ABSTRACT

Tailings are the fine residue of the milling process in the mining industry and appear in slurry form being mixed with water during this process. Large tailings ponds are required to contain them, usually confined by man-made dams. Such tailings facilities pose considerable risk both to the environment and human lives.

A major interdisciplinary research project ('TAILSAFE') supported by European Union has been initiated with the aim to increase attention towards and reduce the risk posed by tailings facilities. Methods of parameter evaluation and measurement are being developed within the project and applied for the detection, assessment and improvement of the safety state of tailings dams and ponds.

One of the workpackages has his focus on non-destructive geophysical investigation methods. Geoelectrical (SIP), seismic and radar methods will be used to get information on the tailings dam structure and water content.

1 Introduction

Tailings are fine-grained wastes of the mining industry. They appear in slurry form being mixed with water during mineral processing. Tailings facilities consist of tailings ponds or lagoons, tailings dams and tailings transport systems (usually pipelines). The large ponds are required to contain the tailings.

Normally, the dams, usually man-made, are the most critical elements of these facilities. They can present a serious threat, especially where there is improper handling and management [Ref 1]. Recent accidents at tailings facilities, such as the Baia Mare (Romania) and the Aznalcóllar (Spain) disasters, resulted in major threats to the environment and human life.

Most of the tailings dams are constructed from the solids coming from the milling process. This is achieved by using hydro cyclones at the end of the pipelines transporting the slurry from the mill. The coarser particles (sandy fraction) are used for dam construction, while the fines are deposited in the pond.

The traditional construction method for tailings dams has been the upstream method (Figure 1, a), providing only a thin shell with instability implications. The downstream method (Figure 1, b) yields better stability but requires considerably larger amounts of solids.

Fig 1: Tailings dam construction schemes: (a) upstream method, (b) downstream method.

Unlike the large dams of water reservoirs, tailings dams are constructed without any impervious core. Process and rainfall water seeps through the dam towards its free face. Uncontrolled water flow through, beneath or - in the worst case - over the dam can lead to a critical loss of stability. In handling large amounts of inhomogeneous wet slurries, water management is a key safety factor. Deficient water management is one of the main causes of accidents and hazards emanating from tailings facilities.

A major interdisciplinary research project ("Sustainable Improvement in Safety of Tailings Facilities", TAILSAFE) supported by the European Union has been initiated with the aim to increase attention towards and reduce the risk posed by tailings facilities. Methods of parameter evaluation and measurement are being developed within the project and applied for the detection, assessment and improvement of the safety state of tailings dams and ponds.

Aspects of probabilistic stability analysis, water management involving paste technology, non-destructive (NDT) and minimally intrusive testing methods and monitoring methods, and intervention and practical remediation options are being considered. The results will be incorporated in a systematic risk reduction framework. The investigations are focussed on the structural parameters of tailings dams stability, their measurement and their evaluation as regards risk factors.

10 research institutions from 6 European countries are partners in the project. A more detailed overview is given by Meggyes [Ref 2].

2 Investigation and Monitoring

Establishing a risk reduction scheme for an existing tailings facility can only be done by a detailed analysis of the state and structure of the dams. Beside review of existing data there are two main tasks:

• Investigation of the structure, water content and water flow paths inside the dam to get input data for reliable stability analysis
• Monitoring of the state of the dam and changes of water level in pond and dam as an aid for the facility management and the prevention of failures.

Most of the tailing dams are not as structured as the examples shown in Figure 1. In reality there are transition zones between the tailings and the dam. Height and thickness of the several parts of the dam vary in the horizontal and vertical direction. Even by the most detailed investigation possible this structure cannot be described exactly. Therefore it is necessary to develop probabilistic methods for geotechnical stability analysis. This will be done within the project and is expected to give more reliable results.

In the TAILSAFE project it is planned to do conventional soil sampling, drilling, chemical analysis and soil property determination on samples collected from drillings. As it is not possible to take more than just a few samples, the highly inhomogeneous structure of the tailings dams cannot be estimated with these methods only. Additionally, further techniques are needed to find representative sampling points, to get data from parts of the dams inaccessible by drill rigs and to interpolate between drillings. In the project geophysical methods will be used in order to fulfil these tasks.

3 Geophysical Methods

The following geophysical methods are going to be tested, improved or enhanced for the investigations in the TAILSAFE framework:

• SIP
• GPR
• Seismics

Geoelectrical techniques are measuring the electrical resistivity of the subsoil from the surface (Figure 2). Data sets with several hundred measurements from different points and with different penetration depths can be processed by computer software to produce sections or cuts through the object under investigation (Figure 3).

