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Predominant failure mechanisms at the Kiirunavaara mine footwall
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Mining and Geotechnical Engineering.ORCID iD: 0000-0002-4189-945X
2015 (English)Licentiate thesis, comprehensive summary (Other academic)Alternative title
Predominant failure mechanisms at the Kiirunavaara mine footwall (Swedish)
Abstract [en]

The Luossavaara-Kiirunavaara Aktiebolag (LKAB) Kiirunavaara mine is a large scale sub-level caving (SLC) mine in northern Sweden. The use of SLC as a mining method inherently causes significant rock mass movements above the extraction level. It has been one of the objectives of LKAB since the early 1990s to accurately forecast the global stability of the footwall in relation to the inherent rock mass movements from the sub-level caving. In the Kiirunavaara case, the dip of the main ore-body entails the footwall to develop as a rockslope confined by cave material from the hangingwall. It has been discussed that the global stability of the footwall is likely related to the interaction of two or more failure mechanisms acting in combination, however, the true footwall failuremechanisms are still debated. The objective of this thesis is to study and evaluate the footwall behaviour and determine the predominant mechanisms by combining data from field observations, numerical modelling and seismic data analysis. Field data was collected through damage mapping on decommissioned levels in the footwall on depths between 120 to 700 m for the full 4 km ore-body length. From the mapping data a conceptual boundary between damaged and undamaged footwall rock was established in the form of a damage boundary surface. The 3D geometry of the damage surface was analysed and a section was extracted and used in calibrating numerical models for simulatingthe footwall behaviour in response to mining. A parametric study was performed to highlight high impact inputs and study plausible origins of the conceptual damage surface. A base case model was adopted to explain the failure evolution and used in the analysis of seismic data. The seismic data was analysed with respect to origin mechanisms as well as temporal and spatial location patterns. The outline of the large scale footwall fracturing interpreted from the conceptual damage surface was geometrically complex. No single principal failure modes could be identified from evaluating the 3D geometry favouring the initial assumption of multiple mechanism interactions. In addition, the mapping data itself indicated changes in failure mode with respect to depth. On higher levels structurally controlled damages were predominant while general rock mass failures became common on lower levels. The parametric study related thehighest influence on plastic response to the internal cohesion followed by internal friction angle. This was interpreted for the base case as the rock mass being more sensitive to shear failures in favour of tensile failures. This indication was further strengthened by the evaluation of the seismic data. The origin analysis of the seismic events pointed to a significant dominance of shear origin events clustered in active fracturing volume indicated by the base case numerical analysis. By combining field observations, numerical modelling and seismic analysis a plausible description of the large scale footwall fracturing could be provided. The structurally controlled failures in the upper and mid portion of the footwall are reactions to active failure on deeper lying levels. Active fracturing of the footwall rock mass occurs based on the numerical and seismic results on levels on and underneath the current mining level. On the levels where active fracturing takes place the rock mass is confined by the support pressurefrom the un-mined ore-body. As mining progresses deeper the confinement is lowered as the ore is replaced by low stiffness cave rock. Due to the loss of support pressure the rock mass expands towards the sub-level cave and the induced weaknesses are activated and manifested as drift damage during rock mass mobilisation. The numerical models showed that the mobilised rock mass above the mining level exhibits the displacement pattern of a potential curved shear failure. This failure path intersects both the footwall slope face and thestructures from the upper footwall and thus enables these structures to shear.

Place, publisher, year, edition, pages
Luleå tekniska universitet, 2015.
Series
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
National Category
Other Civil Engineering
Research subject
Mining and Rock Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-18459Local ID: 8b8661b3-b13c-486e-ba16-feea5d3d0221ISBN: 978-91-7583-236-4 (print)ISBN: 978-91-7583-237-1 (electronic)OAI: oai:DiVA.org:ltu-18459DiVA: diva2:991468
Note
Godkänd; 2015; 20150119 (miknil); Nedanstående person kommer att hålla licentiatseminarium för avläggande av teknologie licentiatexamen. Namn: Mikael Svartsjaern Ämne: Gruv- och Berganläggningsteknik/Mining and Rock Engineering Uppsats: Predominant Failure Mechanisms at the Kiirunavaara Mine Footwall Examinator: Bitr Professor David Saiang, Institutionen för samhällsbyggnad och naturresurser Luleå tekniska universitet Diskutant: PhD Diego Mars Ivars, Itasca Consulting Sweden AB, Kista Tid: Fredag den 27 mars 2015 kl 10.00 Plats: A1545, Luleå tekniska universitetAvailable from: 2016-09-29 Created: 2016-09-29 Last updated: 2017-08-30Bibliographically approved
In thesis
1. Footwall stability in SLC mining
Open this publication in new window or tab >>Footwall stability in SLC mining
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is based on a case study of the Kiirunavaara sublevel cave (SLC) mine. It focuses on footwall stability and damage development in the mining infrastructure on mine scale. Damage to the infrastructure is mappable for the full height of the footwall by access through decommissioned infrastructure associated with earlier mining stages. Damages range from pure structurally controlled failures (wedge failures) in the upper part of the footwall to fracture growth through intact rock combined with micro‑seismic emissions at the active mining depth.

