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Sliding stability re-assessment of concrete dams with bonded concrete-rock interfaces
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.ORCID iD: 0000-0003-4555-0471
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The shear strength of the concrete-rock interface is an important parameter in sliding stability analyses of concrete dams founded on rock and depends, in addition to the normal stress state, on the bonding conditions of the interface; concrete-rock interfaces can be either unbonded, partially bonded or fully bonded.

In the Swedish guidelines for dam safety all dam-foundation contacts are treated as unbonded. This has the benefit of eliminating all uncertainties related to the cohesive strength of bonded contacts but it might also lead to unnecessary strengthening of dams. Other national guidelines deal with the uncertainties related to cohesion by applying higher safety factors, mainly determined based on previous experience, when both cohesion and friction are taken into account.

The main objective of this project is to study if and how cohesion can be included when evaluating the shear strength of bonded or partially bonded interfaces. To accomplish this, uncertainties associated with cohesion are identified and their influence on the assessed stability is investigated.

The results show that the influence on the assessed sliding stability is strongly dependent on the magnitude of the involved uncertainties that might vary significantly for different dams. It is thus questionable if one safety factor applicable for all dams can be established for use in deterministic analyses.

Taking into account cohesion when reliability methods are used is somewhat less complicated because of the possibility of directly incorporating the uncertainties in the analysis. The main challenge in such cases is the quantification of the involved uncertainties due to lack of proper data and, in some cases, knowledge. In this thesis, a framework for quantification of parameter uncertainty is suggested and the model error due to brittle failure in combination with spatial variation in cohesion is analysed. Areas that require more research to further refine the analysis are also identified.

Abstract [sv]

Skjuvhållfastheten i betong-berggränssnittet är en avgörande faktor vid glidstabilitetsutvärderingar av befintliga betongdammar grundlagda på berg och beror dels på normalspänningsfördelningen och dels på kontaktytans status med avseende på vidhäftning, vilken kan delas in i tre separata fall; då vidhäftning existerar och kohesion medräknas (intakt), då vidhäftning aldrig funnits eller förlorats (bruten), samt en kombination av föregående (delvis intakt).

I RIDAS, de svenska riktlinjerna för dammsäkerhet, behandlas alla berg-betonggränssnitt som brutna. Detta förhållningssätt har fördelen att det utelämnar all osäkerhet förknippad med intakta kontaktytors kohesion men det kan också resultera i icke nödvändiga förstärkningar av dammar. I andra nationella riktlinjer för dammsäkerhet beaktas osäkerheterna förknippade med kohesion genom att högre säkerhetsfaktorer, i huvudsak bestämda baserat på erfarenhet, tillämpas då både kohesion och friktion används vid beräkning av kontaktytans skjuvhållfasthet.

Det övergripande syftet med detta doktorandprojekt är att studera om och hur kohesion kan medräknas vid stabilitetsutvärderingar av befintliga betongdammar med helt eller delvis intakta betong-berggränssnitt. För att uppnå detta, identifieras osäkerheter förknippande med kohesionen och deras inverkan på den bedömda glidstabiliteten utvärderas.

Resultaten från projektet visar att osäkerheternas inverkan på den uppskattade glidstabiliteten är starkt beroende av osäkerheternas storlek, vilken varierar för olika dammar. Det är således tveksamt om en säkerhetsfaktor giltig för alla dammar kan bestämmas för användning i deterministiska stabilitetsanalyser. När sannolikhetsbaserade metoder används kan osäkerheterna införlivas direkt i analysen. Kvantifieringen av osäkerheterna förknippade med ett specifikt fall utgör då den huvudsakliga utmaningen. I denna avhandling presenteras ett ramverk för att uppskatta de statistiska parametrarna hos de ingående variablerna. Dessutom studeras modellosäkerheten förknippad med intakta gränssnitts spröda brott i kombination med kohesionens rumsliga variation i detalj. Områden som fordrar ytterligare forskning i syfte att förbättra analysen identifieras också.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2016. , xxiv, 71 p.
Series
TRITA-JOB PHD, ISSN 1650-9501 ; 2031
Keyword [en]
concrete dams, sliding stability, cohesion, shear strength, uncertainty, bond
National Category
Geotechnical Engineering
Research subject
Civil and Architectural Engineering
Identifiers
URN: urn:nbn:se:kth:diva-185144ISBN: 978-91-7595-907-8 (print)OAI: oai:DiVA.org:kth-185144DiVA: diva2:918393
Public defence
2016-05-04, Kollegiesalen, Brinellvägen 8, KTH campus, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20160411