Fig 2: Principle of geoelectrics.

The resistivity depends on material, water content and other influences, so interpretation in terms of parameters needed for geotechnical analysis is sometimes difficult. The use of a more sophisticated technology like SIP, which delivers additional data, is expected to improve the results [Ref 3, Ref 4].

Fig 3: Geoelectrical cross-section trough tailing pond. Low resistivities correspond to fine tailings, high resistivities to cover (top), dams (left and right) and bedrock (bottom).

SIP and other geoelectrical methods have been reported to be valuable tools for investigating tailing dumps [Ref 5, Ref 6]. In theses cases the method was used to give a raw overview of the structure or the content of the dumps.

Figure 3 shows the result of geoelectrical measurements at a tailings pond site in Saxony (Germany). It was investigated by a geophysical contractor by order of the Federal Institute for Geosciences and Natural Resources of Germany (BGR). Electrodes were placed with 5 m distance on a 290 m line, some 200 single measurements were taken. After data inversion with commercial software RES2DINV the results are presented as a cross section through the pond. Black to dark grey colours represent low resistivities, light grey to white high resistivities. The dark zone in the centre represents the fine part of the tailings. It is covered by coarser material. The underlying crystalline rock (gneiss) is marked by a sharp resistivity increase. The right dam seams to be upstream type as it has low resistivities (fine tailings) beneath 5 to 10 m cover. In the TAILSAFE project the resolution and the interpretation methods will be improved.

GPR is a method for near surface investigations. A short electromagnetic pulse is sent in the earth by an antenna, the reflected signals are received by another one. GPR will be used in the TAILSAFE framework for evaluating the near surface structure of the dams and determination of the depth of the phreatic surface.

In addition to these methods seismics will be used at selected points. Seismic methods rely on the generation, propagation and detection of mechanical waves. Common sources are hammers, weight drops and others. The interpretation of the arrival times of reflected and refracted waves from subsurface strata will give additional information on structure and water table.

Data fusion of the results of these methods basing on different physical phenomena and correlation with the results of conventional investigations will lead to an improved, more detailed picture of the dams interior. A strong focus will be on the translation from geophysical properties like resistivity to parameters which can be used by the geotechnical engineers.

4 Field Experiments

Measurement and monitoring technologies adapted for use in tailings facilities will be tested in field trials at case study sites yet to be determined. There will be preliminary test in the laboratory and then at abandoned, well investigated tailing facilities to check measurement and interpretation methods.

At live test sites extensive trailing and refinement of the chosen measurement equipment is planned and the measured data and interpreted results will be incorporated into the TAILSAFE framework. Recommendations for stabilisation, remediation, technology changes and risk management for the test sites will be derived as practicable.

5 Conclusions

With an integrated approach concerning legislation and authorisation procedures, processing techniques, investigation and monitoring techniques and advanced geotechnical methods an improvement in the safety of tailing facilities will be reached in the next years via the TAILSAFE framework. Geophysical techniques for the investigation of tailing dams are expected to give valuable input for the geotechnical engineers. As a result the reliability of stability calculations will improve.

6 Acknowledgement

Figure 3 is used with kind permission of the Federal Institute for Geosciences and Natural Resources of Germany (BGR).

REFERENCES

1. ICOLD International Committee of Large Dams: Tailings Dams Risk of Dangerous Occurrences. Lessons Learnt from Practical Experiences. Bulletin Nr. 121, 144 p., Paris, 2001.
2. Meggyes, T.: Reducing the risks for tailing facilities. Proceedings of Consoil 2003, in press.
3. Niederleithinger, E., Grissemann, Ch., & Rammlmair, D.: SIP Geophysical Measurements on Slag Heaps: a New Way to Get Information About Subsurface Structures and Petrophysical Parameters. Proceedings of ICAM (International Conference on Applied Mineralogy), Göttingen, 2000.
4. Niederleithinger, E.: Spectral Induced Polarization - a tool for nondestructive testing of soils and building materials. Proceedings of NDT-CE 2003 (this volume), Berlin, 2003.
5. Buero für Geophysik Lorenz: Wiederholungsmessungen mit der Methode der Induzierten Polarisation zur Erkundung von Bergbau- und Schlackehalden. Report to the Federal Institute for Geosciences and Natural Resources (BGR), Hannover, 1999 (unpublished).
6. Campbell, D.L., and Fitterman, D.V.: Geoelectrical methods for investigating mine dumps, in: Proceedings from the Fifth International Conference on Acid Rock Drainage (ICARD2000), Denver, Colorado, May21-24, 2000. Society for Mining, Metallurgy, and Exploration, Inc., v. II, p.1513-1523.