A likely result of changes in rock stresses due to progressing mining is an increased number of compressive stress–induced failures. This paper presents the results from numerical analysis and observations of stress-induced fallouts infootwall drifts in the Kiirunavaara underground mine. A brittle-plastic cohesion-softening friction-hardening (CSFH) material model was used for simulating brittle fallouts. To account for mining-induced stress changes, the local model stress boundary conditions were extracted from a global model. The rock mass properties were based on field observations in the footwall drifts as well as on results from laboratory testing. A multi-stage analysis was carried out to gradually change the stresses to simulate mining progress. A parametric study was conducted in which strength properties, location, and shape of the footwall drift were varied. Yielded elements and maximum shear strain were used as damage and fallout indicators. The modelling results were sensitive to the shape of the drift. The location of the predicted fallouts was in good agreement with the location of observed fallouts for the case in which the drift roof was simulated flatter than the theoretical cross section. The results indicate that the true shape of the drift is different from the planned one.

Knowledge of the rock mass strength is important for the design of all types of underground excavations. A frequently applied approach for estimation of the rock mass strength is through an empirical failure criterion, often in conjunction with rock mass classification/characterisation systems. This paper presents a review of existing methods to estimate the rock mass strength using empirical failure criteria and classification/characterisation systems—in this study, commonly denoted as estimation methods. A literature review of existing methods is presented, after which a set of methods were selected for further studies. The selected methods were used in three case studies, to investigate their robustness and quantitatively compare the advantages and disadvantages of each method. A Round Robin test was used in two of the cases. The case studies revealed that the N, Yudhbir-RMR76, RMi, Q-, and Hoek–Brown-GSI methods, appeared to yield a reasonable agreement with the measured strengths. These methods are thus considered the best candidates for realistic strength estimation, provided that care is taken when choosing values for each of the included parameters in each method. This study has also clearly shown the limits of presently available strength estimation methods for rock masses and further work is required to develop more precise, practical, and easy-to-use methods for determining the rock mass strength. This should be based on the mechanical behaviour and characteristics of the rock mass, which implies that parameters that consider the strength of intact rock, block size and shape, joint strength, and physical scale, are required.

Spaceborne radar interferometry is an established and very powerful method of measuring land subsidence over timescales of weeks to years. It has been demonstrated on natural and urban landscapes and is becoming an operational technique with accuracy better than 1 cm yr-1. The technique generally relies on having scatterers (which reflect the radar signal) which have stable properties over the timescale of interest. In some landscapes these scatterers occur naturally. However, at high latitudes there are particular difficulties because of the strong seasonal variation in the landcover – snow cover in particular can vary dramatically over periods of weeks to months – and the satellite orbits have particular features which are not generally significant at lower latitudes. It is unlikely that natural scatterers will be stable over long periods in these areas. The aim of this project is to develop methods of SAR interferometry suitable for use in landscapes with seasonal snow cover. The project has two themes: (1) improved understanding of SAR imaging at high latitudes and the use of interferometry in such landscapes, and (2) the development of artificial radar targets which can provide the necessary stability for long-term surface deformation monitoring. The sponsor runs a large mining operation at high latitudes. The study has a practical focus and is part of a larger project to provide a mining subsidence monitoring service for the sponsor. The project’s aim is to achieve accurate monitoring of subsidence using radar interferometry at high latitudes. Techniques developed should be suitable for operational use. Test site The experiment test site is centred on the town of Kiruna in northern Sweden (67° 51’ N, 20° 13’ E). The ground is covered with snow (to a depth of 1 m or more) from October to May each year, and so the winter and summer periods have very different land cover properties. The site includes the town of Kiruna, a large mine, and areas of natural sparse forest with mainly birch and some coniferous trees up to several metres tall. Datasets Available Several datasets are available to support the research, these include: • GPS measurements from a network of control points • General weather observations • Mapping data for land cover / land use and topography • SAR images at approximately monthly intervals from 2009 (Radarsat-2, using up to 3 satellite tracks for imaging) • A network of corner reflectors across the test area The core data analysis tool is the MDA software package designed for processing Radarsat images (for both backscatter and interferometric products). The key resource for the research is the time series of high resolution SAR images suitable for interferometric processing, which allows seasonal changes in backscatter to be observed directly. Methodology The project requires a mix of simulation and practical design and fieldwork. The main contributions will be in the area of target specification, design and validation. Achievements to date The project started late in 2009 and the main emphasis so far has been to establish the datasets needed for the research. Radarsat-2 images have been acquired and are being processed to derive interferometric products and to register them to standard projections compatible with each other and ancillary data. Field observations complementing the satellite imaging are also being made. Acknowledgements The project is sponsored in full by the LKAB mining company.The project also benefits from technical advice of MDA.

