Sam Abbaszadeh, Ph.D., P.E.

Consolidation behavior of mine tailings governs the material’s density and permeability over time, and these properties have significant impacts on tailings storage facility (TSF) behavior both during operations and post-closure. Rate of consolidation and correspond-ing changes in tailings impoundment dry density impact ultimate TSF capacity, rate of rise, and water flows within and out of a tailings impoundment. When tailings deposition ceas-es, a TSF undergoes post-closure consolidation and… Show more

Consolidation behavior of mine tailings governs the material’s density and permeability over time, and these properties have significant impacts on tailings storage facility (TSF) behavior both during operations and post-closure. Rate of consolidation and correspond-ing changes in tailings impoundment dry density impact ultimate TSF capacity, rate of rise, and water flows within and out of a tailings impoundment. When tailings deposition ceas-es, a TSF undergoes post-closure consolidation and draindown which must be considered in closure planning. Thus, representative characterization of tailings consolidation behav-ior is a crucial component of TSF design. The use of idealized homogeneous/isotropic la-boratory test results can lead to mischaracterization of consolidation behavior in facilities where segregation and layering are known to occur and lead to heterogenous and aniso-tropic conditions. Prior publications have introduced methods of calibrating tailings con-solidation parameters using field measurements to provide more representative forecasting of impoundment density and permeability. This paper expands on the applications of one such methodology in practice. Calibration results for one of the TSFs presented in a 2020 publication are compared to recent field data from through 2022 to demonstrate the pre-dictive performance of the calibrated consolidation properties compared to laboratory da-ta. A new case study is also introduced and compared to field data. Additionally, key ap-plications of calibrated consolidation properties for TSF design and operational planning are presented, and the benefits and limitations of TSF material property calibration are dis-cussed. Show less

Recent failures have brought into focus the importance of the stability and safety of tailings storage facilities (TSFs), and the management of risks associated with operating a TSF. While the causal factors of the Brumadinho TSF failure continue to be evaluated, other recent TSF failures including those at Mount
Polley, Fundão, and Cadia have been attributed to changing field conditions such as water pressures within the foundation and embankment, or unrecognized field conditions such as… Show more

Recent failures have brought into focus the importance of the stability and safety of tailings storage facilities (TSFs), and the management of risks associated with operating a TSF. While the causal factors of the Brumadinho TSF failure continue to be evaluated, other recent TSF failures including those at Mount
Polley, Fundão, and Cadia have been attributed to changing field conditions such as water pressures within the foundation and embankment, or unrecognized field conditions such as the strength of foundation and embankment materials.
This paper discusses effective risk management controls for TSFs during design, operation, and preclosure.
The specific role of geotechnical investigations, Quality Assurance and Quality Control (QA/QC) and instrumentation during design, construction, and operations are also discussed in the context of dam safety and risk management. Considerations for planning, executing, and interpreting these investigations
are provided, along with lessons learned where risk mitigation controls have been implemented. There are
other dam safety factors such as freeboard and water management that need to be continuously monitored on a TSF; however, the focus of this paper will be on providing suggestions for managing changing conditions and geotechnical risks within the embankment and foundation of a TSF. Show less

For many years, Geotechnical engineers have thought of gravelly soils as non-liquefiable
material due to its high hydraulic conductivity and therefore the ability to quickly
dissipate the high pore pressures generated during an earthquake loading. However, a
number of recent failures and damages to dams and other structures caused by
liquefaction of gravely soils have changed this viewpoint drastically and required
reassessment of the behavior and response of gravelly soils to… Show more

For many years, Geotechnical engineers have thought of gravelly soils as non-liquefiable
material due to its high hydraulic conductivity and therefore the ability to quickly
dissipate the high pore pressures generated during an earthquake loading. However, a
number of recent failures and damages to dams and other structures caused by
liquefaction of gravely soils have changed this viewpoint drastically and required
reassessment of the behavior and response of gravelly soils to cyclic loading. During the
past few decades, many professionals from academia collaborated with engineering
practitioners to better understand this phenomena from a micro level (in laboratory) to
macro level (field and large-scale tests). These ongoing investigations have been
insightful and improved our understanding of the behavior of gravelly soils during an
earthquake. However, for practicing engineers, the liquefaction evaluation procedure and
criteria is still not very clear for gravelly soils and engineers have no choice other than
using one of the classical liquefaction assessment approaches that were developed
originally for sandy materials.
This paper will first present the state of knowledge by summarizing the advancements
made to date in the area of gravelly soil liquefaction. The paper will then present a review
of the state-of-the-practice on liquefaction evaluation of gravelly soils. Finally, gaps in
engineering, design and rehabilitation of embankment dams founded on gravelly soils
will be identified and recommendations for future studies will be made. Show less

Evaluating the potential for earthquake-induced liquefaction under and beneath embankment dams continues to be a significant challenge for geotechnical earthquake engineers. A variety of methods are available to estimate the potential deformations associated with liquefaction and range from relatively simple analytical models to fully coupled numerical analyses using advanced constitutive models. Multiple constitutive models are currently available which can simulate the cyclic response of… Show more

