In tailings management, what happens below ground is equally important as what happens above ground.
The Australian Mining Review spoke to Klohn Crippen Berger (KCB) principal hydrogeologist Chris Dickinson about the topic of hydrogeological best practice as it relates to modern tailings management.
Global Industry Standard on Tailings Management (GISTM)
The GISTM (and its accompanying compendium) represent a significant shift in the approach to tailings management worldwide, “striving to achieve the ultimate goal of zero harm to people and the environment with zero tolerance for human fatality”.
Chris says the new standard is a “quantum step forward in the management of mine tailings” aiming to couple engineering and geoscience together in the design, construction, operation and closure of facilities in a manner not typically undertaken in the past.
“It pushes the proactive and integrated mixing of science and engineering towards a more positive operational and closure outcome,” he said.
Principle number 2 of the GISTM formally introduces the broader topic of best practice within geoscience fields, in the development and maintenance of an interdisciplinary knowledge base to support safe tailings management through the tailings facility lifecycle.
This includes preparing, documenting and updating detailed site characterisations such as data on climate, geomorphology, geology, geochemistry, hydrology and hydrogeology (surface and groundwater flow and quality), geotechnical and seismicity.
“This Principle importantly recognises that the management of tailings or mine waste is not just about the engineered components of the facility,” Chris said.
“Understanding the interactions of the tailings facility with the geological environment is very important. Understanding how facilities interact with the natural environment and how to design suitable mitigation or control strategies, whilst complementing the engineering design, is the key to good hydrogeological practice.”
Tailings dam hydrogeology
Hydrogeology is the study of groundwater – pressure and flow, recharge and discharge, seepage, transport and fate, water quality and physical impacts.
“Hydrogeology requires a sound geoscience foundation to understand and predict the performance of a tailings facility on the natural environment,” Chris said.
“Geological conditions, including rock and soil, structure, weathering and alteration, must be understood to an extent that captures those groundwater flow fields likely to be influenced by the structure, during its life.”
Tailings dams are facilities designed to store tailings, produced from the mill after ore has been processed.
Engineers design and construct the complex structures which retain and store tailings that in the most part are the above ground components of the facility. Tailings chemistry, tailings morphology and their water quality, can and do vary greatly, and also need to be understood.
“Tailings dams do not operate in isolation of the environment around them. They are very much embedded within the hydrogeology and it is therefore important to understand this relationship to be able to interpret what risks they might pose,” Chris said.
Tailings vs Foundation Hydrogeology
Chris said it was important to understand the component differences between tailings hydrogeology and foundation hydrogeology, with the former representing how the dam is designed and constructed, and the latter being the understanding of the natural hydrogeological system within which tailings are to be stored.
“Foundation hydrogeology must integrate with engineering design,” Chris said.
“If you don’t get the hydrogeology right, an inefficient or inappropriate design can be very difficult and expensive to mitigate. Advancing those technical elements at the start, and in parallel to engineering design, helps identify and understand operational risk as it relates to hydrogeology.”
Geometric Challenges to Analysis
Geoscientific analysis frequently requires three-dimensional thinking – this is needed to address high variability within the natural environment, as well as to account for geometric uniqueness which can be a challenge to performance evaluation of structures.
Chris said engineering practice had substantially evolved in recent years to include detailed analyses of tailings dams in both two and three dimensions.
“This has allowed a more interactive approach to assessing the relationship between tailings hydrogeology and foundation hydrogeology,” he said.
“Sometimes a two-dimensional analysis simply isn’t appropriate to assess how the natural system is going to respond to the constructed dam. This doesn’t mean all analyses should be 3D however.”
Informing such detailed analyses requires a comprehensive hydrogeological conceptualisation, progressively advanced to a level of technical confidence commensurate to that of the engineering design.
The potential for the natural environment to produce a pressure or seepage response outside of predicted ranges will always remain. Because of this, predicted performance should always be supported with performance observations and a documented process of response in the event such excursions from design do occur.
Most hydrogeological conceptualisations require a similar suite of assessment tools to develop a technical description of a site – geology, structure, hydrology, geochemistry, and hydrogeology are all inter-related components.
Such assessments should also be supported with suitable field testing and deployment of instrumentation to permit an understanding of time variant changes which may occur.
Of additional importance to tailing hydrogeology are the understanding of vertical pressure gradients between the facility (especially dam embankments) and the foundation, the operational water balance (ensuring it accounts for the complex flow mechanics which can result in losses, gains or both in a system), the rate of dam construction and tailings deposition, and how this may translate to impact to the natural system over time.
Recognition of the physical scale of assessment needed is also required to permit performance evaluation of total groundwater flow fields which may be influenced in a facilities life cycle.
Further, the development of operational monitoring records will support the design for future potential raises and/or expansions to the facility that may be required.
Importance of Life Cycles
Chris believes the best way to implement tailings hydrogeology best practice is to address every facility throughout its life cycle, and to do so in close and regular collaboration with design engineers and construction management teams.
“The design, planning and construction phases of a tailings facility might only take a few years but the operation and closure of the facility could be for decades, or longer,” he said.
“The inclusion of hydrogeology and other geosciences as complimentary and permanent technical elements to a tailings facility lifecycle is an important process in the improved management of tailings worldwide.
“In the past, these might have been seen as specialised sciences needed to inform a project application. This has changed, with these specialised fields identified through the GSITM as key lifecycle components of a tailings facility.”
Professional Competency and Experience
KCB is a prominent service provider in the tailings design and operational space worldwide. The company, which has been in operation for more than 70 years, is at the forefront of science and engineering of tailings storage facilities.
Chris said KCB has developed a strong understanding of what needs to be achieved in the hydrogeological space to adequately inform and support engineering and design.
“We have developed hydrogeological skills in all major offices globally. We are able to leverage specialised experience, not just hydrogeology, within our design and construction team dynamic. These specialised fields do not need to be outsourced,” he said.
“In terms of the science of tailings hydrogeology best practice, geoscience brings significant value to the engineering team. Being able to identify and quantify a potential pathway for seepage or mechanisms for pressure generation requires this geoscience background. Having the ability to converse technically with design engineers is of course also a great advantage.”