Available from: 2016-04-11 Created: 2016-04-11 Last updated: 2016-04-11Bibliographically approved
List of papers
1. The influence of correlation between cohesion and friction angle on the probability of failure for sliding of concrete dams
Open this publication in new window or tab >>The influence of correlation between cohesion and friction angle on the probability of failure for sliding of concrete dams
2012 (English)In: Proceedings of the 3rd International Forum on Risk Analysis, Dam Safety Dam Security and Critical Infrastructure Management / [ed] Ignacio Escuder-bueno, Enrique Matheu, Luis Altarejos-garcfa, CRC Press, 2012, 75-80 p.Conference paper, Published paper (Refereed)
Abstract [en]

In the latest years there has been a trend towards an implementation of structural reliability analysis, SRA, in the stability assessment of concrete dams. In these analyses, it is in general assumed that the basic variables are independent of each other. However, this may be incorrect. For example, when the shear strength of the bonded concrete-rock interface is approximated with the Mohr-Coulomb failure criterion, the linear approximation of the curved failure envelope implies that a negative correlation between cohesion and friction could be expected. The aim of this paper is to study how correlation between friction angle and cohesion for a bonded interface may affect the sliding stability of concrete gravity dams founded on rock. For this purpose, experimental data from uniaxial and triaxial compressive tests performed on intact rock were used in order to study the existence and magnitude of a possible negative correlation between friction angle and cohesion. After that, the reliability index for a concrete gravity dam was calculated using negatively correlated and uncorrelated friction angle and cohesion. The results showed that the safety index increased significantly as the correlation coefficient decreased. Therefore, in order to calculate the reliability of dams correctly in the future, it is important to study and determine the magnitude of this correlation.

Place, publisher, year, edition, pages
CRC Press, 2012
Keyword
Adhesion, Concrete dams, Critical infrastructures, Friction, Gravity dams, Hydraulic structures, Public works, Reliability analysis, System stability, Tribology
National Category
Civil Engineering Geotechnical Engineering
Identifiers
urn:nbn:se:kth:diva-75319 (URN)000300401800011 ()2-s2.0-84856750729 (Scopus ID)978-0-415-62078-9 (ISBN)
Conference
3rd International Week on Risk Analysis, Dam Safety, Dam Security, and Critical Infrastructure Management, October 17-21, 2011. Valencia, Spain
Note

QC 20120413

Available from: 2012-02-05 Created: 2012-02-05 Last updated: 2016-04-11Bibliographically approved
2. Theme C: Estimation of the Probability of Failure of a Gravity Dam for the Sliding Failure Mode
Open this publication in new window or tab >>Theme C: Estimation of the Probability of Failure of a Gravity Dam for the Sliding Failure Mode
2011 (English)In: Proceedings of 11th Benchmark Workshop on Numerical Analysis of Dams, 2011, , xvi, 34 p.Conference paper, Published paper (Other academic)
Publisher
xvi, 34 p.
National Category
Civil Engineering
Identifiers
urn:nbn:se:kth:diva-131429 (URN)
Conference
11th Benchmark Workshop on Numerical Analysis of Dams, Valencia, Spain
Note

