Transitioning from mine operations to closure: the dilemma of differing geotechnical design acceptance criteria perspectives (original) (raw)
Related papers
Design acceptance criteria for operating open-pit slopes: An update
CIM journal, 2020
Open-pit slope engineering involves balancing economic imperatives and risk, where an optimum pit slope design seeks to safely steepen the pit slope angles to minimize the mining of waste, therefore maximizing ore recovery. Open-pit Design Acceptance Criteria (DAC) had been proposed and adopted by industry, and the Guidelines for Open Pit Slope Design, published in 2009, finally provided a consistent set of guidelines. Since the publication of these guidelines, it has been the industry experience that practical DAC requires consideration of multiple levels of design confidence, ranging from highly uncertain greenfield sites to mature operations; the unique nature of consequences associated with open pit wall failures as opposed to other civil and mining structures; and differences between operators' risk appetite. This article presents the state of practice for inter-ramp and overall open-pit slope design in the mining industry and develops flexible DAC for operating pits that addresses the considerations outlined. The criteria are rooted in robust risk tolerance principles, previously adopted criteria, a database of open-pit designs, and mathematical validation for criteria in terms of both Factor of Safety (FoS) and Probability of Failure (PoF).
Geotechnical risk management for open pit mine closure: a sub-arctic and semi-arid case study
Proceedings of the 13th International Conference on Mine Closure, 2019
De Beers is currently developing closure plans for two open pit mines. At first glance they appear quite similar; both are relatively remote, have operated for 10 years, have similar pit dimensions (250-300 m deep and 1.5 km wide) and have Palaeozoic sedimentary host lithologies with weak upper units overlying more competent lower materials. However, Victor Mine in the sub-Arctic Canada, is one of De Beers wettest mines (dewatering volume of 75,000 m 3 /d) and is hosted predominantly in good quality limestone with excellent final wall performance. While Voorspoed Mine, in semi-arid Southern Africa required virtually no dewatering, has poor wall performance associated many with mudstone and country rock breccia instabilities. Victor Mine is expected to achieve stable pit lake in less than 10 years (and less than 2 years with supplementation from a nearby river), while Voorspoed will take over 100 years to reach ultimate pit lake level (due to low groundwater inflows, high evaporation and limited opportunity for flow supplementation). This paper describes a process to determine closure stability design acceptance criteria (DAC) and characterise the zone of long-term surface disturbance surrounding the pit (i.e.: potentially unstable pit edge zone to define the closure exclusion zone). This involved: 1) Pit break back-analysis using i) industry guidelines; ii) an empirical approach based on historical slope instabilities; and iii) stability analysis using predicted post-closure phreatic surfaces. 2) Estimates of erosion potential. These results along with the well documented historical slope performance provided the basis for detailed Geotechnical Risk Assessments which addressed two periods. • Active closure with personnel undertaking rehabilitation activities in and surrounding the pits, and • Long-term closure, personnel and equipment not permitted within the long-term break back zone. Risk-based monitoring plans were developed along with Trigger Action Response Plans (TARPs) to ensure that closure of the pit proceeds safely and efficiently while satisfying the regulatory requirements.
Economic significance of geotechnical uncertainties in open pit mines
2019
The major cost associated with open pit mine operations is waste stripping. While the steepening of slope angles reduces the stripping ratio, and hence operational costs, it also increases the likelihood of failure. Major slope failures incur significant cost elements including clean-up, disruption to mine operation, and damage to mining equipment and in some cases loss of reserves. Geotechnical engineers are often faced with the difficult task of finding the balance between slope optimisation and acceptable risk related to the likelihood of large slope failures. Technological advancements have allowed for the development of larger and deeper open pit operations, but have also created higher economic impact from potential slope failures. Given that the aim of mining operators is to maximise overall profits, it is surprising that most slope designs are based on deterministic design approaches, and limited attention is given to quantifying uncertainties in the geotechnical model. As most major decisions in the mining industry are made by senior management staff and financial staff, any attempt on linking slope stability analysis results with monetary values would improve the critical communication between geotechnical designers and decision-makers. Using Cowal Gold Mine as a case study, this paper illustrates economic risk caused by geotechnical uncertainties. The geotechnical risk estimate is generally subjective due to geotechnical engineers having to rely on limited data and engineering judgement. Geotechnical risk is compared against economic factors that are often perceived as important variables in mining operations.
