Geotechnical considerations for the stability of open pit excavations at mine closure – some scenarios

Author(s): 
I de Bruyn, D Prado, J Mylvaganam, D Walker
Date: 
Tuesday, September 3, 2019
First presented: 
Mine Closure 2019
Type: 
Published paper
Category: 
Geotechnical
 
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).

Feature Author

Ian de Bruyn

Ian de Bruyn has 19 years’ experience in the fields of engineering geology and geotechnical engineering over a wide range of projects in both the mining and civil engineering sectors. He has particularly strong expertise in geotechnical assessment and provision of design parameters for open pit mining operations, and has worked on projects involving very large pits in challenging rock mass and hydrogeological conditions. He is well versed in rock mass characterisation and domain modelling for input into fragmentation analysis, selection of mining method and support design for underground mines. Ian’s projects have involved site investigation, characterisation, analysis, evaluation, design, risk assessment and reporting at all levels – from conceptual through pre-feasibility, feasibility and working design. He has also performed geotechnical assessments for due diligence studies. Ian has expertise in civil engineering site investigations / evaluations, including extensive work on karst-related ground instability, and site characterisation for foundation design for civil and mining infrastructure. Ian is currently the team leader of the SRK geotechnical group in the Perth office.

Principal Consultant (Geotechnical Engineering)
BSc Hons (Engineering Geology), Pr.Sci.Nat., MAusIMM
SRK Perth
Diane Walker

Diane Walker has 18 years’ experience as an engineering geologist in both mining and civil engineering projects. Her principal area of expertise is geotechnical investigation, rock mass characterisation and geotechnical domain modelling for open pit and underground mine design. Diane has extensive experience in investigation programme design, project management, geotechnical data analysis and provision of geotechnical design parameters. Her projects have included conceptual studies, feasibility studies and support for operating mines, as well as geotechnical assessments for due diligence studies. In addition, she has performed a planning and management role for geotechnical studies and design outcomes for the iron ore operations of a mining major in Western Australia. Diane has also conducted geotechnical soils investigations and site characterisation for civil and mining infrastructure projects.

Principal Consultant (Geotechnical Engineering)
MSc (Engineering Geology), Pr.Sci.Nat., MAusIMM
SRK Perth
Daniel Prado

Daniel Prado is a geotechnical engineer with more than 11 years of experience in mining consulting in the areas of geomechanics and civil infrastructure work. His experience includes the stability assessment of open pits in various rock mass conditions/ environments, design of portals, underground support system and rock dumps. He has extensively worked in various open pit and underground projects in Australia, and North/ South America. Daniel’s experience also includes numerical modelling using finite and discrete methods such as FLAC2D/3D, MAP3D and Rocscience software (e.g. Slide/ Phase2) for open pits and underground excavations which is complemented by the use of geological software such as Leapfrog.

Senior Consultant (Geotechnical Engineering)
MSc (Civil Engineering), BEng (Civil Engineering)
SRK Perth
Jaya Mylvaganam

Jaya Mylvaganam is a civil /geotechnical engineer with over 16 years of industry and research experience.  He has broad experience in the design of open pits in various rock mass and hydrogeological conditions from conceptual to implementation level studies, and in civil/ mining infrastructure designs. Jaya takes the role of project manager, team leader and peer reviewer for several projects and is responsible for the study outcomes and technical reports. His experience extends into 2D/ 3D stability modelling using several programs including UDEC and FLAC for open pit and underground excavations, and also into groundwater seepage and slope depressurisation modelling. Jaya has conducted site investigations for rock mass ratings and hydrogeological behaviours, and performed extensive interpretations of geotechnical data and data calibrations using numerical models.  His experience in mine waste management includes the designs of tailings storage facilities and rock dumps, and dynamic stability modelling. As part of his PhD research, Jaya developed an experimental set-up for laboratory investigation; a mathematical model to predict the shear behaviour of infilled rock joints and undertook verification analyses using numerical modelling.

Principal Consultant (Geotechnical Engineering)
PhD (Geotechnical Engineering), BSc Hons (Civil Engineering), MIEAust, MIESL, MAGS
SRK Perth
SRK Africa