NQC 2015

Available from: 2013-10-15 Created: 2013-10-15 Last updated: 2016-04-11Bibliographically approved
3. Effects of spatial variation in cohesion over the concrete-rock interface on dam sliding stability
Open this publication in new window or tab >>Effects of spatial variation in cohesion over the concrete-rock interface on dam sliding stability
2015 (Swedish)In: Journal of Rock Mechanics and Geotechnical Engineering, ISSN 1674-7755, Vol. 7, no 6, 659-667 p.Article in journal (Refereed) Published
Abstract [en]

The limit equilibrium method (LEM) is widely used for sliding stability evaluation of concrete gravity dams. Failure is then commonly assumed to occur along the entire sliding surface simultaneously. However, the brittle behaviour of bonded concrete-rock contacts, in combination with the varying stress over the interface, implies that the failure of bonded dam-foundation interfaces occurs progressively. In addition, the spatial variation in cohesion may introduce weak spots where failure can be initiated. Nonetheless, the combined effect of brittle failure and spatial variation in cohesion on the overall shear strength of the interface has not been studied previously. In this paper, numerical analyses are used to investigate the effect of brittle failure in combination with spatial variation in cohesion that is taken into account by random fields with different correlation lengths. The study concludes that a possible existence of weak spots along the interface has to be considered since it significantly reduces the overall shear strength of the interface, and implications for doing so are discussed.

Place, publisher, year, edition, pages
Kexue Chubanshe, 2015
Keyword
Concrete gravity dam, Sliding stability, Cohesion, Brittle failure, Spatial variation
National Category
Geotechnical Engineering
Identifiers
urn:nbn:se:kth:diva-179207 (URN)10.1016/j.jrmge.2015.08.005 (DOI)2-s2.0-84969899033 (Scopus ID)
Note

QC 20151218

Available from: 2015-12-12 Created: 2015-12-12 Last updated: 2017-12-01Bibliographically approved
4. A comparison between two techniques for including the influence of progressive sliding failure in structural reliability analyses of concrete dams
Open this publication in new window or tab >>A comparison between two techniques for including the influence of progressive sliding failure in structural reliability analyses of concrete dams
2014 (Swedish)Conference paper, Published paper (Refereed)
Abstract [en]

The shear strength of concrete-rock interfaces, used in reliability-based analyses of sliding stability of concrete dams, is commonly quantified using a simplified version of the Mohr-Coulomb shear strength criterion. This criterion is based on the assumption of a mean value driven process occurring according to the requirements imposed by perfectly-plastic failure theory. However, tests on cores with bonded interfaces exhibit semi-brittle post-peak behavior, which in combination with a possible spatial variation in cohesion implies that a progressive failure mechanism is a more correct description of the interface behavior. This has to be considered in sliding stability analyses in order to avoid overestimation of dam safety. Uncertainties between model and real behavior like the one described above can be taken into account by introducing a random variable representative of the specific uncertainty into the limit state function. A drawback with this technique is that the real shear strength of the interface is difficult to define since physical observations are not easy to acquire. Krounis and Johansson (2014), therefore, used a technique where numerical analyses were used in order to estimate the real behavior. The technique requires several numerical calculations and is extremely time-consuming. Another, less time-consuming, technique was used by Westberg Wilde and Johansson (2013). In their work, the progressive failure of the bonded interface was taken into account by simulating new distribution parameters for cohesion. This was done by means of an analytical procedure where the cohesive strength of the interface was considered to behave as a brittle parallel system. The stress state of the interface, however, was not considered, which may have significantly affected the results of the analyses. In this paper, a comparison between these two techniques for a hypothetical dam monolith is made in order to study the magnitude of the possible discrepancy between the two techniques. The benefits and limitations of each method are also discussed based on the results of the analyses.