Evaluating the potential for earthquake-induced liquefaction under and beneath embankment dams continues to be a significant challenge for geotechnical earthquake engineers. A variety of methods are available to estimate the potential deformations associated with liquefaction and range from relatively simple analytical models to fully coupled numerical analyses using advanced constitutive models. Multiple constitutive models are currently available which can simulate the cyclic response of sandy soils, but these models use different formulations and calibration procedures which can lead to very different response at the element level. These differences may be exacerbated when elements are consolidated under large confining and/or shear stresses such as those present under embankment dams. This paper seeks to examine how model differences at the element level influence the overall response of a hypothetical embankment dam. This paper utilizes nonlinear deformation analyses (NDAs) to examine the effects of the choice of constitutive model on the response of a hypothetical embankment dam founded on liquefiable alluvium. The paper briefly describes each constitutive model under consideration and their respective calibration procedures. Two-dimensional NDAs are utilized to examine how the different constitutive models affect the overall response of the dam. Effects of soil density, shaking magnitude and ground motion selection on differences in the model responses are examined using a limited sensitivity study. Implications of these findings for practice are discussed.
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Abstract: Significant effort has been placed on the determination of swelling properties of expansive soils, such as swell pressure and swell potential. However, there are only a limited number of studies of expansive soil response that include the effect of desiccation cracking; most studies and methods of analysis have simply neglected the effect of cracks. At the same time that cracks provide a path for flow of water under positive head, the crack network in an expansive soil functions as… Show more

Abstract: Significant effort has been placed on the determination of swelling properties of expansive soils, such as swell pressure and swell potential. However, there are only a limited number of studies of expansive soil response that include the effect of desiccation cracking; most studies and methods of analysis have simply neglected the effect of cracks. At the same time that cracks provide a path for flow of water under positive head, the crack network in an expansive soil functions as swell-absorbent media (due to open spaces within the soil) which can reduce the total amount of vertical swell that the soil would experience. In this study, a laboratory investigation was carried out to quantitatively evaluate the effect of cracks on swell potential and swelling pressure. The presence of desiccation cracks was found to reduce the amount of swell and also reduce the swell pressure compared to intact, uncracked, specimens. The percentage reduction of swell potential and swell pressure was found to be a function of the volume of cracks and the applied net normal stress. Show less

ABSTRACT: The hydraulic conductivity for unsaturated soil conditions is more difficult to estimate than for the saturated condition. In addition, as the soil transitions from intact to cracked, the difficulty in estimating the unsaturated hydraulic conductivity increases. One critical step in the determination of unsaturated flow hydraulic conductivity is the evaluation of the Soil-Water Characteristic Curve (SWCC). In this paper, a series of laboratory studies of direct measurements of… Show more

ABSTRACT: The hydraulic conductivity for unsaturated soil conditions is more difficult to estimate than for the saturated condition. In addition, as the soil transitions from intact to cracked, the difficulty in estimating the unsaturated hydraulic conductivity increases. One critical step in the determination of unsaturated flow hydraulic conductivity is the evaluation of the Soil-Water Characteristic Curve (SWCC). In this paper, a series of laboratory studies of direct measurements of cracked soil SWCCs is presented, including challenges associated with the control of very low suction levels associated with crack dewatering. An oedometer-type SWCC apparatus, capable of suction and net normal stress control, and volume change measurement, was used in these experimental studies. It is common that SWCCs are com-prised of matric suction values below about 1500 kPa, and total suction values for suctions higher than about 1500 kPa (Fredlund, Rahardjo, and Fredlund, 2012). In this study, all measured or controlled suction values were less than 1500 kPa and obtained using the axis translation method, and the curve in the higher suction range was projected by forcing the SWCC through 106 kPa for completely dried conditions (Fredlund, Rahardjo, and Fredlund, 2012). Volume change corrections were made to the reported volumetric water contents, which is of particular importance when the soil under consideration undergoes volume change in response to wetting or drying. A technique for the determination of the SWCC for cracked clay soils is presented. Test results validated the fact that the SWCC of a cracked soil can be represented by a bimodal function due to the Air Entry Value (AEV) of the cracks being much lower than the AEV of the soil matrix. It was also found that differences between the SWCC for cracked and intact soil appears only in the very low suction range. Show less

Gold Fields La Cima’s (GFLC) Cerro Corona Tailing Storage Facility (TSF) includes a centerline rockfill dam that has been in operation since 2008. The dam has a low permeability core with transitional filters on its downstream side and supporting rockfill both upstream and downstream of these zones. To monitor deformations of the dam throughout operation and closure, GFLC implemented an instrumentation program in 2012 that include twelve inclinometers on both the downstream and upstream sides… Show more

