Open Pit geotechnical engineering and slope stability

Venetia Open Pit Mine, after 30 years of production, will be transitioning to underground production in Q4 2022. This presents an interesting conundrum: aggressively maximise pit ore recovery and short-term revenue or follow a more conservative, high reliability slope design and defer ore to later underground recovery. Preferences are weighted towards high reliability plans with limited 'surprises', as there exists limited stockpile contingency options if unplanned slope failure occurs. Ore delays cannot be afforded. An inter-ramp scale slope failure, 130 m high and 85 m wide, occurred on the south wall of Venetia Diamond Mine in July 2020. The failure impacted the principal haulage ramp to the orebody as well as a substantial resource of high-grade ore. Back analyses were subsequently completed to verify failure mechanisms as well as refine foliation shear strength contributing to slope failure. The results of the back analyses were then used to determine an acceptable mine plan for recovering the ore below the failed section of slope, without compromising further slope instability and minimising deferred ore volumes. The back analyses process applied three-dimensional limit equilibrium slope stability models to replicate the behavior of a strong, but highly anisotropic rock mass and cross-cutting geological features that contributed to the failure. Once a calibrated model was developed a series of pore pressure scenarios were applied to a suite of final pit slope designs at incremental face angles (with veracious pore pressure scenarios) for bench-by-bench progression to ultimate pit geometry. These model scenarios provided the mining operation to follow quantitative risk-based decisions as ground conditions and slope performance unfolded in real time mining. This paper outlines the back analysis, model calibration and forward planning post slope failure. The risk management and mining processes used to successfully mine the remaining ore from open pit methods is also documented.

Interferometric synthetic aperture radar (InSAR) application has recently benefited from an increased number of service providers (with more diverse satellite constellations), advances in algorithm processing methods and, with reductions in costs, is becoming a widely accepted method of surface deformation monitoring in the mining industry. InSAR monitoring is consequently being applied to a wide array of mine infrastructure and geotechnical risk management scenarios ranging from construction to operating and closed mines, including natural slopes traversed by mine access roads, rail and pipelines, to engineered open pits, waste dumps and tailings dams, as well as identification of subsidence and onset of potential collapse due to either active or legacy underground mining and/or karstic terrain sinkhole development. With this increased interest from a growing array of diverse technical disciplines, it has been shown that InSAR monitoring is complex and there are many variables to consider and levels of monitoring possible. Furthermore, making sense of vendors’ claims on system deliverables versus demonstrated outcomes can be challenging. These aspects need to be considered and aligned with the anticipated mode of instability, size, magnitude and rate of movement, and the business risk. Using a premium InSAR product (as high resolution data with two look directions and high frequency reports) when budget is not a limiting factor or using a budget constrained product (such as low resolution freely available Sentinel data), unless appropriately matched to the business risk profile (and deformation characteristics), will likely lead to underwhelming and possibly misleading results. This paper describes how decision trees were developed to assist in selecting the level of InSAR monitoring considering the asset infrastructure risk and the physical characteristics of the area of interest. The decision trees were built such that the user, without extensive technical knowledge of how InSAR functions, can make an independent evaluation of what InSAR product is adequate. A simple cost versus risk trade-off tool is discussed, outlining how the decision trees were developed to determine whether InSAR would be a viable solution at the site and what the appropriate resolution, acquisition frequency, report frequency, and orbit/s should be. This provides a consistent framework for firstly evaluating and matching monitoring rigour with geotechnical risk, secondly a process to facilitate alignment and ideally optimisation of monitoring outcomes between disciplines, and finally for communicating these to management to demonstrate an effective business case for monitoring. Keywords: InSAR, slope stability, tailings dams, displacement monitoring, risk management, decision tree

Working stress design (WSD) has long been used in geotechnical practice. However, in the past decade this has, to a significant extent, been replaced by limit state design (LSD) which is the basis for Eurocode 7, and presents a more robust approach in dealing with uncertainties. Factually, however, none of these approaches quantify the degree of uncertainty in design parameters or their cross correlation; aspects which directly affect the reliability of a design, especially geotechnical designs since soil is amongst the most variable of engineering materials. This paper introduces underlying concepts for the response surface - first-order reliability method (RS-FORM) and provides a demonstration of how this reliability technique can be implemented to analyse the stability of homogeneous slopes in practice.

