Dr. Sanjay Rana, Director, PARSAN Overseas Pvt. Ltd.
New Delhi, India, sanjay@parsan.biz
Abstract
Tailings dams, critical infrastructures in the mining industry, play a pivotal role in the containment of waste by-products. Their significance is underscored by the potential environmental and societal impacts they can have if not properly managed. This paper delves into the application of geophysical methods throughout the life cycle of a tailings dam project, emphasizing their importance from site selection to sustainable management.
At the site selection phase, geophysical methods offer invaluable insights into site characterization, enabling the identification of optimal locations while highlighting potential geological hazards. Techniques such as Seismic Refraction Tomography, MASW and Electrical Resistivity Imaging provide a comprehensive understanding of subsurface conditions, ensuring that the chosen site meets the safety and operational requirements of a tailings dam.
During the construction phase, the real-time application of methods like MASW (Multichannel Analysis of Surface Waves) and Seismic Refraction Tomography ensures the structural integrity of the dam. These techniques aid in monitoring construction processes, identifying anomalies, and addressing challenges promptly, thereby mitigating risks associated with dam failures.
Operational safety is paramount once the dam is functional. Continuous geophysical monitoring, using methods like Electrical Resistivity Imaging, Streaming Potential and MASW, safeguards against potential risks, ensuring the safety of both the surrounding environment and communities. These methods provide early warning of any structural weaknesses or potential breaches, allowing for timely interventions.
Lastly, from a sustainability standpoint, the long-term management of tailings dams necessitates consistent monitoring and assessment. Geophysical methods play a crucial role in environmental impact assessments, ensuring that the dam’s operations remain environmentally friendly and sustainable in the long run.
Incorporating real-world case studies, this paper underscores the transformative impact of geophysical methods in tailings dam projects. By offering a holistic view of their application across different phases, it emphasizes the need for their widespread adoption in the industry. The future of tailings dam projects hinges on the integration of these advanced techniques, ensuring safety, sustainability, and operational excellence.
Introduction
Tailings dams, often referred to as tailings storage facilities, are engineered structures designed to store the by-products of mining operations. These by-products, known as tailings, consist of a mixture of water, chemicals, and finely ground rock. The safe and efficient management of these tailings is of paramount importance, not only for the mining industry but also for the environment and surrounding communities. The failure of a tailings dam can result in catastrophic consequences, including loss of life, environmental degradation, and significant financial implications.
1.1 Background on tailing dams and their significance
The mining industry has witnessed a surge in the demand for minerals and metals, driven by technological advancements and global economic growth. This increased demand has led to the extraction of vast quantities of ore, resulting in the generation of significant volumes of tailings. Tailings dams are constructed to contain these tailings, preventing them from entering the environment and causing pollution. These dams are often massive in size and can contain millions of cubic meters of waste material. Their significance lies in their ability to:
* Safeguard the environment by preventing the release of harmful chemicals and sediments into water bodies.
* Reclaim water from the tailings for reuse in the mining process, promoting water conservation.
* Provide a long-term storage solution for mining waste, ensuring the safety of surrounding ecosystems and communities.
1.2 The role of geophysical methods in tailings dam projects
Geophysical methods offer a non-invasive approach to understanding the subsurface conditions of a site. These methods have found extensive applications in tailing dam projects, aiding in various phases from site selection to operational monitoring. The primary advantages of employing geophysical methods include:
* Providing detailed subsurface imaging, enabling the identification of geological features and potential hazards.
* Offering real-time monitoring capabilities during the construction and operational phases of the dam, ensuring its structural integrity.
* Assisting in the long-term sustainable management of the dam by identifying areas of seepage, potential breaches, and other anomalies.
In the context of tailings dams, the application of geophysical methods is not just a technical necessity but also a commitment to environmental stewardship and community safety.
2. GEOPHYSICAL METHODS OVERVIEW
Geophysical methods are a collection of scientific techniques used to study the Earth’s subsurface. In the context of tailings dams, these methods provide invaluable insights into the geological and geotechnical properties of the site, ensuring the safety and efficiency of the dam throughout its lifecycle. The choice of a particular geophysical method depends on the specific requirements of the project, the geological conditions of the site, and the depth of investigation required.
