IAP2-22-460

New Design Methodologies for Open Pit Mines of Reduced Carbon Footprint for the Extraction of Green Economy Metals

A key policy objective of the United Kingdom government is to reach carbon neutrality by 2050. This requires the decarbonization of all the major sources of emissions direct and indirect, i.e. the supply chain. A major source of carbon emissions is mining and the demand to extract several industrial metals is predicted to increase due to the requirements of the green economy. This proposal seeks to develop a novel design for open pit mines that allows achieving a significant reduction of rock excavation leading to a substantial reduction of emissions and increased profitability which is important to attract the interest of the mining industry in the proposed novel design concept. Open pit mine pitwalls are currently designed to be planar either along the overall profile or between ramps (Hustrulid et al., 2013); the novel concept pursued involves instead the adoption of geotechnically optimal profiles for the mine pitwalls (Utili et al., 2021). These profiles are employed for the design of each sector of an open pit mine. Optimal profiles are geometrically more complex since non-planar in elevation but allow the pitwall to be of greater steepness without compromising the safety of the mine (see Figure I). At present, a key hurdle for the adoption of such profiles is the presence of jointed rock masses exhibiting non-negligible anisotropic behaviour which affects most open pit mines. The proposal aims to solve this scientific issue to make the adoption of optimal profiles possible for all open pit mines.

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Image Captions

Figure 1,Figure 2

Methodology

In the last 10 years, 4 independent systematic studies on the mechanical stability of slopes (Utili & Nova, 2007; Jeldes et al., 2015; Vahedifard et al., 2016), concluded that non-linear concave slope profiles are significantly more stable than the equivalent straight profiles. Recent work has led to a software, OptimalSlope (Utili, 2016) produced by OptimalSlope Ltd, the non-academic partner of the proposal, that systematically determines the optimal profile from a mechanical stability point of view for a given lithology, rock properties, and prescribed Factor of Safety (FoS). OptimalSlope seeks the solution of a mathematical optimisation problem where the overall steepness of the pitwall, from crest to toe, is maximised. Bench geometries (bench height, face inclination, minimum berm width) are imposed in the optimisation as constraints that bind the maximum local inclination of the sought optimal profile. The obtained optimal profiles are always steeper than their planar counterparts up to 8° depending on rock type and severity of constraints on local inclinations. The optimal pitwall profile is defined as the overall steepest safe profile, i.e. OSA=OSAmax, with OSA being the inclination over the horizontal of the line joining the pitwall toe to the crest (see Figure 2).
So far OptimalSlope has been adopted in four case studies of mines featured by isotropic rocks: a copper mine to be excavated in Chile (Utili et al., 2021), an existing North American gold mine to be enlarged (Agosti et al., 2021a), the McLaughlin mine (Agosti et al., 2021b), a copper and gold mine whose data are available from a public repository, and the Marvin gold mine (Agosti et al., 2022). From these works emerges that the adoption of optimal profiles realises reductions of the carbon footprint of up to 18.7% of the emission related to mining activities. To provide some context, in the case of the McLaughlin mine a reduction of 1.5 billion kg CO2 eq is realised. This is equivalent to the carbon sequestered by 24.6 million tree seedlings grown for 10 years and the greenhouse gas emissions avoided by 309 wind turbines producing electricity for a year, as calculated by using Environmental Protection Agency (2021).

Project Timeline

Year 1

i) Training in the use of C/C++ and Matlab programming and analytical methods;
ii) Training in the theory behind the design of open pit mines and commercial software packages for open pit mine design (e.g. Datamine, Geovia Whittle, RPMGlobal SOT, Hexagon mining);
iii) Ability to work out optimal pitwall profiles in the presence of any geological discontinuities;
iv) Extension of OptimalSlope to account for various geological discontinuities.

Year 2

i) Identification of 2 case studies in complex geological conditions (ideally including a deep mine and a multi-pit mine);
ii) Design of the mines of the case studies according to traditional methods;
iii) Design of the mines of the identified case employing optimal pitwall shapes;
iv) Pitwall stability verification via state-of-the-art geotechnical software, evaluation of average carbon footprint saved by adopting optimal pitwalls and identification of any potential design issue.

