Tailings 2019 is a seminar that provides a great setting for executives, professionals and students to learn and analyze recent innovations on tailings management. Members of Golder’s global Mine Waste team will be attending and presenting innovations in mine waste practice. Golder is also a proud sponsor of Tailings.
Golder’s Technical Presentations:
Liquefaction Assessment of Deposited Thickened Tailings using CPTu Soundings Calibrated with Laboratory Testing: A Critical State Soil Mechanics Application
Thursday, July 11, 2019
Authors: Edgar Bard, Senior Mine Waste Consultant, Chile; and Guillermo González, Geotechnical Engineer, Chile.
Application of thickened tailings technologies sometimes require for the impounded tailings to perform as a competent engineered fill (e.g. become the foundation of upstream raises, ensure stability of tailings beach stacked above the dam crest, etc.). In such cases, the geomechanical behavior of tailings is a key issue, with liquefaction often becoming the most influencing aspect. When tailings beaches have a high degree of saturation, stability of the impounded tailings must be demonstrated under post-liquefaction conditions (specially in seismic regions). This requires that liquefaction susceptibility and undrained residual shear strengths of tailings be well understood.
To evaluate liquefaction susceptibility of tailings deposits, CPTu soundings are currently the most used tools. Although CPTu can measure the response of tailings associated with their truly undisturbed in-situ state it does not directly provide geotechnical parameters related to tailings behavior. In contrast, while laboratory testing does provide those parameters, they often relate to disturbed conditions. The integration of both CPTu and laboratory data provide a means to characterize tailings behavior for the undisturbed in-situ condition.
This paper presented a case study where CPTu and laboratory testing data were combined using the framework of critical state soil mechanics. The differences between completing a liquefaction assessment purely based on CPTu results (i.e., based on average properties of soils and typical correlations for CPTu interpretation) and the assessment of the CPTu results combined with isotropic, oedometer and triaxial compression monotonic paths are presented.
Approach and Considerations for OMS Manual Development
Thursday, July 11, 2019
Authors: Paul Bedell, Principal, Senior Geotechnical Engineer, Canada; and Ivette de Jesús (Quebrada Blanca – Teck).
This paper describes the recommended approach and key considerations for the development of an operation, maintenance, and surveillance (OMS) manual for tailings management facilities based on the Mining Association of Canada’s (MAC) guidelines and a case study. The OMS manual constitutes a foundational component for tailings management systems as recommended by recognized international good practice. Early development of an OMS manual ensures timely compliance during the operation as well as more focused training and onboarding for personnel in addition to having operations align with the design intent of the tailings management facility.
Typically, a consultant, usually the engineer of record, leads the development of the OMS manual. This, in turn, is then handed over the Owner for completion and use. This approach typically leads to Owner´s personnel not internalizing the document or making proper use of it. The new revision (2019) of the guideline for Developing an Operation, Maintenance and Surveillance Manual for Tailings and Water Management Facilities, issued by the MAC, advocates for Owner-led development; this approach was adopted as part of the operational readiness activities for the Quebrada Blanca Fase 2 Project (QB2). This paper discusses recommendations for approach (such as using Last planner® approach for the development planning) and key considerations to bear in mind based on the lessons learned through the development of the QB2 OMS manual. A practical overview of the OMS manual development for QB2 will serve as the case study discussed in this paper.
Application of Probabilistic and Deterministic Earthquake Ground Motions to Tailings Dam Design
Thursday, July 11, 2019
Authors: Marcelo Martinez, MEng, Associate and South America Earthquake Engineering Leader, Chile; and Alan Hull, PhD, PG, CEG, Principal and Global Seismic Hazard Practice Leader, USA.
Current international tailings dam guidelines provide earthquake ground motion selection criteria for the analysis and design of tailings dams. It is not always clear, however, what ground motion level is applicable, leading to confusion, inconsistencies, and arbitrary selection of ground motions for seismic analysis and design. For an operating basis earthquake (OBE) performance level, ground motions with an annual exceedance probability (AEP) of 1/475 are generally recommended. OBE ground motions are evaluated using probabilistic seismic hazard analysis (PSHA).