2.1 Seismic Refraction Tomography
Seismic Refraction Tomography (SRT) is a geophysical method that measures the time it takes for seismic waves to travel through the subsurface. By analysing the refracted waves, this method provides a detailed image of the subsurface layers, including their thickness and P-wave velocity distribution.
Applications: SRT is particularly useful in identifying bedrock depth, determining soil layer thickness, and detecting voids or anomalies in the subsurface.
Advantages:Offers high-resolution imaging and can cover large areas, making it ideal for tailings dam site investigations.
2.2 MASW (Multichannel Analysis of Surface Waves)
MASW is a seismic method that analyses surface waves to determine the shear wave velocity of the subsurface. This method is effective in mapping variations in soil stiffness and identifying soft zones that might be prone to liquefaction.
Applications : Used extensively in geotechnical investigations to determine soil properties and assess ground stability.
Advantages: Non-invasive, cost-effective, and provides rapid data acquisition.
2.3 Seismic Tomography
Seismic Tomography is a technique that uses the travel times of seismic waves to create a 3D image of the subsurface. It’s particularly effective in mapping complex geological structures.
Applications: Identifying fault zones, cavities, and variations in rock quality.
Advantages: Provides detailed 3D imaging and can penetrate deeper than most other methods.
2.4 Electrical Resistivity Imaging
This method measures the electrical resistivity of the subsurface to create an image of its geological layers. Different materials have distinct resistivity values, allowing for clear differentiation between them.
Applications :Detecting groundwater, mapping soil types, and identifying areas of seepage in tailings dams.
Advantages: Versatile and can be used in various terrains and conditions.
2.5 Streaming Potential
Streaming Potential measures the voltage generated when a fluid flows through porous materials. In tailings dams, it can be used to detect fluid movement, which can be an indicator of potential seepage or breaches.
Applications :Monitoring seepage paths, detecting leakages, and assessing dam integrity.
Advantages: Real-time monitoring capability and early detection of potential issues.
2.6 Additional Methods
While the above methods are commonly used in tailings dam projects, the field of geophysics is vast, and there are several other techniques that might be relevant based on specific project requirements. These could include Ground Penetrating Radar (GPR), Magnetometry, Micro-gravity, and Electromagnetic methods, among others.
3. IMPORTANCE OF GEOPHYSICAL METHODS IN TAILINGS DAM PROJECTS
The application of geophysical methods in tailings dam projects is not just a matter of technical necessity but is integral to the safety, efficiency, and sustainability of these structures. These methods provide a comprehensive understanding of the subsurface conditions, ensuring informed decision-making at every phase of the dam’s lifecycle.
3.1 Site Selection Phase
The initial phase of any tailings dam project involves selecting an appropriate site. The choice of location is crucial, as it determines the dam’s safety, operational efficiency, and environmental impact.
Site Characterization and Selection: Geophysical methods provide detailed insights into the geological and geotechnical properties of potential sites. Techniques like Seismic Refraction Tomography and Electrical Resistivity Imaging help in mapping the subsurface layers, identifying bedrock depths, groundwater presence, and potential fault zones.
Identifying Potential Hazards and Geological Features: Before construction, it’s essential to be aware of any geological hazards that might affect the dam’s stability. Seismic Tomography and MASW can detect fault lines, cavities, and zones of weak soil, ensuring that these areas are addressed during the design and construction phases.
3.2 Construction Phase
Once a site is selected, the construction phase begins. Here, the focus shifts from site characterization to monitoring the dam’s construction and ensuring its structural integrity.
Monitoring and Ensuring Structural Integrity: As the dam takes shape, geophysical methods play a pivotal role in monitoring its construction. Streaming Potential can detect fluid movement, indicating potential seepage paths, while Seismic Tomography can identify any structural anomalies that might compromise the dam’s integrity.
Identifying and Addressing Construction Challenges: Construction often comes with unforeseen challenges. Soft zones prone to liquefaction, detected using MASW, might require soil stabilization. Similarly, unexpected groundwater detected through Electrical Resistivity Imaging might necessitate changes in the dam’s design or construction methods.