The objective of this work was to (1) increase the knowledge of the behavior of large scale rock slopes, (2) improve the ability to estimate rock mass strength for such slopes, and (3) develop an improved design methodology for forward design of large scale slopes in open pit mining. A fourth objective included the application of the results to the Aitik open pit in northern Sweden. A large number of case studies were collected into a database and empirical design guidelines were developed. A procedure for determining the strength of large scale slopes was developed, based on the Hoek-Brown failure criterion. Failure mechanisms of high slopes were studied using numerical modeling. The procedure for strength estimation and the modeling methodology, were applied to selected case studies-this enabled verification of the design methodology. Finally, the design methodology was applied to the Aitik open pit mine, and slope design recommendations given.

Design of open pit slope angles is becoming more and more important as the mining depths of open pits continuously increase. Small changes in the overall pit slope angle have large consequences on the overall economy of the mining operation. A case in particular is the Aitik open pit mine in northern Sweden, which currently faces the design of the overall slope angles for continued mining toward a depth of around 500 meters. This report constitutes the first phase in a research project aimed at developing design methods for large scale pit slopes. In this report, the stability and design of large scale pit slopes in open pit mining is reviewed, with special reference to slopes in hard, jointed, rocks, similar to the rock types found at the Aitik mine. The review covers the mechanics of pit slopes, existing design methods for large scale slopes, remedial measures and mining strategy to cope with slope failures, and a compilation of case studies from open pits worldwide. Finally, suggestions for future research in this area are presented. The factors governing large scale slope stability are primarily: (1) the stress conditions in the pit slopes, including the effects of groundwater, (2) the geological structure, in particular the presence of large scale features, (3) the pit geometry, and (4) the rock mass strength. Observed failure modes in rock slopes are of a wide variety. On a bench scale, structurally controlled failures such as plane shear and wedge failures are common. However, as the scale increases, simple structurally controlled failures are less dominate, and more complex failures such as step-path failures start to develop. From observations, it appears that for large scale slopes, two failure modes are especially important to consider. These are (1) rotational shear failure, and (2) large scale toppling failure. Rotational shear failure in a large scale slope involves failure both along pre-existing discontinuities and through intact rock bridges, but where the overall failure surface follows a curved path. Large scale (or deep seated) toppling failures have been observed in several large scale natural slopes and high open pit slopes. The mechanisms behind large scale failures are, however, not well known, in particular for hard, strong rocks. Criteria for the shape and location of the failure surface are lacking, as is detailed knowledge regarding failure through intact rock versus failure along discontinuities. Knowledge of the kinetic behavior of failing rock slopes is mostly empirical and requires more studies, in particular for hard and brittle rock masses in which rapid failures can be expected. This review has shown that the strength of a large scale rock mass is very difficult to assess. At the same time, the required accuracy for the strength parameters which are needed for the design is very high. For large scale rock masses, back-analysis of previous failures proves to be the only practical means of obtaining relevant strength parameters. However, the interpretation and translation of such data from one geological environment to another, is very cumbersome and lined with problems. Design methods for rock slopes can divided into mainly four categories, namely: (1) limit equilibrium methods, (2) numerical modeling, (3) empirical methods, and (4) probabilistic methods. The advantages and disadvantages of each of these methods are discussed in the review. For the design of large scale slopes, it appears that the choice of design method is less important than the choice of input parameters to the design, in particular the rock mass strength parameters. Remedial measures for controlling the stability of slope include support and drainage. While support can work for small scale slopes, only drainage is feasible for increasing the stability of large scale slopes. Monitoring of displacements, preferably using survey networks, should be carried out routinely in all open pit mining. Provided that the failure is slow and stable, it is also possible to continue to mine a failing slope. This requires that contingency plans are being made at an early stage in the mine planning process. From the collection of a number of case studies from North and South America, Africa, Asia and Europe, several examples of large scale failures were found, although mostly occurring in weak rocks. There are much fewer examples of slope failures in hard, brittle rocks. The few cases found indicate that failures in this type of rock is more uncontrollable. A compilation of slope height, slope angles, rock strength, and stability conditions for the studied cases concludes this chapter. Future research in the field of large scale slope design must be focused on quantifying the mechanisms for large scale slope failures. Once the mechanisms are better known, design methods based on the actual slope mechanics can be employed. Also, better and more reliable methods for determining the strength of large scale rock masses are important to develop.

