theLower Laminated Tuff/Basal Clay/bedrock contacts of both instabilities simultaneously. Since buttress material could not be placed at a rate sufficient toconstruct two buttresses, it was decided to first complete the Challenger/Gray-Tuff Graben buttress. An additional benefit derived from completioof this buttress was that it decreased the haulage distance of buttress material to the Nine Points slope toe by 2 km, thereby reducing the sloptoe exposure time if unplanned movement occurred.

The buttress design mitigated the potential development of increased pore pressure in the adjacent highwall with two passive methods. Firsa series of vertical drains were constructed behind the buttress to intercept groundwater flow and direct it downward into the dewatered bedrockSecond, a drainage blanket was incorporated into the buttress design to provide a permeable zone for flows that were not captured by the verticadrains. The drainage blanket provided a path for flow underneath the buttress to the toe, where additional vertical drains were installed to divert anoutflow into the bedrock. The drainage blanket was constructed with selectively blasted and stockpiled material, the majority of which haa minimum diameter of12 m. The drainage blanket was designed to be 6 m thick on the bedrock foundation and on the two Carlin Fm. benches immediately above thefoundation; additionally, the design drainage blanket thickness along bench faces was to be 15 m. The remaining buttress fill material was competenwaste rock with minimal fines.

Southern Portion of the Nine Points ButtressMining activity resumed on the Nine Points slide when the Challenger/Gray-Tuff Graben buttress was nearly complete in December 2010

The Nine Points slide mass was split, allowing remediation activity to focus initially on the southern extent of the instability. The mining anconstruction plan required that no more than approximately 45 m of the contact, measured along strike, should be exposed before placing buttresmaterial. This dimension was based on previous experience at Gold Quarry with buttress construction to stabilize the Carlin Fm. However, due to thgeometry of the contacts and the slope, portions of the buttress were constructed from the top elevation (1520 m.r.s.l.) down to the base elevatio(1480 m.r.s.l.), while maintaining sufficient width on the pit side of the bench for access around the buttress. Bench-scale stability analys

showed that1015 m of buttress width could be constructed without impacting the stability of the underlying bench that was to be subsequently excavatedAfter the last foundation slot was excavated and backfilled, buttress construction was extended pitward to the limits on the lower two lifts.

Northern Portion of the Nine Points ButtressThe geometry and access width hindered the ability to mine slide material and concurrently place buttress material while maintaining acces

to excavate the foundation. Simply stated, the majority of the cut (along the prescribed 45 m strike length) had to be excavated prior to buttresconstruction. Safety protocol stated that buttress construction was to begin immediately after excavation. However, this protocol was not followein one instance, resulting in a clear demonstration of the sensitive nature of Carlin Fm. stability.

On the morning of January 12th, 2011, excavation of the foundation elevation for the northern portion of the buttress began. By that afternoon thexcavation had advanced to expose nearly 100 m of the Basal Clay/bedrock contact. An equipment operator, working 36 m above and 150 m to theast, recognized new cracks that had developed since the beginning of shift. A subsequent inspection of the slope toe found that the Basal Clay hadisplaced outward 15 cm along portions of the bedrock contact. Mining activity was temporarily halted to evaluate displacement data from thslope monitoring systems. Mine operations initiated buttress construction in the evening.

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The following morning the first lift of the buttress had extended to the mining dig face, after which there was only negligible slope movement.

Mining activity and buttress construction progressed thereafter without incident. After the base lift of the northern portion of the buttresswas completed in the middle of January 2011, a second lift advanced from south to north and was completed in late January. Completion of the secondlift provided sufficient confidence in the stability of the Carlin Fm. to allow mining of the slide material that had overflowed onto the bedrock workingbenches below to begin. The final lift was constructed from both the south and east, and was completed by the middle of February. The Nine Pointsbuttress near completion is shown in Figure 10. Initial mining progress through the overflow slide material is shown in the lower half of the photograph.