3.3 Operational Safety
With the dam constructed, the emphasis is on its safe operation. Continuous monitoring is essential to detect and address any potential risks promptly.
Continuous Monitoring for Potential Risks: Geophysical methods offer real-time monitoring capabilities. Streaming Potential, for instance, can provide early warnings of seepage or breaches, allowing for timely interventions.
Ensuring Safety of Surrounding Environment and Communities: The dam’s impact extends beyond its boundaries. Methods like Electrical Resistivity Imaging can monitor groundwater movement, ensuring that surrounding water sources remain uncontaminated. Similarly, Seismic Tomography can detect any ground movement, ensuring the safety of nearby structures and communities.
3.4 Sustainable Management
Beyond operational safety, the long-term sustainability of the dam is of paramount importance. This involves its environmental impact, as well as its ability to function efficiently over the years.
Long-term Monitoring and Maintenance: The dam’s lifecycle extends for several decades, necessitating continuous monitoring. Geophysical methods can detect changes in the subsurface conditions, ensuring that any potential issues are addressed before they escalate.
Environmental Impact Assessment and Mitigation: Tailings dams have a significant environmental footprint. Geophysical methods can assess the dam’s impact on the surrounding environment, from groundwater contamination to soil erosion, ensuring that mitigation measures are in place.
CASE STUDIES
Real-world applications of geophysical methods in tailings dam projects provide invaluable insights into their practicality, challenges, and outcomes. This section delves into specific case studies where these methods were employed, highlighting their transformative impact and the lessons learned.
4.1 Case Study-1
In a mining area, filling of slime was commenced in one of the deepest pits. Initially it was noticed that some of water was seeping from the bed of mine pit. After almost a year a major sinkhole/crater got created in western part corner of pit and a vast amount of slime (about 60000 to 70000 m3) went underground. The oozing or coming out from the surface was not noticed anywhere in the surrounding area. This phenomenon therefore mandated an investigation to understand as to where the huge amount of water and slime had gone or where it had accumulated inside the subsurface.
Fig-1: Major sinkhole/crater created in western part corner of pit.
The geophysical investigations comprising hydrogeological study, Induced Polarization and Electrical Resistivity Imaging (ERI) were conducted which successfully achieved the objectives and provided an in-depth understanding on the phenomenon.
Fig-2: Layout of various lines used for electrical resistivity & Induced Polarization
The ERI profiles demonstrated various localized low resistivity zones in and around the slime dam, which were clearly marked in various sections. Good correlation of geophysical anomaly and geological features was also observed.
Fig-3: Example-1 of resistivity anomaly associated with anomalous zones.
Fig-4: Example-2 of resistivity anomaly associated with anomalous zones.
The results of this case study are summarised as follows:
Surrounding the sick slime pit, various geological features existed on the surface terrain revealing the presence of several feeble zones in the form of weathered surfaces, differential contact planes, joint planes, fissures, fractures, faults and folds etc. These structures may have largely induced the secondary porosity in and around the slime pit. Continuous blasting applied for mining of the ore since many years may also be a reason to enhance the opening of deep-seated fracture and subsequently increase the effective permeability. This phenomenon could have created a channel for possible leak of slime from the pit to deep seated fractures. It is obvious that more concentration of these geological tectonic structures in southwest of sick slime pit may be reason to create crater/sinkhole slime pit and drained the slimes and water in deeper fracture zone.
The permeability test of drilled boreholes, conducted during previous geotechnical investigations by other agencies for down to depths of 20 m, have been analysed and it was found that the permeability value increased towards N-W direction which revealed that weaker zones existed in this direction. This was also confirmed on ground survey.
Indication of higher range of value than permissible limit for colour, turbidity, Iron and total suspended solid parameters in piezometer during the time of leakage of slimes from the pit and also more Iron concentration afterward may be indicative for percolation of slimes water towards northwest direction of slime pit because no significant change had been noticed in other piezometers.