Year 3

i) Validation and refinement of the design procedure of open pit mines in the presence of complex geological conditions;
ii) Compilation of improved design guidelines for mine pitwalls;
iii) Publication of the results in peer-reviewed Journals and conferences;
iii) Presentation of the results in mining society workshops and courses for mining professionals.

Year 3.5

i) Thesis write-up;
ii) Dissemination at various conferences/workshops.

Training
& Skills

The School of Engineering requires each student to collect at least 60 PGRDP credits, corresponding to the attendance of in-school delivered workshops, taught modules and other activities that display further engagement. The training in (a) research skills and techniques and (b) research environment are provided through four mechanisms: (i) a programme of taught modules; (ii) internal training ‘workshops’ that focus on key geographical research skills and techniques; (iii) input from supervisors; and (iv) School and academic Group seminars by visiting and internal speakers and presentations by postgraduate students themselves.

In addition to generic training offered by the University, the School also provides training through a series of in-house ‘workshops’. Engineering research postgraduates normally take the following Workshops: ‘Scientific Writing’, ‘Research Ethics (Theory)’, ‘Data Management’, ‘Time management’, ‘Document Management – Content and Layout’, ‘Introduction to Learning and Teaching’ during their first year. Also, it is envisaged that the student will undertake from 2 to 4 taught modules depending on the academic background of the appointed student of the MSc in ‘Geotechnical Engineering’ and ‘Software Engineering’. Modules particularly relevant for the project are ‘Slope stability assessment’, ‘Applied rock mechanics’, ‘C/C++ Programming’. Most of these modules are delivered in one or two intensive weeks so well suited for PhD students.

Research training continues through the second and third years, and is based around a number of themes: (i) Recognition and validation of problems; (ii) Demonstration of original, independent and critical thinking, and the ability to develop theoretical concepts; (iii) Knowledge of recent advances within the research field and in related areas; (iv) Understanding relevant research methodologies and techniques and their appropriate application within the research field; (v) Ability to analyse and critically evaluate findings and those of others; and (vi) Summarising, documenting, reporting and reflecting on progress.

Bespoke technical training will also be provided by the research supervisors (numerical and analytical modelling of jointed rock masses, geomechanical principles for the design of pitwall profiles, C/C++ programming) and technical staff in the School of Engineering.

References & further reading

Agosti A., Utili S., Gregory D., Lapworth A., Samardzic J., Prawasono A. 2021. Design of an open pit goldmine by optimal pitwall profiles. CIM Journal, in press.
Agosti A., Utili S., Valderrama C., Albornoz G. 2021. Optimal pitwall profiles to maximise the Overall Slope Angle of open pit mines: the McLaughlin mine. ACG Second Int Slope stability in mining conference, Perth (Australia).
Hustrulid W., Kutcha M., Martin R. 2013. Open pit mine planning and design. 3rd edition CRC Press.
Society for Mining Metallurgy & Exploration 2011. Mining Engineering Handbook P. Darling ed., 3rd edition.
Environmental Protection Agency of the United States. 2021. https://www.epa.gov/energy/greenhouse-gas-equivalencies-calculator
Jeldes I.A, Drumm E.C., Yoder D.C. 2015. Design of stable concave slopes for reduced sedimentary delivery. ASCE Jnl Geotech & Geoenv Eng, 141:4040-93.
Utili, S., 2016. OptimalSlope: software for the determination of optimal profiles for slopes and pitwalls.
Utili S., Agosti A., Morales N., Valderrama C., Pell R. 2021. Optimal pitwall shapes to maximise financial return and decrease carbon footprint of open pit mines. Mining Metallurgy & Exploration, under review.
Utili S., Nova R. 2007. On the optimal profile of a slope. Soils and foundations, 47(4): 717-729.
Vahedifard F., Shahrokhabadi S., Leshchinsky D. 2016. Optimal profile for concave slopes under static and seismic conditions. Canadian Geotechnical J., 53:1522-32.
Agosti A., Utili S., Tasker J., Zhao C., Knights P., Nerhing M., Zia S. (2022) The effect of carbon tax and optimal profiles on profitability and emissions of open pit mines. Mining Technology, in press

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