Documents published in Scipedia

• Development of a 3D ground model to design the stabilisation of a downstream dam founded on weak and liquefiable units

  J. Rola, M. Sottile, N. Rivas, L. Roldan*, A. Sfriso

  ISC2024.

  
  

  Abstract

  Athree-dimensional (3D) ground model was developed to design the stabilisation of a dam founded on both weak and liquefiable units up to about 18 m below ground surface. The ground model covers a linear extent of approximately 800 m and was developed from five separate site investigations completed over a four-year period and digitisation/georeferencing of historic drawings/plans. Combined, the investigation comprised 206 cone penetration tests (CPTs), 37 boreholes and 36 test pits, including several vane shear tests, ball penetrometer tests and sampling. CPT data was processed to identify different material behaviours, generally based on the following features: corrected tip resistance; sleeve friction resistance; pore water pressure ratio; state parameter; and the soil behaviour type index. Each CPT interpretation was compared with information from the nearest borehole using a purpose-built python code. This information was reviewed manually in an iterative process to delineate the various geotechnical unit based on CPT response and the physical logs. This process identified a continuous weak organic layer across the site which had not been previously picked-up by the Engineer of Record (EoR). The works identified nine separate geotechnical units, with one of these subdividable based on its CPT response/grain size. The 3D ground model was built in Seequent Leapfrog Geo using the following information: unit levels specified from each CPT, digitised historic drawings/plans and a topographic survey. This paper describes the process of development and presents the full 3D ground model used as critical input to the stabilisation design of the dam

  Abstract
  Athree-dimensional (3D) ground model was developed to design the stabilisation of a dam founded on both weak and liquefiable units up to about 18 m below ground surface. The [...]

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• Evaluation of the susceptibility to flow liquefaction of an iron ore tailings using the state parameter and Yield Stress Ratio approach

  A. Faria, M. Junior*, J. Carneiro, M. Dias, G. Pinto

  ISC2024.

  
  

  Abstract

  Flow liquefaction is observed in saturated or nearly saturated geomaterials, showing a strain-softening response during undrained shear, primarily in very loose sands and silts, as well as in very sensitive clays. The Cone Penetration Test (CPTu) has been gaining popularity among the geotechnical community to evaluate the state of a soil profile due to its good repeatability, detailed information on the soil stratigraphy and extensive detailed scientific studies to guide the application of the test results. Susceptibility to flow liquefaction is typically evaluated by estimating the in-situ state parameter (𝜓) since it directly correlates to the soil behaviour at large strain, as shown by Jefferies & Been (2016). Mayne & Sharp (2019) suggested using the yield-stress-ratio (YSR = σp’/σv0’) to estimate the soil state using a threshold of YSR ≈ 3. Currently, in the Brazilian Mining Industry, it is common to find the application of different approaches to evaluating the susceptibility of mining tailings to flow liquefaction (e.g., Plewes et al., 1992; Olson, 2001; Shuttle & Cunning, 2008; Robertson, 2016). This paper presents an evaluation of the susceptibility of an iron ore tailings (IOTs) to flow liquefaction using the following recently published approaches: i) Mayne and Sharp (2019), using the yield stress ratio (YSR) approach; ii) Smith et al. (2021), using a generalised CPTu state parameter inversion method based on the NorSand Widget; and iii) Robertson (2022), with the updated Kc. The main results show a good convergence between the three methodologies used.

  Abstract
  Flow liquefaction is observed in saturated or nearly saturated geomaterials, showing a strain-softening response during undrained shear, primarily in very loose sands and [...]

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• Difficulties in Building a Ground Model When Lacking Historic Data Archives and its Impact in TSF Safety Assessment

  I. Lopes*, M. Garcia, G. Tavares

  ISC2024.

