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Uncertainty in Sliding Stability Analyses of Existing Concrete Gravity 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
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Safety assessment of existing dams becomes more and more important with their increasing age. In addition, new regulations, due to climate changes and changes in knowledge, demand a re-evaluation of the existing dams’ safety. One of the failure modes considered in safety assessments of concrete gravity dams is sliding at the dam-foundation contact. Sliding failure is assumed to occur when the applied load exceeds the shear strength of the interface. Thus, the shear strength of the concrete-rock interface will ultimately determine if sliding of the dam will occur or not and it is, therefore, of utmost importance to be able to quantify it.

The shear strength of concrete-rock interfaces is in general defined based on the Mohr-Coulomb shear strength criterion, which is governed by the cohesion, friction angle and normal stress along the interface. This simplified model of the shear strength is subject to several uncertainties, due for example to the natural variability of the involved parameters, statistical errors etc. In addition, analyses are commonly performed using the averaged values of cohesion, friction angle, normal and shear stresses, based on the assumption of ductile failure. This may be incorrect for bonded or partly bonded interface since test results show that bonded concrete-rock cores exhibit brittle behaviour.

The uncertainties related to bonded or partly bonded interfaces are taken into account in the Swedish guidelines, RIDAS, for by treating all concrete-rock interfaces as unbonded, i.e. the effect of cohesion is not included when evaluating the shear strength of the interface. This is a conservative method, but it may lead to expensive and unnecessary strengthening of existing dams. Other deterministic guidelines/regulations, e.g. FERC, allow the use of cohesion but apply higher target safety factors when both the cohesive and the frictional strength are taken into account. To evaluate the adequacy of using cohesion in sliding stability analyses of concrete dams, the effect of the uncertainties on the calculated sliding stability of the dam has to be assessed.

This thesis highlights several uncertainties related to bonded concrete-rock interfaces. However, the thesis focuses mainly on increasing the knowledge regarding the model uncertainty due to the brittle failure mechanism in combination with a possible spatial variability of cohesion. The magnitude of the model uncertainty is studied using numerical analyses. Its influence on the assessed behaviour of a hypothetical dam monolith is then evaluated using probability based methods. A conclusion drawn from this particular case, but which is likely to be generalized to other dam-foundation systems, is that the ductile sliding failure is too coarse an approximation of the failure behaviour of dams with bonded interfaces and could lead to an overestimation of dam safety. In addition, the potential spatial variability of cohesion along the interface further diminishes the validity of the ductile failure model.

Areas that require further research in order to take into account the most significant uncertainties related to bonded interfaces include the definition of the ratio of bonded area to total area and its influence on sliding stability. The statistical uncertainty due to the limited number of tests also needs to be studied.

Place, publisher, year, edition, pages
Stockohlm: KTH Royal Institute of Technology, 2013. , xvi, 34 p.
Series
Trita-JOB. LIC, ISSN 1650-951X ; 2022
National Category
Civil Engineering
Identifiers
URN: urn:nbn:se:kth:diva-131430OAI: oai:DiVA.org:kth-131430DiVA: diva2:656171
Presentation
2013-10-11, Sal B1, Brinellvägen 23, KTH, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20131015

Available from: 2013-10-15 Created: 2013-10-15 Last updated: 2013-10-15Bibliographically approved
List of papers
1. 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
2. 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
3. Model uncertaintyin sliding stability analyses of concrete gravity dams with bonded concrete-rock interfaces
Open this publication in new window or tab >>Model uncertaintyin sliding stability analyses of concrete gravity dams with bonded concrete-rock interfaces
(English)Manuscript (preprint) (Other academic)
National Category
Civil Engineering Geotechnical Engineering
Identifiers
urn:nbn:se:kth:diva-131441 (URN)
Conference
3rd International Week on Risk Analysis, Dam Safety, Dam Security, and Critical Infrastructure Management, October 17-21, 2011. Valencia, Spain
Note

QS 2013

Available from: 2013-10-15 Created: 2013-10-15 Last updated: 2013-10-15Bibliographically approved

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