Figure 10. Nine Points buttress progress on February 8, 2011;the extent of the buttress is indicated by the dashed red line.

CARLIN FM. POST-REMEDIATION

The remedial cut through the Carlin Fm. totaled approximately 60Mt. An additional 5 Mt of slide material were removed to allow mining to resume on the bedrock working benches that were covered by debrisduring the December 2009 failure. As of August 2013, mining activity has advanced into the slide material that filled the bottom 40 m of the pit; theslide material will be removed during 2013 as to complete the current layback. To date, no significant deformation within the Carlin Fm. hasbeen observed following the remediation. An April 18th, 2013 photograph of the Carlin Fm. highwall is shown in Figure 11. It has been over 3.5 yearssince the last major Carlin Fm. slope failure (December 2009), and nearly 2.5 years (February 2011) since buttress construction was completed. Thisfollows over 10 years of regular Carlin Fm. slope failures that adversely impacted the mine development. This outcome validates the geotechnical andhydrogeological characterization and modeling work, and points to the importance of this type of work during the early stages of a new laybackevaluation or new open pit development. At Gold Quarry, it has improved potential to decrease the overall mining costs and provide a higherprobability of completing an open pit as scheduled.

Subsequent to the post-instability geotechnical and hydrogeology investigation program conducted throughout 2010, investigation of the Carlin Fm.has continued in advance of the next planned layback, particularly to identify the contacts between the Lower Laminated Tuff/Basal Clay/bedrock. Theobjectives are to expand the Carlin Fm. model to the east and southeast of Gold Quarry where the planned layback would cut a new highwall. In

addition to drilling, geophysical techniques have been implemented. The methods that have been successful with the Carlin Fm. include controlledsource audio- frequency magnetotelluric (CSAMT) surveys and gravity surveys. Data from this work have expanded the modeled CarlinFm./bedrock contact, identified additional structures, and provided a map of likely groundwater within the upper sands and silts. These new datahave also provided potential targets for wells to depressurize the upper sands and silts in advance of the layback development. Removing this

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groundwater will prevent it from draining into the deforming Lower Laminated Tuff and Basal Clay that may otherwise enhance the strainsoftening deformation of these units.

With the increased understanding of the hydrogeology of the Carlin Fm., new drains and pumping wells targeting structural intersectionhave been installed. As a result the potentiometric surface has been drawn down between 15 and 30 m in the Nine Points slope area. Asensitivity analysis with respect to groundwater on overall slope stability found that for every 15 m of average drawdown the FOS would improvby nearly ten percent. With more accurate information, geotechnical and hydrogeological staff will be able to develop more confident and accurate pslope design inputs for future Gold Quarry laybacks. Until the necessary investigation and analysis work is complete, the projected design for the nexlayback through the Carlin Fm. maintains a conservative overall 12 s lope angle, along with a foundation bench on the bedrock to allow construction oa buttress across the exposed critical Carlin Fm. contacts.

CONCLUSIONS

The impact of the Nine Points instability on Gold Quarry required that the investigation of the large-scale slope failure be conductedsimultaneously with mining activity necessary to remediate the instability and prevent it from propagating into nearby infrastructure. Thicase study summarizes important aspects that were encountered to safely implement this dual approach at Gold Quarry. In order to accomplish thisthe preliminary remediation recommendations had to leverage the experience and judgment of on-site personnel from observations of previous CarlinFm. instabilities. The information necessary to finalize the remediation design details was collected throughout 2010 and implemented as it becameavailable.

Remediation of the Nine Points slope instability resulted in a Carlin Fm. highwall that has remained stable, in contrast to nearly ten years oslope instability experience. Moreover, the performance of the highwall since the remediation has reinforced the need to conduct thorough geologicageotechnical, and hydrogeological investigations prior to mining larger, deeper open pit expansions. These efforts will significantly increase thcapability of geotechnical and hydrogeological personnel to develop recommendations that support reliable open pit slope designs, which increasethe probability of mine operations to safely and efficiently achieve the mine plan.