  
  

  Abstract

  Sometimes, when working in TSF safety analysis, the historic archives with the original ground topography or details about the TSF design and construction are few or inexistent. The knowledge of the bedrock position, as the details about the embankment construction are essential to know, for instance, the tailings thickness and the construction type. For the analysis of TSF current state stability the ground/structure model is essential, and is, in many cases, very difficult to define with few and low-quality data. The original ground topography is, often, obtained from aerophotogrammetric reconstruction, from satellite images or aerial photos, originally with low resolution and uncertainty of more than 10 m for the elevation. In this paper is presented a case-study from a TSF where the initial data package had only the feasibility design and some very simplified reports that checked the stability of the dam before an upstream raising, i.e. it didn’t exist much information and the details about the site and about the structure were very limited. To “add” difficulties to the process of defining the bedrock surface and to establish the TSF design, the embankments were built with local rocks (mainly schist and phyllite), and during the initial analysis of historical satellite images it was noticed that the original ground was excavated in different areas to increase storage area and obtain construction materials. This paper presents the steps developed to establish the definition of the bedrock ground surface and the difficulties felt and its impacts on TSF safety assessment are discussed.

  Abstract
  Sometimes, when working in TSF safety analysis, the historic archives with the original ground topography or details about the TSF design and construction are few or inexistent. [...]

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• CPTu for Assessment of Flow Liquefaction of Tailings with Similar Physical Characteristics

  C. Lebron*, M. Pio dos Santos Junior, A. Viana Da Fonseca

  ISC2024.

  
  

  Abstract

  A geotechnical assessment of mine tailings state conditions using the static cone penetration test performed at different tailings storage facilities (TSF’s) will be presented in this paper. Both tailings evaluated herein are deposited as a slurry (hydraulic deposition) and have similar grain-size distribution curves. A set of Cone Penetration Tests (CPTu) with pore pressure measurements were performed at each site to evaluate the state of the tailings. To assess the contractive-dilative behavior classical methodologies were adopted such as i) the contractive/dilative boundary suggested by Robertson (2016) ii) the approach suggested by Plewes et al. (1992) and cited by Jefferies and Been (2016) and iii) the yield stress ratio method proposed by Mayne and Sharp (2019). Partial drainage effects will be identified with classical methodologies. The results were compared to evaluate the difference and limitations of each methodology. Comments on the similarity between the two tailings evaluated herein will also be presented to explain the differences in behavior due to aspects such as mineralogy, gradation, stress history and deposition.

  Abstract
  A geotechnical assessment of mine tailings state conditions using the static cone penetration test performed at different tailings storage facilities (TSF’s) will be presented [...]

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• Geotechnical Parameters of Filtered Tailings and Waste Rock from the Itabira Complex used in Stacking Projects

  D. Eloi*, M. Paganin, D. Bastos, W. Maciel, M. Filho, R. Silva

  ISC2024.

  
  

  Abstract

  The complex where we used the materials ins this study, explores iron ore in an open pit process and is formed by a significant number of mines, and all their geotechnical structures such as waste rock stackings, tailings dams and, recently, the stackings that are being constructed with waste rock and tailings. For the piles formed by the shared disposal of waste rocks and filtered tailings, the characterization of the materials and the strength and critical state parameters determined the executive methodology and the safety factors required for the projects. The waste rocks and filtered tailings are mechanically disposed and divided into zones defined not only according to the geotechnical characteristics of the materials, but also the structural and operational needs of the stacking, which are divided into the confining zone and the confined zone. In the confining zone, compacted waste is deposited. The aim of this procedure is to enhance the structural performance by employing materials with higher resistance. In the confining zone, as closer to the outer layer of the pile, greater shear strength is mobilized. In the confined zone, friable waste, and mining tailings are disposed. In this zone, there is less mobilization of shear strength. For tailings, control and evaluation adhere to parameters outlined by the Critical State Line, employing the void ratio control of layers. This ensures dilatant mechanical behaviours for all confinement stresses specified in the project.

  Abstract
  The complex where we used the materials ins this study, explores iron ore in an open pit process and is formed by a significant number of mines, and all their geotechnical [...]

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• Effects of Vane Size and Aspect Ratio on the Measurement of Undrained Shear Strength of a Fine-Grained Soil

  T. Chatfield*, R. Rinehart, C. Fontaine

  ISC2024.

