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Mine-scale numerical modelling, seismicity and stresses at Kiirunavaara Mine, Sweden
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Mining and Geotechnical Engineering.
Queen's University.
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Mining and Geotechnical Engineering.
2014 (English)In: Deep mining 2014: proceedings of the seventh international conference on deep and high stress mining :16-18 september 2014, Sudbury, Ontario, Canada / [ed] Marty Hudyma; Yves Potvin, Nedlands, WA: Australian Centre for Geomechanics, 2014, 363-376 p.Conference paper, Published paper (Refereed)
Abstract [en]

LKAB’s Kiirunavaara Mine, located in northern Sweden, has exhibited seismic behaviour since the mining production extended below 700 m depth. Iron ore is mined from the 4.5 km long orebody via sublevel caving at a production rate of 28 million tonnes per annum. The deepest current production level is at approximately 800 m depth, and current mining plans call for mining to about 1200 m depth. It is thus of critical importance for LKAB to gain a deeper understanding of the stress and rock mass behaviour at the mine.The Kiirunavaara orebody has complex geometry and geology, which is represented using the discontinuum distinct element code 3DEC. As part of a larger series of models investigating the influence of strength and structural geology on rock mass behaviour, the results of multiple continuum models are presented. The goals of these continuum models included: i) obtain a better understanding of the virgin stress field and redistribution of stresses caused by mining, ii) further define the extent of mining induced plastic failure, and iii) increase the understanding of existing failure mechanisms at the mine.The elastic and plastic continuum models accurately produced principal stresses similar to measurements recently conducted at two sites in the mine, confirming the previously estimated virgin stress state. Spatial correlations between plastic failure in the model and seismicity in the hangingwall and footwall were found. However, these correlations were not consistent throughout either material for any evaluated set of material properties; either the plastic failure in the footwall or hangingwall corresponded well with seismicity. This may be because a set of rock mass properties which represent rock mass failure at this scale have not been evaluated or that some underlying failure mechanisms causing seismicity are not represented in the models, for example, failure along discontinuities. Some events larger than moment magnitude of 1.2 in the hangingwall, in particular shear source mechanisms events, do not correspond well with plastic failure from the model. These results potentially indicate that geological structures, which are not represented in these models, influence mine behaviour.The improved understanding of input data, rock mass behaviour, and failure mechanisms as a result of these models has a direct impact upon mine excavation design and future rock behaviour investigations, and will be used in the continued research, as well as in mine planning.

Place, publisher, year, edition, pages
Nedlands, WA: Australian Centre for Geomechanics, 2014. 363-376 p.
National Category
Other Civil Engineering
Research subject
Mining and Rock Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-37263Local ID: b3cb328a-bb40-4dd9-9dc6-0990aefd0973ISBN: 9780987093790 (print)OAI: oai:DiVA.org:ltu-37263DiVA: diva2:1010761
Conference
International Conference on Deep and High Stress Mining : 16/09/2014 - 18/09/2014
Note
Godkänd; 2014; 20150612 (jesvat)Available from: 2016-10-03 Created: 2016-10-03 Last updated: 2017-09-11Bibliographically approved
In thesis
1. Listening to the story of the rock mass: The integration of conventional and unconventional data to understand rock mass behaviour at the Kiirunavaara Mine
Open this publication in new window or tab >>Listening to the story of the rock mass: The integration of conventional and unconventional data to understand rock mass behaviour at the Kiirunavaara Mine
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The need to understand rock mass behaviour has never been greater; as mining globally progresses deeper to extract the precious resources humankind needs to maintain and forward our lives, we encounter rock behaviours associated with deep mining. These environments typically experience seismicity (including fault slip), rock bursting, strain bursting, spalling, aseismic movement along pre-existing discontinuities, and ground falls. These behaviours can have significant consequences to an operation’s safety and profitability. An understanding of the rock mass enables risk mitigating design.