Figure 11. A 27 August 2013 photograph of the Carl in Fm. highwall and butt resses.

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Feb. 23 - 26, 2014, Salt Lake City, UT4. Call & Nicholas Inc., 2011, Nine Points Failure GeotechnicalInvestigation Failure Mechanisms Report,April 2011, p 261.

5. Harlan, J.B., Harris, D.A., Mallette, P.M., Norby, J.W., Rota, J.C. and Sagar, J.J., 2002, Geology and mineralization of the Maggie Creekdistrict, Gold Deposits of the Carlin Trend, T. Thompson, L. Teal, and R. Meeuwig (eds), Nevada Bureau of Mines and Geology, Bulletin 111,Bear Industries, Sparks, Nevada, USA, pp.115142.

6. Itasca Denver Inc., 2010, Gold Quarry Pit Design Section 600- Summary of MINEDW and FLAC Modeling Technical Memorandum, August13, 2010, p. 20.

7. Itasca Inc., 2011, Gold Quarry Pit Design Sections 600, III, VI, D, and G Technical Memorandum, April 26, 2011, p. 37.

8. Mesri, G. and Shahien, M., 2002, Residual shear strength mobilized in first-time slope failures, Journal of Geotechnical andGeoenvironmental Engineering, American Society of Civil Engineers, Vol. 129, No. 1, pp. 1231.

9. Regnier, J., 1960, Cenozoic geology in the vicinity of Carlin, Nevada,Geological Society of America Bulletin, Volume 71, No.8, pp. 11891210.

10. Sheets, R.J., 2011, Lessons from Carlin Formation slope instability at the Gold Quarry operation,Mining in Saprolites Workshop, P.Dight (ed), September 18, 2011, Vancouver, British Columbia, Canada, Australian Centre for Geomechanics, Digital Publication.

11. Sheets, R.J., 2013, Nine Points slope failure at the Gold Quarry open pit,April 2013 pre-publication case study submitted for inclusion in thepending Large Open Pit Mine Slope Stability Project guidelines for weak rocks book.

12. Sherman, C.S. and Sheets, R.J., 2008, Gold Quarry Phase 4North slope failure, Proceedings of the 41st Symposium on Engineering Geology and Geotechnical Engineering, T. Weaver and S. Sharma(eds), April 911, 2008, Boise, Idaho, USA, pp.269275.

13. Skempton, A.W., 1964, Long-Term Stability of Clay Slopes, Fourth Rankine Lecture,Gotechnique 14(2) pp. 77-101.

14. Yang, D.Y., Brouwer, K.J., Sheets, R.J., St. Louis, R.M. and Douglas, S.J., 2011, Large-scale slope instability at the Gold Quarry mine,Nevada, Proceedings for the 2011 International Symposium on Rock Slope Stability on Open Pit Mining and Civil Engineering, E. Eberhardtand D. Stead (eds), September 1821,2011, Vancouver, British Columbia, Canada, Canadian RockMechanics Association, Digital Proceedings.

REFERENCES

1. Acorn, T., 2011, Geotechnical slope remediation via rc-drilling and controlled slope failure blasting, Proceedings ofthe 37th Annual Conference on Explosives and BlastingTechniques, February 69, 2011, San Diego, CA, USA,International Society of Explosives Engineers, pp. 663776.

2. Bates, E., St. Louis, R., Douglas, S. and Sheets, R., 2006,Slope monitoring and failure mitigation techniques applied inthe Gold Quarry open pit, Proceedings of the 31st U.S. RockMechanics Symposium, D.P. Yale et al. (eds), July 17-21,2006, Golden, Colorado, USA, American Rock Mechanics

Association, Digital Proceedings.

3. Call & Nicholas Inc., 2010, Preliminary Runs for theAccelerated

Phase 1 Design,June 23, 2010, p. 18.

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