Proceedings of the International Conference on Mine Closure, 2019
In the current mining industry climate, early and detailed planning for mine closure is becoming increasingly important. Geotechnical inputs relating to stability of open pit excavations were often limited or cursory in past mine closures. Although broad guidelines based on general observations do exist in some countries, detailed deterministic approaches and design criteria specifically applicable to open pit excavations during mine closure have not been well established. Large open pits may be most sensitive to the economic impacts of geotechnical requirements/constraints for closure, particularly if unduly conservative approaches are employed. It is therefore necessary for these requirements and constraints to be carefully assessed for each pit, and for the most cost-effective measures to be identified. It is important that slope instabilities are well-documented during operations, and that good communication between various technical departments is maintained for closure planning and coordination. This paper presents examples of several scenarios in which pit slope stability assessment may be required at mine closure. These are focused specifically on the long-term stability of the open pit excavations, not the stability of any adjacent structures. The paper does not attempt to propose a set of guidelines but discusses techniques that may be used for assessment under various scenarios. Topics include estimation of rock mass degradation over time, assessment of long-term instability around pits, evaluation of slope buttressing options, and determination of exclusion zones around pit crests (and in-pit floors that must remain open to access).
A risk consequence approach to open pit slope design
The Journal of the Southern African Institute of Mining and Metallurgy, 2006
Open pit slope design has conventionally been effected as a bottom up function utilizing available geotechnical information. This results in a decision criterion based on probability of failure and factor of Safety with a risk assessment being carried out on the proposed design slopes. The design approach recommended reverses the process using fault event tree decision methodology. The consequences of this approach are that acceptable risk criteria have to be determined by mine owners. Slopes are then designed by the technical staff to achieve these corporate goals. Benefits that arise from the process are that the owners take a proactive decision on the risk-benefit relationship allowing the technical staff to optimize the geotechnical exploration programme and design.
Critical challenges impacting the advancement of slope design reliability
SSIM 2023: Third International Slope Stability in Mining Conference
There have been many recent and ongoing advances in the area of slope stability, including tools and processes for slope management, rock mass characterisation, stability assessments, and selection of design acceptance criteria. Despite the many advances and ongoing research in the field, there remain several fundamental unresolved challenges that impact the design and assessment of pit wall stability. In this paper, these have been categorised into six key areas: • Assessment and communication of risk: a risk-based approach to slope design has become more common practice, particularly with advances in approaches to calculating a Probability of Failure (PoF). However, the PoF is not quantified in a consistent manner within our industry and can present a false or misleading view of project risk to stakeholders and decision-makers. • Reliance on technology: new and emerging technologies have provided many benefits to our industry but, in some cases, technology has advanced faster than the ability to answer more fundamental and basic questions related to material characterisation and slope behaviour. • Treatment of damage/overbreak zone: the rock mass strength and hydraulic conductivity of the near-surface 'rind' around the pit wall is impacted by damage resulting from blasting and stress relaxation. The depth and extent of damage/disturbance are very difficult to test/evaluate. Although the assumptions may significantly influence predictions of inter-ramp scale Factors of Safety (FoS), there are no consistent approaches or standards for the treatment of this zone in stability analyses. • Integration/communication: within our complex mining environments, disconnects often exist between disciplines but also between the technical teams, management and stakeholders. There are also obstacles to information sharing and collaboration. These can reduce efficiency, design reliability, and the speed of innovation. • Increased demand for minerals: electrification and the demand for resources are arguably greater than they have ever been. With this increase in demand comes additional challenges related to developing deeper mines and steeper pit walls in increasingly remote environments. • Reducing technical workforce: the industry is currently under-resourced. The talent pool is shrinking while mine complexity is increasing. It is critical that the industry take steps in the short term to address this skills shortage. This paper is not intended to prescribe or present solutions to these issues. Rather it is intended as a means to engage and challenge the industry to discuss and consider these issues. Based on research along with the author's experience and discussions with industry leaders, this paper explores each of these issues, how they may impact the reliability of slope designs, and ideas for consideration to move the industry along a path to overcoming these challenges.
2020
A transparent, pragmatic geotechnical design system is outlined that presents a selection of risk options with associated risk/reward for decision makers. Risk options termed ‘robust’, ‘balanced’ and ‘aggressive’ have been defined appropriate to ‘critical infrastructure’, ‘typical industry’ and ‘low risk’ mining environments (where the safety risk and the consequences of failure on the budgeted mine plan are acceptably low), respectively. The geotechnical model includes ‘most realistic’ and ‘reasonable lower case’ conditions. A ‘realistic’ design principle requires reporting a Factor of Safety on the realistic case, rather than to reduce design inputs due to uncertainty. Uncertainty is transparently covered by the ‘lower case’ in sensitivity analyses. Indicative probabilities of failure are estimated and a simple empirical tool estimates the consequence of failure in terms of the area of mining floor impacted. These together with indicative value or tonnage estimates are presented t...