The thesis addresses four distinct research questions;

(i) What are the predominant failure mechanisms for the Kiirunavaara footwall?

(ii) What is the role of confinement on the damage development in the footwall?

(iii) How does the SLC relate to the footwall damage development?

(iv) How can infrastructure damage associated to the future mining be estimated using currently available data?

Two sets of calibrated numerical models were used to study the damage evolution processes using damage mapping data as the main calibration parameter. Validation of the models was achieved by correlation of model output to micro-seismic locations. The modelling and damage mapping results were used as the basis for the development of a simple prognosis tool for estimating the ultimate extent of infrastructure damage associated to the mining advance for future mining steps.

A literature review on slope failure modes, large scale failures in cave mining and failure tracking using micro-seismic locations is included to provide background and definitions. The literature describes principal failure modes as well as mechanism combinations such as structurally controlled failures initiated by deep seated rock mass failures or relaxation. Cases are presented where previously stable structures become destabilised by cave advance and examples where micro‑seismic recordings were used to track deformations and the initiation and growth of newly formed fractures.

The Kiirunavaara SLC mine is presented in detail as the main case study of the work. The mine has been in operation since the early 20th century with a transition to underground operation over 50 years ago. The extent of the orebody is 4 km in length with an average width of 80-90 m, the termination at depth has yet to be determined. The ore has an average dip of 60˚ east and a dip-along-strike to the north. Both the footwall and hangingwall rock masses are considered hard and competent with UCS values for the footwall ranging from ca. 130 MPa to extreme cases of 600 MPa. The ore is mined in production blocks about 400 m wide (along strike), Mining of the northernmost blocks, situated in the Lake ore, did not start as open pit operations but has been accessed from the underground via SLC only.

The instabilities in the footwall has been addressed by several research studies in the past, with the predominant failure mechanisms in different studies being suggested as large scale tensile failure, complex wedge failure, or rotational shear failure, i.e., some type of principal slope failure.

In this work, conceptual numerical models in UDEC were calibrated to fit underground damage mapping data by tracking numerical shear strain concentrations. The conceptual models suggested rock mass damage without the indications of development of large scale slope failure mechanisms such as shear bands. Mine scale PFC models were calibrated with respect to the rock mass strength parameters derived by the conceptual UDEC models and used to study rock mass fracturing in the absence of large scale failure. It is shown that damage to the rock mass occurs mainly close to the active mining in a seismically active zone. This is suggested to weaken and soften the rock mass to allow the development of infrastructure damage in this volume to occur as the rock mass relaxes when entering the stress shadow of the SLC as mining progresses.

The damage to the rock mass at the production depth is argued, based on seismic records and a parametric study in UDEC, to constitute of large quantities of local shear failures coalescing to appear as a large scale step-path or rotational shear failure in mapping records. The extent of the associated infrastructure damage is predicated to be limited by the extent of the damaged rock mass zone. A simple bi-linear equation is suggested using ore-width and mining depth as input to estimate the ultimate extent of the damaged zone for each mining stage and thus the limit of later infrastructure damage development.

The thesis is concluded with recommendations for future work and potential for continued research.

Abstract [sv]

Denna avhandling baseras på en fallstudie av skivrasgruvan Kiirunavaara. Fokus ligger på liggväggstabilitet och skadeutveckling på gruvans infrastruktur i gruvskala. Infrastrukturskador kan karteras längs med hela liggväggens höjd där tillgång till bergmassan ges via urdrifttagna ortar och ramper drivna i samband med tidigare brytningssteg. Dokumenterade skador varierar med djupet – från strukturstyrda brott i den övre delen av liggväggen till ny sprickbildning genom intakt berg kombinerat med mikro-seismik vid nuvarande brytningsdjup.

Fyra distinkta forskningsfrågor avhandlas;

(i) Vilka är de dominerande brottsmekanismerna i Kiirunavaaras liggvägg?

(ii) Vilken roll spelar inspänning för brottsutvecklingen i liggväggen?

(iii) På vilket sätt relateras skivrasbrytningen till brottsutvecklingen?