Gold Fields La Cima’s (GFLC) Cerro Corona Tailing Storage Facility (TSF) includes a centerline rockfill dam that has been in operation since 2008. The dam has a low permeability core with transitional filters on its downstream side and supporting rockfill both upstream and downstream of these zones. To monitor deformations of the dam throughout operation and closure, GFLC implemented an instrumentation program in 2012 that include twelve inclinometers on both the downstream and upstream sides of the core. Additional instrumentation was installed to monitor displacements as well as validate inclinometer data and provide further insight for deformation interpretation. Due to the centerline design, the dam is limited to annual staged raises to maintain upstream slope stability which is supported by the consolidated, sub-aerial tailing beach. In October 2012, a nine meter raise was completed over a six month period to elevation 3755 masl; providing a dam height of approximately 125 meters. This paper describes the measured deformations of the dam with focus on the recent raise. Furthermore, the nine meter raise of the dam is simulated using a finite difference modeling tool, FLAC, to calculate the deformations induced by recent construction. The numerical analysis deformations are then discussed and compared to measured field deformations. Show less

Existing mining facilities typically have fixed property boundaries, permit limits, or economic considerations that constrain the mining infrastructure to a limited area. Expansion projects at these mines often must fit into these fixed areas and compete with the space requirements of ongoing activities. An alternative for a facility expansion being developed at a copper and gold mine in South America is to locate a new gold heap leach on top of an existing waste storage facility. Because the… Show more

Existing mining facilities typically have fixed property boundaries, permit limits, or economic considerations that constrain the mining infrastructure to a limited area. Expansion projects at these mines often must fit into these fixed areas and compete with the space requirements of ongoing activities. An alternative for a facility expansion being developed at a copper and gold mine in South America is to locate a new gold heap leach on top of an existing waste storage facility. Because the waste facility was not originally constructed as a heap leach foundation, a concern is that the various types and depth of materials in the waste facility will have different settlement responses. Differential settlement beneath the leach pad could lead to (1) tears in the leach pad liner due to excessive liner strains and (2) poor drainage of pregnant leach solution due to reversal of gradient in the collection pipes. In this paper a series of possible differential settlement scenarios are simulated using the finite difference modeling software, FLAC. The location and extent of the differential settlement is what differentiates between these scenarios. The total differential settlement and the stresses and strains developed in the liner are calculated for each scenario. The modeling demonstrates strategies to mitigate the risk of differential settlement beneath the heap leach. Show less

The Cerro Corona gold and copper mine is located in the Cajamarca district of Peru and owned by Gold Fields La Cima. Tailing produced by the processing plant is stored in the Tailing Storage Facility, located just northwest of the plant site and includes several dams. To support future mining operations, the dams will be raised, the tallest to an ultimate height of 170 meters. The dam is a centerline construction with a central low permeability core and transitional filters located on its… Show more

The Cerro Corona gold and copper mine is located in the Cajamarca district of Peru and owned by Gold Fields La Cima. Tailing produced by the processing plant is stored in the Tailing Storage Facility, located just northwest of the plant site and includes several dams. To support future mining operations, the dams will be raised, the tallest to an ultimate height of 170 meters. The dam is a centerline construction with a central low permeability core and transitional filters located on its downstream side, with supporting rockfill both upstream and downstream of these zones. The integrity of the dam and its core under earthquake loading were of particular concern during the design of the dam. This paper presents a summary of seismic analysis of the dam performed to address this concern. The mine is located in a seismically active zone with a history of significant earthquake events. A site-specific seismic hazard analysis was conducted and two design earthquakes were identified: a M8 intraslab event with a PGA of 0.55 g at a distance of 75 km, and a M9.2 interface event with a PGA of 0.29 g at a distance of 118 km, for which design ground motions were developed for use in the deformation analysis. The seismic deformation of the dam under the postulated ground motions was then evaluated using a two-dimensional nonlinear time domain analysis. General analysis approach is discussed and the detailed results of the seismic analysis for the most critical section are presented. Show less

Heap leaching of gold, silver and copper ores is being conducted worldwide to extract these precious metals. A key component of successful heap leaching is the permeability of the stacked ore (heap). A South American mine is studying the feasibility of extracting gold from a stockpiled oxide ore by means of heap leaching. Due to the high percentage of fines, and specifically clays in the ore, the native permeability of the ore is low for heap leaching. Ore agglomeration is being proposed to… Show more

Heap leaching of gold, silver and copper ores is being conducted worldwide to extract these precious metals. A key component of successful heap leaching is the permeability of the stacked ore (heap). A South American mine is studying the feasibility of extracting gold from a stockpiled oxide ore by means of heap leaching. Due to the high percentage of fines, and specifically clays in the ore, the native permeability of the ore is low for heap leaching. Ore agglomeration is being proposed to improve the permeability of the ore. Bench-scale agglomeration tests showed that the agglomerate quality is sensitive to the moisture of the ore; agglomeration is poor or fails when the moisture content of the ore is outside a certain range. Field studies showed that the in situ ore moisture content of the stockpiled ore can be as high as 27%; thus, demonstrating the ability to effectively agglomerate wet ore up to the maximum moisture content was essential to establish the feasibility of the heap leach project. Show less