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On the behaviour of tailings dams: Management in cold regions
Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Mining and Geotechnical Engineering.ORCID iD: 0000-0003-1672-6159
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Associated to mining activities there is mine waste generated. The residues left over from mineralextraction processes are referred to as tailings, normally stored in surface tailings facilities. Thefacilities are usually confined by embankment dams, so called tailings dams. Sufficient stability of thetailings dams is essential, and a proper management of the tailings is needed in order to maintain asafe storage. However, optimal strategies for tailings management are very much site specific, andthere is no universal answer on how tailings management should be applied. Despite the availableguidelines, recommendations and current trends in the industry on how to handle tailings, theremust be a pertinent approach to optimize tailings management to the site-specific conditions.

The aim with the work presented in this thesis is to enhance current practice in tailings managementin cold regions. Three sets of research objectives are presented, from which recommendations will begiven on how management can be adjusted for site-specific conditions.

Firstly, depositional aspects in cold climate are studied. A methodology is proposed where thethermal regime is studied in a tailings facility with active deposition, i.e. raised tailings surface, andconcurrent freezing and thawing. Despite the fact that there is no natural permafrost in Sweden, thedeposition can imply generation of “man-made” permafrost in the facilities. The model is simple andeasily accessible data are used as input. With a proposed methodology, deposition schedules can bemodelled, and the deposition scheme can be adjusted in order to prevent permafrost generation intailings facilities.

In addition to the depositional aspects in cold climate, a study is presented focusing on thaw stabilityof tailings beach slopes. The tailings beach is the inclined surface of settled tailings without pondedwater. For conventional tailings deposition, the beach slopes are relatively flat and the need for thawstability analyses is trivial. However, there is a current trend of adopting thickened tailingstechnology where steeper beach slopes are one of the intended merits. The increased slopeinclination might be unstable with major seasonal freezing and thawing. In rapid thaw excess porewater pressure might be generated, creating instability where masses of tailings can slide along a stillfrozen interface. Increased storage capacity, or freeboard, along the surrounding dams might beneeded, and the intended merits with the thickened tailings technology might be lost.

Secondly, a study on the determination of strength parameters on granular soil is presented. Simpleshear tests are widely used particularly in Sweden for soil strength determination. However, recentstudies on tailings indicate large differences on the evaluated strength compared to what isdetermined via triaxial testing. In this thesis a laboratory study is presented, where simple sheartesting and triaxial testing were conducted. The results were used for examining the Swedishguidelines on simple shear testing and associated strength determination. It was found that directapplication of strength determined by simple shear testing leads to incorrect design assumptions.

Thirdly, a study on prediction and verification of tailings dam stability is presented. Dam stability iscentral in tailings management, and a convenient way to describe the safety is via the factor of safety(FoS). Although the FoS can be predicted via calculations, it cannot be measured directly in the field.Therefore, in order to verify the calculated stability, field measurements must be used to comparewith the anticipated performance. With good agreement the prediction indicate reliableunderstanding of the dam, and the predicted stability can be verified. Without good agreement, thestability is easily questioned.

Based on findings from the three sections presented in the thesis, recommendations are given ontailings management in cold regions. With increased knowledge in the field of tailings deposition incold climate, shear parameter determination and overall dam stability assessment, current practice intailings management is enhanced.

Abstract [sv]

I samband med gruvdrift genereras gruvavfall. Restprodukter som återstår efter att malm har brutitsoch förädlats, vanligen kallat anrikningssand (eng. tailings), deponeras i sandmagasin där det förvaras.Magasinen omges vanligen av dammar, s.k. gruvdammar. Management av gruvdammar ochsandmagasin är emellertid väldigt beroende av platsspecifika förutsättningar, och det finns ingetgenerellt svar till hur ett fullgott management ska se ut. Även då det finns riktlinjer, krav och trenderi hur anrikningssand bör hanteras, bör dess management anpassas utifrån platsspecifika förutsättningar.

Målet med detta arbete är att bidra till att förbättra dagens praxis gällande management avanrikningssand i kallt klimat. Tre delmål presenteras i avhandlingen, från vilka rekommendationerkommer ges med tanke på hur management kan anpassas utifrån platsspecifika förutsättningar.