However, a number of hindrances exist along the path to gaining this understanding. Limited information in the literature is available on which data to collect, the most appropriate analysis techniques, and how one combines varied data sources into a mine-scale understanding of the rock mass. In essence, we do not yet understand how to listen to the story of the rock mass. This thesis explores these research topics and develops and extends techniques and methodologies to address these issues. Data from Luossavaara-Kiirunavaara AB’s (LKAB) Kiirunavaara Mine was used, however a strong focus was given to developing general methodologies that can be applied to many mining environments.

The typical situation at most mine sites is one where many forms of data exist, a subset of which is conventional rock mechanics data. These data typically include, for example, laboratory testing for strength and stiffness of geological units, rock mass characterization from underground mapping, geological and geomechanical core logging, and positions and orientations of mapped discontinuities from underground mapping and/or oriented core logging. These data are often used for empirical design. It is common with particularly damaging seismic and/or ground fall events that additional data be acquired from underground damage mapping, but this information is rarely used on a larger scale. However, there is a subset of data that tends to be overlooked from a geomechanical perspective, even though the potential exists that this data can give us information about the characteristics of the rock mass. In the case of the Kiirunavaara Mine, this includes seismic data (including tomographic velocity structure), a database of information regarding ground falls, and laser imaging data.

A variety of conventional and unconventional data from the Kiirunavaara Mine were analysed using existing, and extensions of existing, analysis techniques. A 3-D geomechanical model was developed, incorporating geological and geomechanical core logging data, underground mapping data, and laboratory testing data. The resulting mine-scale model showed some particularly interesting results, including 1) a very large variation in intact rock strength within each statistical grouping of geological units, combined with much overlap between the strengths of geological units, and 2) a 3-D model of volumes of clay alteration built on geological core logging data calibrated favourably to data of underground mapping of clay. Unconventional data sources were also analysed, in particular the use of behavioural data to infer characteristics of the rock mass. Spatial and temporal patterns of ground falls were evaluated, alongside spatial patterns in overbreak, which were identified using new, mine-scale analysis techniques of laser scanning data.

Interestingly, correlations were apparent when comparing the analyses of distinct and separate data sources. Ground falls were concentrated in the intact rock between the clay volumes. The rock quality designation (RQD) had lower values in the same volume. Seismicity in the area was concentrated in the intact rock, with very few events in the clay volumes. 3-D velocity tomography models (developed by another researcher within this project) showed 1) no correlation with the RQD model (this result may highlight a limitation of scaling RQD data beyond it’s intended use), and 2) possible correlation with the clay volumes.

A methodology was developed using numerical stress analysis to identify, at an early stage of investigation, if a volumetric feature is of significance to seismic rock mass behaviour. With so much surmounting evidence that the clay volumes influence rock mass behaviour at the Kiirunavaara Mine, this feature was well-suited to be tested. Results showed that the clay volumes: 1) influence the mine-scale stress field, 2) have the potential to extend the volume of the rock mass which is expected to experience crack initiation (one underlying cause of seismicity), and 3) result in a stress field that enables slip along many of the orientations of mapped discontinuities (another underlying cause of seismicity).

Particularly valuable data were found to share the following characteristics: 3-D coverage, mine-scale, long-term, and multiple and distinct sources. It was important that the different data sources and their analyses led to the same conclusions even though they were separate and distinct. Unconventional data sources that were of particular use in this case include: spatial and temporal ground fall patterns and spatial patterns of seismicity. With further research, other unconventional data sources may show their value to rock mechanics analyses, such as spatial and temporal patterns of: seismicity, focal plane solutions from moment tensors, seismic stress inversion from moment tensors, overbreak, and possibly even tomography (the lack of strong correlation in this case may be specific to this environment and models used). Opening our minds to the possibilities, collaboration with experts in other fields, and a little creativity in our approach will help us in our quest to understand the stories of rock masses.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2017
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Other Civil Engineering
Research subject
Mining and Rock Engineering
Identifiers
urn:nbn:se:ltu:diva-65438 (URN)978-91-7583-959-2 (ISBN)978-91-7583-960-8 (ISBN)
Public defence
2017-11-06, A1547, Luleå Tekniska Universitet, Luleå, 10:00 (English)
Opponent
Supervisors
Available from: 2017-09-11 Created: 2017-09-11 Last updated: 2017-10-16Bibliographically approved

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