Map of the potential geotechnical susceptibility for operational pit slopes
REM - International Engineering Journal, 2019
This article proposes a procedure to elaborate a map that presents the potential risk of failure occurrence in the operational slopes of open pit mines. First, it is necessary to collect the available geological-geotechnical data and perform a field mapping, in order to verify and validate the most representative parameters and to characterize the discontinuity families of the rock mass. Then, the mine should be sectorized, considering all the data collected, the geometry of the operational slopes and its development until the final pit. The next step will be to define and to evaluate which failure modes have greater or lesser potential to occur in the pit and to assign weights to them. In this study, the weathering, planar failure, and plane circular failure potentials were evaluated. As a result, it is possible to develop a map with the susceptibility level of the sectors. This map will help make technical and managerial decisions in order to reduce the risk level of the sectors and to promote an increase in the operational safety of the mine.
Red Dog Mine is a zinc-lead-silver mine located in northwest Alaska. The Red Dog deposit consists of three sub-horizontally stacked, folded and faulted thrust sheets. The structural geology of the mine area is highly complex and extremely variable. Monitoring and failure mitigation measures have been implemented in the Main Pit at the Red Dog Mine. These measures include a revision of the structural geological model, daily field wall inspections and data collection, monitoring of slope movement, and on-going geotechnical stability assessment and analysis. Particular emphasis has been given to identifying potentially adversely oriented major faults and geologic contacts which may exist behind the proposed ultimate pit walls. Pit walls inspections, together with slope movement monitoring data, allows for small scale (bench size) to large scale (multi-bench) instabilities to be proactively identified. The monitoring of slope displacement is undertaken using a robotic total station prism monitoring system. Formerly, the updated structural geology interpretation has been used to update stability assessments, re-design certain areas of the Main Pit, and carry out hazards and risk assessment of the pit walls. Latterly, the interpretation of survey data has allowed for long term hazard assessments, stability assessments and analysis, and day-to-day geotechnical support for the on-going safe operation of the open pit. This paper briefly describes the geologic model that was developed to guide the forecasting, assessment, monitoring, and mitigation of geotechnical hazards and risks in the Main Pit at Red Dog Mine. A discussion is provided regarding the ground control management plan which has been implemented to mitigate the risks associated with potential slope instability. The identified zones of instability, known as geotechnical hazard zones (GHZ), are presented, including a geotechnical description, anticipated future behaviour, actual slope performance, required pit design modifications and remedial measures being implemented. Results of the stability analyses including the underlying assumptions, methodology, input data and limitations are also discussed.
Pit Slope Configuration for Open Pit Mining – A Case Study
American journal of science, engineering and technology, 2024
To achieve stable pit wall slopes, it is imperative to obtain a fair knowledge of the rock mass characterisation before designing the pit. Insufficient knowledge of the competency of the country rock could lead to using unsupported slope configuration in the design process which can consequently lead to slope failure. In this study, the geomechnical properties of the Bremen-Nkosuo concession are analysed using Bieniawski's classification scheme to determine the Rock Mass Rating (RMR) for defining safe pit slope configuration of the Nkosuo pit. The findings show that the rockmass are best described as 'fair' for the two main lithologies existing at the concession. Subsequently, localised adjustment factors are applied to the calculated RMR to arrive at Mining Rock Mass Ratings (MRMR). These MRMR values are correlated with 50 m fixed stack height and 1.2 safety factor to determine optimum Bench Slack Angle (BSA) of 54° and 57° for host sedimentary and granitic rocks respectively. For individual benches, optimum slope design configurations were 10 m, 800, and 6.6 m respectively for bench height, bench face angle and catch berm for metasedimentary rocks. Likewise, that for granitic formation were 10 m bench height, 800 face angle and 6.0 m catch berm width. These configurations are in conformance with mineral and mining regulations of Ghana. Slope stability assessment was performed which included Slope Mass Rating (SMR), Kinematic and Limit equilibrium analysis. From the analysis, multi-bench scale slope instability occurrence was found to be rare but single-double scale could be possible at the western wall of the planned pit with probability of failure of about 0.4. Presplit and trim shots perimeter blasting techniques are recommended to maintain the integrity of the final pit walls at certain areas.