Electrical resistivity tomograms of the profiles have indicated remarkably different geo-stratigraphic units of various geoelectrical layers. The hard and compact rock formation has been characterized with very high resistivity vales whereas the saturated rocks have been characterized with low resistivity values. Results of resistivity imaging conducted surrounding in-pit slime area have demonstrated the presence of various subsurface high and -low resistivity anomalous zones at varying depths in investigated depth range of 160 m bgl in the area.
The significant observation of ERT results indicated the occurrence of saturated zones at 570 m above MSL over many ERT profiles located in northwestern part of the area. An inference can be drawn that these zones may have provided the seepage path to slimes and water from the inpit slime dam or seeped slimes, and water may be accumulated in these zones.
4.2 Case Study-2
Seepage was suspected in a tailings dam located in the state of Jharkhand, India. The geophysical investigations were carried out to find out potential seepage paths (if any) in the earthen embankments (tailings dam). There are various techniques available for such inspections like, electrical imaging, streaming potential, Ground Penetrating Radar, seismic tomography, radar tomography, cross hole surveys etc. For this case following investigations were chosen:
* 2D-Electrical Imaging: To get a picture of internal resistivity distribution of the tailings dam, identifying areas of water saturation, if any.
* Streaming Potential: The investigations were carried out to identify the zones through which seepage, if any, is taking place. This is important to identify the seepage paths for any possible seepage not yet noticed by visual inspection/ observation.
ERI profile clearly detected the anomalous zones within the dam body.
Fig-5: Example of resistivity anomaly associated with anomalous zones (Blue Zones).
Correlation with streaming potential further enabled detection of low resistivity zones through which actual flow of water is taking place:
Fig-6: Example of streaming potential anomaly associated with flow zones (negative anomaly).
4.3 Case Study-3
This case study involved geophysical investigations (Electrical Resistivity Imaging, MASW & SP) along proposed locations at a tailings dam in Odisha, India, to detect seepage zones through the tailings dam body and reason for observed subsidence in certain areas.
A total of seven ERI profiles, seven MASW profiles and seven SP profiles were undertaken. ERI and SP examples have been discussed in previous case studies and hence only MASW example result is being presented here.
Fig-7: Example of MASW anomaly associated with weak zone (blue anomaly between 200-230m).
The study successfully detected the anomalous zones within the tailings dam, including the active flow zone, helping tailings dam owners plan the repair and rehabilitation works. Low shear wave velocity zones were detected at locations having visible subsidence, further confirming the findings.
5. CONCLUSION
The integration of geophysical methods in tailings dam projects has proven to be transformative, ensuring not only the structural integrity of these massive infrastructures but also their long-term sustainability and minimal environmental impact. As the mining industry continues to grow, the challenges associated with managing its by-products become even more pronounced. This section summarizes the key takeaways from the paper and offers a forward-looking perspective on the future of geophysical methods in tailings dam projects.
5.1 The Future of Geophysical Methods in Tailings Dam Projects
The rapid advancements in geophysical technology, combined with the increasing complexity of tailings dam projects, suggest a future where these methods become even more integral. Some potential future trends include:
Advanced Real-time Monitoring : With the advent of IoT (Internet of Things) and AI (Artificial Intelligence), real-time monitoring of tailings dams can become more sophisticated, offering predictive insights and early warnings.
Integration with Other Technologies: Combining geophysical methods with other technologies, such as drones or satellite imaging, can provide a more comprehensive overview of tailings dam sites, from the surface to the subsurface.
5.2 Recommendations for Best Practices
Based on the insights gathered from the paper, several best practices can be recommended for future tailings dam projects:
Early Integration: Geophysical methods should be integrated from the earliest phases of a project, ensuring informed decision-making from siting to construction
Continuous Training: As geophysical technologies evolve, continuous training for professionals in the field becomes crucial, ensuring that they are equipped with the latest knowledge and skills.
Community Engagement: Especially for tailings dams near residential areas, transparency and community engagement are essential. Sharing geophysical data and findings with the public can build trust and ensure a harmonious coexistence.
5.3 Final Thoughts
The application of geophysical methods in tailings dam projects is a testament to the mining industry’s commitment to safety, sustainability, and innovation. As challenges arise, these methods offer solutions, ensuring that the industry can continue to grow while minimizing its environmental footprint and ensuring the safety of all stakeholders involved.
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