  
  

  Abstract

  The field Vane Shear Test (VST) is a widely used in-situ test method to measure undrained shear strength and sensitivity of saturated, fine-grained soils. The United States Bureau of Reclamation (Reclamation) commonly performs this test method to help inform numerical modeling of earth embankment dams when undergoing a risk analysis or design. Although the test method itself has been a geotechnical tool for quite some time, its primary use has traditionally been limited to sites with soft and/or relatively shallow clay soil. Typically, embankment and foundation materials of interest at Reclamation facilities are at greater depths, under higher effective stresses, and can be relatively stiff. Testing of stronger soils poses issues when performing the VST; typical commercially available equipment has a limited torque capacity to cause yielding of the soil. As a solution, modifying the dimensions and aspect ratio of the vane is an economic means of increasing the measurable range of undrained strength. Yet, the effects of these modifications are not well understood. Soil strength anisotropy is one of the primary components of this uncertainty. Testing on a sandy lean clay has been conducted to enable side by side comparisons of traditional aspect ratio vanes versus the proposed modified vanes to quantify the potential differences in measured undrained strength. In addition, measured undrained strengths from the various vanes are compared to results of laboratory testing on the same sandy lean clay (i.e., direct simple shear and triaxial compression) to provide a better understanding of the differences between the in-situ and laboratory test methods. This paper presents the apparatus developed to allow full scale vane shear tests to be conducted in the laboratory and summarizes the results of tests on a normally consolidated sandy lean clay.

  Abstract
  The field Vane Shear Test (VST) is a widely used in-situ test method to measure undrained shear strength and sensitivity of saturated, fine-grained soils. The United States [...]

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• Analysis of Shear and Consolidation Behaviour of a Clay Foundation Below a Tailings Storage Facility

  T. Armstrong*, T. Grobler

  ISC2024.

  
  

  Abstract

  Historically, the analyses of tailings storage facilities (TSFs) have primarily focused on understanding the characteristics of tailings, while often overlooking a comprehensive evaluation of the foundation, as seen at Mount Polley in 2014. The Global Industry Standard on Tailings Management (GISTM) Requirement 5.4 stresses the importance of comprehensively addressing all potential failure modes. Numerous platinum TSFs in Southern Africa are underlain by residual clay, specifically a residual mafic rock from the Bushveld Complex. Similar soils, known as "tropical black clay soils", are found in other regions of the world. Surprisingly, there's limited public information on testing and modelling the behaviour of this clay foundation. This paper presents a case study that employs a novel approach to assess the impact of loading from tailings deposition on the underlying clay foundation of an upstream TSF. The analysis investigates how shear behaviour and consolidation characteristics of the clay foundation change with varying TSF heights and construction rates. The approach includes quantifying excess pore water pressures and their influence on the clay foundation's effective stress. The analyses primarily employ traditional limit equilibrium methods to assess TSF stability and foundation behaviour, with the potential to expand to numerical modelling. The study concludes that excess pore water pressures will significantly affect the Factor of Safety (FoS) of a TSF, particularly with adverse consolidation characteristics and increasing TSF height. This is primarily due to the low permeability and changing consolidation coefficient (cv) of the clay layer as the surcharge load increases. Furthermore, the research reveals that, depending on the rate of rise, tailings deposition may induce excess pore water pressures, potentially reducing the FoS. The ability to quantify excess pore water pressures using this novel approach enables a more accurate estimation of the FoS for facilities underlain by low-permeability materials, either residual or transported.

  Abstract
  Historically, the analyses of tailings storage facilities (TSFs) have primarily focused on understanding the characteristics of tailings, while often overlooking a comprehensive [...]

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• Temperature based Leakage Detection and Monitoring Systems in view of Tailings Storage Facilities

  G. Dumont, A. Fabritius*

  ISC2024.

  
  

  Abstract

  Characterizing the in-situ state of soils is essential for evaluating their vulnerability and the consequences of failure. The cone penetration test (CPT) enables efficient, repeatable, and continuous soil characterization based on the recorded response to penetration. Particularly for waste storage facilities, analysis of CPT results can help avoid failures that could lead to significant socio-economic and environmental impacts. Human-made soils in waste storage facilities, like coal combustion products and mine tailings, can have a large fraction of silt-sized particles, which makes them prone to experiencing partial drainage during CPT soundings at standard penetration rates. However, the current state of practice still predominantly adopts the assumption of fully drained or undrained conditions, which may lead to inaccurate interpretation of soil properties and state. This study aims to explore a new CPT-based characterization framework for intermediate silty soils using cone tip resistance values to determine the soil state. To do so, CPT soundings were performed in-flight in centrifuge models of a coal combustion product with different initial densities at varying penetration velocities. Soils with a low density and contractive behavior experience a decrease in tip resistance as the penetration velocity is increased due to the generation of excess pore pressures, resulting in high ratios of drained to undrained tip resistance (Qtn,drained/Qtn,undrained). In contrast, the tip resistance increases with penetration velocity, resulting in low Qtn,drained/Qtn,undrained ratios in soils of a high density and dilative behavior. The proposed framework uses Qtn,drained/Qtn,undrained to identify contractive layers and is expected to help assess the vulnerability of soil layers to experience liquefaction failure.