The Citybanan commuter train tunnel will comprise a 6 km long, double-track train tunnel to be constructed in rock under Stockholm City. In this paper, analysis of the intersections between the planned train tunnel and so-called "energy tunnels" is described. The objective of this analysis was twofold: (i) to predict deformations and stability of the energy tunnels caused by excavation of the train tunnel, and (ii) to determine required rock reinforcement for the train tunnel. Analysis was conducted primarily using the three-dimensional finite difference program FLAC3[). The analysis results showed that only minor deformations can be expected in the energy tunnels due to the excavation of the train tunnel, and that the overall stability of the tunnels was satisfactory. Furthermore, the results could be used to confirm a proposed rock reinforcement scheme for the train tunnels, thus securing the stability of the load-bearing structure for the rock tunnel.

With increasing mining depths and excavation volumes comes not only increased rock stresses and more difficult underground mining conditions, but also increased surface effects, in particular from cave mining. The surface effects of deep sublevel cave mining are not well understood and are further explored in this paper, through a case study of the LKAB Kiirunavaara Mine. Two different numerical modelling approaches were used to quantify potential surface effects. The first approach was applied to Sjömalmen (Lake Orebody). This is a non-daylighting portion in the northern end of the mineralisation, above which surface cratering has developed. Three-dimensional (3D) numerical modelling, using the Itasca caving algorithm, was applied to study future mining of Sjömalmen down to Level 1365 m. In the second approach, 2D modelling of the main portion of the Kiirunavaara orebody was conducted, using a caving simulation scheme initially developed at the Luleå University of Technology. This model enabled simulating caving to large depths, in this particular case down to Level 1800 m, for prediction on hangingwall deformations. The actual caving is simulated implicitly in these continuum models. Observational data on cave development and surface cratering, as well as measured ground surface deformations, were used to calibrate the numerical models. For both approaches, deeper mining was shown to significantly affect the ground surface. Ground deformations are not arrested by bulking and/or increased confinement as mining goes deeper. Both modelling approaches have distinct pros and cons. The 2D approach is only applicable to the main portion of the orebody, where 2D geometrical conditions can be reasonably assumed, but calculation times are faster compared to the 3D approach. The models were fairly sensitive to the geomechanical properties and choice of constitutive model. This facilitated calibration, but also implies that an improved characterisation of the rock mass in the cap rock and hangingwall is important for increased reliability in predictive analyses.

Using blasting caps with electronic delay units, it has become possible to employ wave superposition in rock blasting. This paper presents computer simulations to investigate the hypothesis that fragmentation is improved in areas between blast holes where the tensile waves meet, overlap and interact. In this study, a numerical methodology using the code LS-DYNA was developed. LS-DYNA is a commercially available multi-purpose finite-element code, which is well suited to various types of dynamic modeling. Two different element formulations were used — Euler formulation in, and close to, the blast hole, and Lagrange formulation in the rock volume farther from the blast hole. The models used have a resolution (element size) of 50 mm and comprise approximately 20 million elements. Single and dual blast hole configurations have been studied, and a methodology to calculate possible fragmentation based on model interpretation was developed. The results showed that the amount of explosives and the blast hole spacing had the largest effect on fragmentation. The effect of varying delay times was small and local, implying that a significant increase in fragmentation should not be expected through wave superposition.