Först behandlas deponeringsaspekter i kallt klimat. En metodik föreslås där termiska förhållanden hosanrikningssand studeras i samband med deponering och samtidig tjälning/upptining. Trots att detnormalt ej finns permafrost i Sverige, så visar studien att deponering kan innebära att lager avpermafrost kan byggas in i sandmagasinen. Modellen som presenteras är enkel att använda och byggerpå att lättillgänglig information används som indata. Med ett föreslaget angreppsätt kandeponeringsscheman enkelt alterneras för att hitta det mest optimala för sandmagasinet i fråga. På såsätt kan inbyggnad av permafrost undvikas.

Därefter behandlas en studie rörande töstabilitet av sandmagasinets yta. För konventionell deponeringär lutningen på sandmagasinets yta generellt sett flack, och behovet av töstabilitetsutredning är litet.En nuvarande trend inom deponeringsteknik är så kallad förtjockad deponering, där ökadesandlutningar är en av fördelarna. Men det finns en risk för instabilitet hos branta lutningar i kalltklimat. Vid snabb upptining kan höga porvattenövertryck genereras, vilket kan leda till otillräckligstabilitet längs det underliggande frusna planet och att glidning sker i magasinet. Ytterligarelagringskapacitet, dvs. ökat fribord, kan behövas utmed dammarna, vilket innebär att fördelarna medförtjockad deponering minskar.

Ett andra delmål i avhandlingen rör utvärdering av hållfasthetsparametrar. Direkta enkla skjuvförsök(Simple shear) är ett vanligt försök som används för hållfasthetsutvärdering av jord, inte minst iSverige. Men många tidigare studier har visat stora skillnader i hållfasthetsparameterar som bestämtsmed denna metod jämfört med motsvarande utvärderade från triaxialförsök. I denna avhandlingpresenteras en laborationsstudie, där både direkta enkla skjuvförsök och triaxialförsök utfördes pånaturlig sand. Resultaten användes för att granska dagens riktlinjer rörande hållfasthetsutvärderingarvia direkta enkla skjuvförsök. Resultat visade att hållfasthetsutvärdering via direkta enkla skjuvförsökkan leda till felaktiga antaganden vid dimensionering.

Ett tredje delmål i avhandlingen rör dammstabilitet och säkerhet, där en fallstudie presenteras. Ettvanligt begrepp inom dammsäkerhetserbete är säkehetsfaktorn, vilket är ett mått på hur stabildammen är gentemot ett stabilitetsbrott. Trots att säkerhetsfaktorn kan beräknas, är det samtidigt enstorhet som ej kan mätas i fält. För att bekräfta måttet av stabilitet hos en damm, måste hänsyn tas tilldammens beteende. Dammens förväntade beteende bör simuleras, och sedan jämförs beräknadestorheter med fältmätningar. God överenstämmelse mellan förväntade och uppmätta värden tyder pågod förståelse av dammen, och dess stabilitet kan bekräftas. Om det däremot saknas en godöverenstämmelse mellan förväntat och uppmätt beteende, tyder osäkerheterna på ofullständigförståelse av dammen, och säkerheten kan ifrågasättas.

Utifrån resultaten som presenteras i avhandlingen kommer rekommendationer ges för förbättratmanagement av gruvdammar i kallt klimat. Med ökad förståelse inom deponeringsteknik i kalltklimat, utvärdering av hållfasthetsparametrar, samt utredning av dammstabilitet, kan dagens praxisgällande hantering av anrikningssand förbättras.

Place, publisher, year, edition, pages
Luleå: Luleå tekniska universitet, 2018.
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Geotechnical Engineering
Research subject
Soil Mechanics
Identifiers
URN: urn:nbn:se:ltu:diva-67218ISBN: 978-91-7790-029-0 (print)ISBN: 978-91-7790-030-6 (electronic)OAI: oai:DiVA.org:ltu-67218DiVA, id: diva2:1172525
Public defence
2018-03-20, F1031, Luleå, 10:00 (English)
Opponent
Supervisors
Available from: 2018-01-11 Created: 2018-01-10 Last updated: 2018-02-27Bibliographically approved
List of papers
1. Stability considerations for thickened tailings due to freezing and thawing
Open this publication in new window or tab >>Stability considerations for thickened tailings due to freezing and thawing
2016 (English)In: Paste 2016: Proceedings of the 19th International Seminar on Paste and Thickened Tailings / [ed] Sergio Barrea & Richard Jewell, Santiago: GECAMIN, 2016, p. 567-577Conference paper, Published paper (Refereed)
Place, publisher, year, edition, pages
Santiago: GECAMIN, 2016
National Category
Geotechnical Engineering
Research subject
Soil Mechanics
Identifiers
urn:nbn:se:ltu:diva-61174 (URN)
Conference
19th International Seminar on Paste and Thickened Tailings, Santiago, Chile, July 5-8 2016
Available from: 2016-12-20 Created: 2016-12-20 Last updated: 2018-02-27Bibliographically approved
2. Benefits of Advanced Constitutive Modeling when Estimating Deformations in a Tailings Dam
Open this publication in new window or tab >>Benefits of Advanced Constitutive Modeling when Estimating Deformations in a Tailings Dam
2018 (English)In: Journal of Earth Sciences and Geotechnical Engineering, ISSN 1792-9040, E-ISSN 1792-9660, Vol. 8, no 1, p. 1-19Article in journal (Refereed) Published
Abstract [en]

