A Novel design methodology for open-pit mines to reduce carbon footprint for the extraction of green economy metals

Overview

A key policy objective of the United Kingdom government is to reach carbon neutrality by 2050. This requires the decarbonisation of all the significant sources of emissions, direct and indirect, i.e. the supply chain. An essential 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 significantly reduce rock excavation, leading to a substantial decrease in emissions and increased profitability, which is essential to attract the mining industry’s interest in the proposed novel design concept. Open-pit mine pitwalls are currently designed to be planar either along with the overall profile or between ramps (Hustrulid 2013); the novel idea pursued involves adopting 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 1).

Methodology

At present, a key hurdle for adopting such profiles is the presence of jointed rock masses exhibiting non-negligible anisotropic behaviour, which affect most open-pit mines. The proposal aims to solve this scientific issue and make optimal profiles possible for all open-pit mines. The resulting algorithms will be designed and implemented using software-based solutions relying on object-oriented programming languages such as C++ (Balsamo et al., 2017).
In the last 10 years, 4 independent systematic studies on the mechanical stability of slopes (Utili and 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 (i.e. the planar profiles exhibiting the same FoS, see Fig. 1a) 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 line joining the pitwall toe to the crest (see Figure 2).
So far, OptimalSlope has been adopted in three 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), and the McLoughlin mine (Agosti et al., 2021b), a copper and gold mine whose data are available from a public repository. These works reveal that adopting 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 using Environmental Protection Agency (2021).

Project Timeline

Year 1

i) Training in the use of C/C++ and Matlab;
ii) Training in the theory behind the design of open-pit mines and commercial software packages for open pit mine design (e.g. Datamine, Micromine);
iii) Establishment of the procedure to work out equivalent continuum anisotropic rock strength parameters for jointed rock masses encountered in typical open-pit mines;
iv) Extension of OptimalSlope to include anisotropic jointed rock masses.

Year 2

i) Identify 3 case studies of metalliferous open-pit mines in jointed rock masses exhibiting anisotropic behaviour (ideally including a deep mine and a multi-pit mine);
ii) Design of the mines of the case studies according to traditional methods;
iii) Structure of the mines of the identified case employing optimal pitwall shapes;
iv) Pitwall stability verification via state of the art geotechnical software, evaluating average carbon footprint saved adopting optimal pitwalls and identifying potential design issues.

Year 3

i) Refinement of the procedure to work out equivalent continuum anisotropic rock strength parameters;
ii) Refinement and validation of the design procedure of open-pit mines employing optimal pitwall shapes in light of the results;
iii) Publication of the results in peer-reviewed journals and conferences;
iv) Presentation of the developments and results in mining society workshops and courses for mining professionals.

Year 3.5

i) Write up the PhD Thesis;
ii) Continue publishing results in peer-reviewed journals and conferences.

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 critical 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 the university’s generic training, the School also provides training through a series of in-house ‘workshops’. For example, engineering research postgraduates typically 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 and UGs in ‘Geotechnical Engineering’ and ‘Electrical and Electronic Engineering’. Modules relevant to the project are ‘Slope stability assessment, MSc’, ‘Applied rock mechanics, MSc’ and ‘C/C++ Programming, UG’ ‘Computer Systems and Microprocessors, UG’. 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 many 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 computers architecture) 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).
Balsamo D. et al. 2017. Wearable and autonomous computing for future smart cities: Open challenges. 25th International Conference on Software, Telecommunications and Computer Networks (SoftCOM).
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.

Further Information

Dr Domenico Balsamo: domenico.balsamo@newcastle.ac.uk
Prof Stefano Utili: stefano_utili@optimalslope.com

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