For the higher performance safety evaluation earthquake (SEE), the dam failure consequence needs to be considered because it influences the final ground motion value. SEE ground motions can be evaluated using both PSHA and the maximum credible earthquake (MCE) evaluated by deterministic seismic hazard analysis (DSHA). We suggest that the lower of the MCE 84th-percentile, or the 1/10,000 AEP and 1/5,000 AEP ground motion be used for analysis and design of extreme and high failure consequence dams, respectively. For a low dam failure consequence, the controlling ground motion can be the lower of the MCE 50th-percentile (median), or the 1/2,500-AEP.
When DSHA is not possible because of the lack of identifiable fault sources in very low to low seismic-hazard regions, then engineering judgment should be used to select the most appropriate PSHA-based ground motion associated with a low AEP.
The Use of Mobile Paste Technology for Crown Pillar Stabilization in a Flooded Stope
Friday, July 12, 2019
Authors: Sue Longo, Principal and Project Manager, USA; Thomas Skocir, Process Design Specialist, USA; and Jonathon Taylor, Senior Mining Engineer, USA.
The use of paste for tailings disposal and stabilization of underground voids in an active mine is a well-established technology practiced the world over. Paste can also be used for mine site closure and remediation. In 2018, a mobile paste plant was operated at a closed mine site in Ontario, Canada. Decades of mining activity at the site had left some of the near surface stope crown pillars with long term stability concerns. A backfill program was designed to include the use of historic tailings that were excavated from the tailings impoundment area. The tailings were used to produce paste for backfilling the stopes from the surface via a series of targeted boreholes. One stope posed unique challenges in that it was flooded with mine water. While mobile technology has been employed at other closed sites, backfilling flooded voids is rare. Evaluation of the underground voids on site was done with a Cavity Monitoring Survey (CMS) using a laser scanner. This technology, however, is not suitable for flooded applications. The flooded void had to be evaluated using sonar technology. Typical paste recipes had to be modified for subaqueous placement by using admixtures. A strategy for capturing and dealing with water displaced out of the underground void by paste was required to avoid contamination of the local watershed. These modified methods can be applied not only to other closed sites, but also to active sites trying to deal with flooded stopes.
Impacts of Topography Quality on Dam Breach Assessments
Friday, July 12, 2019
Authors: Alexandra Halliday, Project Engineer, Chile; and Alfredo Arenas, Geotechnical Civil Engineer, Chile.
Dam breach assessments (DBA) include estimating the location at which a dam may fail, the volume and properties of material that may be released and finally predict where the released material may travel. The purpose of a DBA will vary, however they generally aim to support mine operators in devising emergency action plans and classifying the consequence of their TSF, assisting in the implementation of design measures and operational controls on their facilities.
The area impacted during a dam breach will vary depending on the volume released and the flow properties, but also on the availability and quality of the topographic/bathymetric data downstream of the TSF. The impacted area can be amplified if tailings find their way into a watercourse but can be stopped short if a breach occurs on an uphill topography, these changes in topography are critical to the results of a DBA and need to be captured in the topographic survey for the assessment to be an accurate representation of the site conditions and potential breach.
Common industry practices adopt the use of numerical modelling software as tool to undertake DBA. This software allows users to input site specific topography for each DBA. Generally, a survey will be localized to TSF site boundaries, excluding susceptible receptors and flow channels that a breach may impact.
Like any modelling software, these packages each have their sensitivities which, for the most part, are well understood. A sensitivity which has not yet been established is the impact that a low-quality survey can have on the results of a DBA. This paper presents the impact that a low-quality survey can have on the results of an analysis by comparing topographies from two site specified aerial surveys with the corresponding free-sourced surveys.
The results of this study show that the topographic survey quality can have a significant impact on the dam breach characteristics and inundation area, potentially causing larger runout areas, greater flow volumes and more rapid flood streams when a free-sourced and low-resolution topography is utilized as a substitute for a high-resolution site-specific survey.
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