Behavior of tailings dams are often controlled in dam surveillance programs where horizontal deformation is one of the key aspects. When evaluating field data, there is a necessity for comparison with anticipated deformations in order to relate field behavior to dam stability. With numerical modeling, these predictions can be made. This paper presents a case where horizontal deformations in a tailings dam have been simulated for a six-year period, using two-dimensional finite element modeling. Yearly dam raises have been simulated as staged constructions according to activities at site. Tailings materials have been simulated with an elasto-plastic constitutive model with isotropic hardening, called Hardening Soil and the conventional linear-elastic, perfectly plastic Mohr-Coulomb model. Soil parameters used for input were calibrated to laboratory data. Results from simulations were compared with data obtained in situ by a slope inclinometer. Results obtained by the Hardening Soil model indicate good agreement with respect to field measurements. However, this was not reached with the Mohr-Coulomb model. The results presented indicate benefits by using an advanced constitutive model for tailings in order to estimate in situ deformations in a tailings dam. The methodology presented can be used for prediction of future deformations, in order to relate the dam behavior to its stability. This is important in dam safety assessment, and will lead to a better understanding of the dam safety, being of great importance for the dam owner and the society in general.

Place, publisher, year, edition, pages
Scienpress Ltd., 2018
National Category
Geotechnical Engineering
Research subject
Soil Mechanics
Identifiers
urn:nbn:se:ltu:diva-67191 (URN)
Note

Validerad;2018;Nivå 1;2018-01-22 (rokbeg)

Available from: 2018-01-08 Created: 2018-01-08 Last updated: 2018-03-15Bibliographically approved
3. The use of numerical modelling in alert level set-up for instrumentation in tailings dams
Open this publication in new window or tab >>The use of numerical modelling in alert level set-up for instrumentation in tailings dams
2015 (English)Conference paper, Oral presentation only (Refereed)
Abstract [en]

In dam safety operations for tailings dams, instrumentation has a major role. High pore-water pressure or large deformations can both be observed with good monitoring programs, and are covered in many dam safety guidelines. A key aspect when evaluating field data is the way to compare values with expected dam behavior (including a certain safety margin). This is needed in order to determine the safety of a dam, but there is lack of methods for this in the dam industry. With the use of finite element modeling, the behavior and stability of tailings dams can be simulated. Simulated behavior can be used in dam safety operations, where field data is compared with numerical results. In this paper, a case study is presented where a method for instrumentation alarm-level set-up is proposed.

Keyword
tailings dam, safety of dams, finite element method, pore pressure, deformation measurement, Civil engineering and architecture - Geoengineering and mining engineering, Samhällsbyggnadsteknik och arkitektur - Geoteknik och gruvteknik
National Category
Geotechnical Engineering
Research subject
Soil Mechanics
Identifiers
urn:nbn:se:ltu:diva-34109 (URN)837e03f7-1262-4d9f-9ff2-3341ec10b705 (Local ID)837e03f7-1262-4d9f-9ff2-3341ec10b705 (Archive number)837e03f7-1262-4d9f-9ff2-3341ec10b705 (OAI)
Conference
ICOLD Congress 2015 : International Commission on Large Dams 15/06/2015 - 16/07/2015
Note
Godkänd; 2015; 20150705 (svek)Available from: 2016-09-30 Created: 2016-09-30 Last updated: 2018-02-27Bibliographically approved

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