Place, publisher, year, edition, pages
International Society for Rock Mechanics, 2014
National Category
Geotechnical Engineering
Identifiers
urn:nbn:se:kth:diva-184137 (URN)978-4-907430-03-0 (ISBN)
Conference
ISRM International Symposium-8th Asian Rock Mechanics Symposium. International Society for Rock Mechanics
Note

QC 20160329

Available from: 2016-03-28 Created: 2016-03-28 Last updated: 2016-04-11Bibliographically approved
5. Influence of cohesive strength in probabilstic sliding stability re-assessment of concrete dams
Open this publication in new window or tab >>Influence of cohesive strength in probabilstic sliding stability re-assessment of concrete dams
2016 (English)In: Journal of Geotechnical and Geoenvironmental Engineering, ISSN 1090-0241, E-ISSN 1943-5606, Vol. 143, no 2, 04016094Article in journal (Refereed) Published
Abstract [en]

For concrete dams, cohesive strength often constitutes a significant share of the overall shear strength of partially bonded concrete-rock interfaces. However, cohesive strength is also associated with great uncertainties that may have a significant impact on the assessed stability of the analyzed structure. In this paper, the merits of including cohesion are evaluated using a probability-based approach to analyze the sliding stability of an existing concrete gravity dam. The shear strength properties of the interface are inferred from a limited number of site-specific tests and previous knowledge from similar structures using Bayesian updating. The study shows that the potential gain from cohesive strength is strongly related to the involved uncertainties and identifies the bonding percentage and basic friction angle as the most influential parameters. The importance of testing, both with regard to the specific project and for future projects, is also highlighted.

Place, publisher, year, edition, pages
American Society of Civil Engineers (ASCE), 2016
Keyword
Cohesive strength, Concrete dam, Concrete-rock interface, Partially bonded, Sliding stability
National Category
Geotechnical Engineering
Research subject
Civil and Architectural Engineering
Identifiers
urn:nbn:se:kth:diva-185143 (URN)10.1061/(ASCE)GT.1943-5606.0001583 (DOI)000394257800001 ()2-s2.0-85010665903 (Scopus ID)
Note

QC 20160411

Available from: 2016-04-11 Created: 2016-04-11 Last updated: 2017-11-30Bibliographically approved
6. Shear strength of partially bonded concrete-rock interfaces for application in dam stability analyses
Open this publication in new window or tab >>Shear strength of partially bonded concrete-rock interfaces for application in dam stability analyses
2016 (English)In: Rock Mechanics and Rock Engineering, ISSN 0723-2632, E-ISSN 1434-453X, Vol. 49, no 7, 2711-2722 p.Article in journal (Refereed) Published
Abstract [en]

The shear strength of the concrete–rock interface has a substantial influence on the sliding stability of concrete gravity dams founded on rock. While several studies have been done on concrete–rock contacts, there remains uncertainty regarding the peak shear strength of partially bonded interfaces. There exists, in particular, an uncertainty regarding the contribution from surface roughness of the unbonded parts to the peak shear strength of the interface due to the dependency of mobilized strength on shear displacement. In this study, a series of 24 direct shear tests are performed under CNL conditions on concrete–rock samples with different bonding conditions. Tests on samples with fully bonded and unbonded interfaces are conducted to study the strain compatibility of the different contacts, while the results of samples with partially bonded interfaces are evaluated in the context of linking the joint roughness of the unbonded parts to the peak shear strength of the interface. The results indicate that a significant part of the surface roughness of the unbonded parts is mobilized prior to degradation of bond strength, in particular for interfaces with low bonding percentages. It is recommended that further research should be conducted to understand how the contribution from roughness change with an increase in scale and degree of matedness.

Place, publisher, year, edition, pages
Springer, 2016
National Category
Geotechnical Engineering
Identifiers
urn:nbn:se:kth:diva-184115 (URN)10.1007/s00603-016-0962-8 (DOI)000379015700013 ()2-s2.0-84962169543 (Scopus ID)
Note

QC 20160516

Available from: 2016-03-27 Created: 2016-03-27 Last updated: 2017-11-30Bibliographically approved

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