  Abstract
  Characterizing the in-situ state of soils is essential for evaluating their vulnerability and the consequences of failure. The cone penetration test (CPT) enables efficient, [...]

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• Characterization of coal combustion products using variable rate CPT in a geotechnical centrifuge

  J. Chen, A. Martinez

  ISC2024.

  
  

  Abstract

  Characterizing the in-situ state of soils is essential for evaluating their vulnerability and the consequences of failure. The cone penetration test (CPT) enables efficient, repeatable, and continuous soil characterization based on the recorded response to penetration. Particularly for waste storage facilities, analysis of CPT results can help avoid failures that could lead to significant socio-economic and environmental impacts. Human-made soils in waste storage facilities, like coal combustion products and mine tailings, can have a large fraction of silt-sized particles, which makes them prone to experiencing partial drainage during CPT soundings at standard penetration rates. However, the current state of practice still predominantly adopts the assumption of fully drained or undrained conditions, which may lead to inaccurate interpretation of soil properties and state. This study aims to explore a new CPT-based characterization framework for intermediate silty soils using cone tip resistance values to determine the soil state. To do so, CPT soundings were performed in-flight in centrifuge models of a coal combustion product with different initial densities at varying penetration velocities. Soils with a low density and contractive behavior experience a decrease in tip resistance as the penetration velocity is increased due to the generation of excess pore pressures, resulting in high ratios of drained to undrained tip resistance (Qtn,drained/Qtn,undrained). In contrast, the tip resistance increases with penetration velocity, resulting in low Qtn,drained/Qtn,undrained ratios in soils of a high density and dilative behavior. The proposed framework uses Qtn,drained/Qtn,undrained to identify contractive layers and is expected to help assess the vulnerability of soil layers to experience liquefaction failure.

  Abstract
  Characterizing the in-situ state of soils is essential for evaluating their vulnerability and the consequences of failure. The cone penetration test (CPT) enables efficient, [...]

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• Cone penetration test assessment to identify fluid-like tailings to support a tailings storage facility deconstruction

  G. Quaglia*, I. Cueto

  ISC2024.

  
  

  Abstract

  This paper presents the implementation of a Cone Penetration Test (CPT)-based methodology to identify fluid-like tailings in the Tailings Storage Facility (TSF) pond to support a TSF deconstruction. The TSF discussed in this paper was partially raised upstream and classified as very high risk due to its location, next to a creek and a short distance to the downstream community. The owner's preferred mitigation strategy was to proceed with the TSF deconstruction. To inform the deconstruction planning, a comprehensive geotechnical site investigation was conducted, which included CPTu and boreholes with continuous thin wall tube sampling from a floating platform. The investigation identified tailings in the pond as high plasticity clays (CH) with a geotechnical fines content exceeding 95 %. The CPT results indicated low tip resistance, nearly linear pore pressure measurements, and minimal sleeve friction, suggesting the presence of fluid-like tailings in a significant portion of the TSF. The implementation of this methodology allowed to systematically confirm the presence of fluid-like tailings across the pond and define their extent. This information supported the owner’s decision to employ mechanical excavation taking advantage of the tailings flowability. The observations made during the tailings excavation confirmed a good agreement between the CPT assessment and the actual material behaviour. These findings underscore the efficacy of the CPT-based method in guiding safe TSF deconstruction projects

  Abstract
  This paper presents the implementation of a Cone Penetration Test (CPT)-based methodology to identify fluid-like tailings in the Tailings Storage Facility (TSF) pond to support [...]

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