The LKAB Malmberget Mine is mined using sublevel caving. This mining method is cost-effective but results in successive caving of the host rock and mining-induced ground deformations. Consequently, re- locations of residential areas have been in progress in Malmberget ever since iron ore extraction on industrial scale commenced about a century ago. This study seeks to increase the understanding of the intrinsic characteristics of the rock mass governing deformation and caving activities. Rock mass characterizations were done in two selected orebodies — Printzsköld and Fabian. Two drill holes were drilled in each orebody from the surface. Geotechnical core logging was performed using the RMR system. Weakness zones were categorized to determine what role they played in the caving process. Point load testing was conducted for a sampling interval of about 5 m and selected uniaxial compressive strength tests were conducted to calibrate the point load index. Tunnel mapping was conducted in the hangingwall of the Printzsköld orebody. The finite element modeling code Phase2 was used for a sensitivity analysis of rock strength parameters and tostudy factors that may influence initiation of caving of the hangingwall.

As with many other mining environments, the frequency of ground falls at Luossavaara-Kiirunavaara AB’s Kiirunavaara Mine has increased with the progression of mining depth. These instabilities, which are unevenly distributed throughout the rock mass, have failure modes primarily including spalling, strainbursting, structurally controlled failure, and combinations thereof. Although caused in part by the mine-wide stress redistribution and geomechanical features of the rock mass, the exact manner in which these factors control the spatial distribution and characteristics of the ground falls not well understood. The objective of this paper is to describe the development of a geomechanical basis for how and why the distribution and characteristics of the ground falls differ throughout the rock mass. Spatial and temporal characteristics of ground falls at the mine-scale were analysed using two main forms of data: 1) a database of ground fall events, and 2) laser imaging data. A methodology was developed specifically for the use of three-dimensional laser imaging data for mine-scale analysis of overbreak and falls of ground. In conjunction with geomechanical characterisation of the rock mass, these results can be used to assist with: identification of areas with higher risk of instabilities, production planning from an induced stress management perspective, location-based support system design in advance of drifting, evaluating the performance of drift development practice in different geomechanical conditions, and data collection and usage recommendations.

An understanding of the relationship between the geological environment and rock mass behaviour induced by mining activities can lead to hazard reduction through knowledge-based design. However, characterisation of complex and heterogeneous rock masses that typify mining environments is difficult. A methodology to characterise these types of rock masses, based largely on classical statistics, geostatistics and an extension of previous quantitative structural domaining work, is presented and applied to the Kiirunavaara Mine, Sweden. In addition to a new perspective on intact rock strengths of geological units at the mine, a correlation was found between modelled volumes of clay, modelled RQD, newly identified structural domains and falls of ground. These relationships enabled development of a conceptual model of the role of geology in rock mass behaviour at the mine. The results demonstrate that the proposed methodology can be useful in characterisation of complex rock masses.

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.

Mining induced seismicity and rockbursting significantly increased in the LKAB Kiirunavaara Mine when mining production progressed beyond 700 m depth. Since 2008, significant work has been done at LKAB to better understand their induced seismicity. It has been identified that the majority of seismic events in the mine are likely caused by the interaction of mining excavations and structural geology. Two complimentary PhD projects (funded by LKAB) are underway at Luleå University of Technology to address the cause of the seismicity experienced at the mine, with one concentrating on mine seismology and one on rock mechanics.The rock mechanics project, the focus of this paper, concentrates on quantifying relationships between mining sequences, geomechanical and geological conditions, stress changes and induced seismicity at the mine. A series of numerical models will be developed based on an extensive data acquisition campaign to examine the interaction between the mining and geological systems. The role of structural geology in mine behaviour and its application to mine planning is of particular focus within these models. This paper presents the methodology of the rock mechanics project, including: data acquisition, data analysis, and numerical stress analysis models and modelling techniques

With the application of electronic detonators and with short delay times, it may be possible to achieve improved fragmentation through stress wave superposition. This hypothesis was studied through a series of small scale laboratory tests. The results from these tests have subsequently been modeled using the numerical FEM code LS-DYNA and the RHT (Riedel-Hiermaier-Thoma) material model, applying a newly developed methodology for three-dimensional computer simulation of blasting. This work also involved simulating initial damage to the rock through previous blasting, and analyzing the resulting effects. The effect of different delay times showed that through a properly chosen delay time, improved fragmentation could be inferred. Moreover, the initial damage (from the previous row) clearly affected the fragmentation; however, the results indicated that longer delay times (in which the stress wave would have passed the boreholes) also resulted in improved fragmentation, implying that stress wave superposition may not be the primary factor governing fragmentation