(iv) Hur kan skador på gruvans infrastruktur kopplat till framtida brytningssteg uppskattas med data tillgängliga idag?

Två omgångar med kalibrerade numeriska modeller togs fram för att studera skadeutvecklingen i liggväggen med skadekarteringsdata som primär kalibreringsparameter. Validering av modellerna uppnåddes genom att studera samstämmigheten mellan modellresultaten och lokaliseringen av mikro-seismiska händelser. Modellresultaten och skadekarteringsdatabasen användes som grund för att utveckla ett enkelt prognosverktyg för att uppskatta den slutgiltiga utbredningen av infrastrukturskador direkt associerade med gruvbrytningen för framtida brytningssteg.

En litteraturstudie av släntbrott, storkskaliga brott i samband med rasbrytning samt brottsövervakning med mikro-seismik är inkluderad som bakgrund och för att definiera terminologier som används genom avhandlingen. Literaturstudien beskriver principiella brottsmekanismer samt kombinationer av mekanismer såsom strukturstyrda brott pådrivna av djupt belägna bergmassebrott eller minskad inspänning. Fallstudier presenteras där tidigare stabila strukturer destabiliseras av rasbrytningens framskridande och exempel där mikro-seismikdata använts för att följa deformationer samt initiering och tillväxt av nya sprickor i intakt berg och bergmassa.

Kiirunavaaragruvan presenteras i detalj som den huvudsakliga fallstudien för arbetet. Gruvan har varit aktiv sedan tidigt 1900-tal med övergång till underjordsbrytning för över 50 år sedan. Malmkroppens utbredning är 4 km längs strykningen med en genomsnittlig vidd av 80-90 m, och malmkroppens fortsättning mot djupet är öppen. Malmen har en genomsnittlig stupning av 60 grader öst med en fältstupning mot norr. Bergmassan i både liggvägg och hängvägg anses vara hård och kompetent med UCS värden för liggväggen mellan ca. 130 MPa till extrema fall av 600 MPa. Malmen bryts i produktionsblock med ca 400 m bredd (längs malmens strykning). Brytning av de nordligaste blocken, belägna i Sjömalmen, har inte skett i dagbrott utan har utförts enbart via skivrasbrytning.

Instabiliteten i liggväggen har avhandlats i ett flertal tidigare studier. De dominerande brottsmekanismerna har föreslagits i tidigare arbeten som storskaligt dragbrott, komplext kilbrott eller cirkulärt skjuvbrott d.v.s. någon typ av principiellt släntbrott.

I arbetet för denna avhandling kalibrerades konceptuella numeriska modeller i UDEC mot skadekarteringsdata från liggvägens underjord, med avseende på koncentrationer av skjuvtöjningar. De konceptuella modellerna visade på bergmasseskador utan indikationer på storskaligt släntbrott, exempelvis koncentrationer av numeriska skjuvband. PFC-modeller i gruvskala kalibrerades gentemot bergmasseparametrarna från de konceptuella studierna i UDEC för att direkt studera upprickningen av bergmassan i frånvaro av storskaliga brottsindikationer. Modellerna visade på att skador i bergmassan främst uppkommer nära brytningsområdet i en seismiskt aktiv zon. Detta föreslås försvaga och mjukgöra bergmassan vilket i sin tur leder till utveklingen av infrastrukturskador i den skadade volymen när berget avlastas då området hamnar i spänningsskugga från skivraset.

Ovanstående studier visar att skadorna som uppkommer i bergmassan, baserat på de konceptuella UDEC-modellerna och mikro-seismiska data, består av ett stort antal lokala skjuvbrott vilka samverkar till att framstå som ett storskaligt trappstegsbrott eller cirkulärt skjuvbrott i skadekarteringsdatat. Utbredningen av de relaterade infrastrukturskadorna förutspås begränsas av utbredningen av bergmasseskadorna uppkomna vid bryningen. Ett enkelt bi-linjär samband föreslås vilket använder malmbredd och brytningsdjup för att uppskatta den slutgiltiga utbredningen av skadezonen i bergmassan för varje brytningssteg, och i förlängningen begräsningen av senare uppkommande infrastrukturskador.

Avhandlingen avslutas med rekommendationer för fortsatt arbete samt framtida forskningspotential.

Place, publisher, year, edition, pages
Luleå: Luleå tekniska universitet, 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-65420 (URN)978-91-7583-949-3 (ISBN)978-91-7583-950-9 (ISBN)
Public defence
2017-10-27, F1031, LTU, Luleå, 10:00 (English)
Opponent
Supervisors
Available from: 2017-08-30 Created: 2017-08-30 Last updated: 2017-09-01Bibliographically approved

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