Closure planning is a fundamental requirement for all existing and planned future mines. However, there are no accepted industry guidelines for how to assess options for open pit closure or for advancement of the closure plan in parallel stages with overall project development and operation. Ideally, closure planning needs to consider the site setting, develop the “big-picture” strategy, and work downward from there; balancing the objectives of the operator, regulator and community. At present, too much inappropriate detail is being included in “early-stage” closure plans that, when approved, become committed. This paper outlines the “State-of-Practice” geotechnical and hydrogeological guidelines for closure planning and implementation being developed by the Large Open Pit (LOP) project research group. The new guidelines are intended for use by geotechnical and hydrogeological mining professionals addressing closure design criteria, risk management, detailed planning and implementation. Benchmarks are provided for stakeholders, including regulators and the community, to judge whether adequate investigations and planning have been completed for appropriate stages of overall project development. Three case studies are provided to illustrate how changes of closure approach produced a substantially better outcome.

This paper presents a case study on the slope design optimization studies for the De Beers Victor Mine open pit in northern Ontario. The bench geometries appeared to have the potential for steepening; however, the necessary increased level of data confidence required for acceptance of the design improvements and to deviate from empirically derived catch berm width designs was lacking. Together with conventional investigation, two unconventional techniques were implemented to collect information inexpensively and efficiently with minimal disruption to the operation. The first technique trialled was 3D digital photography of the pit walls, obtained by helicopter drone, allowing photogrammetric assessment of bench geometries. The findings of this effort were combined with geotechnical domain-specific rockfall trials on bench geometry, which indicated that narrower catch berms would be reliable. The second unconventional technique targeted in-pit rock core sampling for laboratory testing using a hand drill more typically used to obtain concrete cylinders. This avoided the need for disruptive in-pit geotechnical drilling. The hand drilling results were questionable and are shared so that future studies can benefit from the experience. These unconventional techniques, combined with conventional geotechnical investigation, improved confidence in the data on rock strength and potential bench geometries, and provided justification for steeper bench geometries without compromising reliability on rockfall containment. The pros and cons of the drone survey and hand drilling are highlighted in the context of the overall study.

Slope optimization for hard rock open pits with favourable kinematics requires documented understanding of the engineering geology and specifically the interaction between bench scale structural fabric and the blasting and scaling procedures that control the achievable bench geometry. Such was the case in 2008 for the 200 m deep Tio Pit in Quebec, owned and operated by Rio Tinto Fer et Titane, where the inter-ramp angle was flattened from 58 to 52 degrees and benching reduced from triple to double for future laybacks and for deepening of the current layback because of rockfall hazard. This has implications for the life of mine and size of the ultimate pit which would be on the order of 400m deep. This paper reviews the investigations, the integrated operational leadership, mapping, controlled blasting and rock fall trials, wall scaling and innovative bench cleaning efforts using the spider shovel, carried out between 2010 and 2016 that significantly improved pit slope performance in the anorthosite. The result was procedures and insight as to what an adequate safe and steeper double bench geometry, inter-ramp angle 56 degrees, could consistently be achieved when combined with an empirical design for the catch berm width. The results of the rock fall trials provided validation of the conservatism of empirical catch berm width but also identified opportunity for optimization based on site-specific rock fall evidence. For practitioners with similar hard rock slopes, combining site-specific catch bench width and achieved bench face angle information as done on Tables 1 and 2 can facilitate discussion on risks and opportunity when a design team or review board are considering a design change based on evidence and risk tolerance. Key to the success was having dedicated ground control engineers supported by the mine engineering team and by constructive external review.

The De Beers Canada Victor Diamond Mine is located in the James Bay lowlands of Northern Ontario. This case study presents the evaluation of geotechnical stability and pit lake filling. The work was used to support decisions that informed risk assessments and the closure plan for two key phases: 1. Active closure, with personnel undertaking rehabilitation activities in and surrounding the pit. 2. Post-closure, when personnel and equipment have been demobilised from the mine site. 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 regulatory requirements. Active mining operations in the open pit ceased in mid-2019 and pit filling is underway. The pit required the installation of a major dewatering system, with up to 94,000 m 3 /day, pumped mostly from dewatering wells. Considerations for closure included the site remoteness, safety, global and local stability, water quality of the pit lake, permitting commitments, and closure regulations in the province of Ontario. A major consideration was the rate of pit filling. Rapid pit lake filling using water from the nearby Attawapiskat River leads to more favourable stability and environmental outcomes. A simple hydrogeological model was used to predict the filling rate and the final pit lake level for a number of potential closure options. This was used to schedule a phased geotechnical monitoring approach to ensure the safety of the operators as the pit walls became increasingly pressurised. A trade-off study has informed the preferred approach for pit lake development. Active pit closure has recently been completed and closure reclamation is ongoing.

This paper discusses the engineering, hydrogeology, operational practices and TARPs that facilitated deeper mining and goodbye cuts at the De Beers Canada Victor Diamond Mine. A number of technical challenges related to water management and adverse geology late in the mine life were overcome in order to successfully meet and exceed the planned mining depth. Extensive pit dewatering and a pit water management program were implemented not only to maintain dry working conditions for operations but to ensure highwall stability and reduce the formation of hazardous ice columns along the highwalls. After a 15 m tall ice column failure, bench design was optimised to ensure perched aquifers occurred as low on the highwall as possible. Where the uppermost ice columns formed, strategic reinforced meshing was installed to eliminate the possibility of sudden ice column failures that could not be predicted with the monitoring systems in use. Excellent limits blasting outcomes and better than expected conformance to design also allowed for inter-ramp limestone slope optimisation. Additionally, a 'ploughing' style failure mode was of concern due to blocky limestone underlain by saturated clays and mudstones. Here, a pit redesign and highwall depressurisation thresholds were used to prevent this failure mechanism from occurring, as well as developing vibrating wire piezometer trigger action plans. The overall results of the integrated hydrogeological and geotechnical designs and slope management practices were positive and contributed significantly to slope design optimisation and safely recovering additional ore. What worked technically and in terms of people and procedure are shared.

The performance of soils exposed on open pit slopes is adversely affected by subarctic climatic conditions. This paper outlines a case study of how geotechnical conditions were reassessed from the design stage and remedial measures implemented to control erosion and maintain long term stability at the De Beers Canada's Victor Diamond Mine open pit, located in northern Ontario. During development of the Victor pit, exposed deposits of sands on upper slopes were encountered where only limestone bedrock had been anticipated. Because of their weakly cemented characteristics, the sands were excavated without blasting, using the bench geometry intended for the disturbed limestone domain on the perimeter of the kimberlite pipe. Performance was always initially good, due to the sand's weak cementation and also by the below freezing temperatures. With each spring freshet and significant rain event the slopes degraded, sloughed and eroded causing concerns for haul road traffic below and for the performance of overlying glacio-marine clays. The remedial measures considered in the evaluation included; water diversions, slope flattening, laybacks, various types of buttresses and rip rap. The remedial measures were organized in the form of a decision matrix that was used to address exposed or possible future sand zone exposures, taking into account their size, accessibility, and location in relation to the overburden slopes and ramp. The case study describes the buttressing of a 100m long exposure with an additional design challenge when locally the sand was two benches (i.e. 20m) deep and displaying cross-beds, referred to as the River Sand Lens.