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The influence of correlation between cohesion and friction angle on the probability of failure for sliding of concrete dams
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.ORCID iD: 0000-0003-4555-0471
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.ORCID iD: 0000-0002-8152-6092
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 (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. 75-80 p.
Keyword [en]
Adhesion, Concrete dams, Critical infrastructures, Friction, Gravity dams, Hydraulic structures, Public works, Reliability analysis, System stability, Tribology
National Category
Civil Engineering Geotechnical Engineering
URN: urn:nbn:se:kth:diva-75319ISI: 000300401800011ScopusID: 2-s2.0-84856750729ISBN: 978-0-415-62078-9OAI: diva2:490455
3rd International Week on Risk Analysis, Dam Safety, Dam Security, and Critical Infrastructure Management, October 17-21, 2011. Valencia, Spain

QC 20120413

Available from: 2012-02-05 Created: 2012-02-05 Last updated: 2016-04-11Bibliographically approved
In thesis
1. Uncertainty in Sliding Stability Analyses of Existing Concrete Gravity Dams with Bonded Concrete-Rock Interfaces
Open this publication in new window or tab >>Uncertainty in Sliding Stability Analyses of Existing Concrete Gravity Dams with Bonded Concrete-Rock Interfaces
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.
Trita-JOB. LIC, ISSN 1650-951X ; 2022
National Category
Civil Engineering
urn:nbn:se:kth:diva-131430 (URN)
2013-10-11, Sal B1, Brinellvägen 23, KTH, Stockholm, 13:00 (English)

QC 20131015

Available from: 2013-10-15 Created: 2013-10-15 Last updated: 2013-10-15Bibliographically approved
2. Sliding stability re-assessment of concrete dams with bonded concrete-rock interfaces
Open this publication in new window or tab >>Sliding stability re-assessment of concrete dams with bonded concrete-rock interfaces
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.
TRITA-JOB PHD, ISSN 1650-9501 ; 2031
concrete dams, sliding stability, cohesion, shear strength, uncertainty, bond
National Category
Geotechnical Engineering
Research subject
Civil and Architectural Engineering
urn:nbn:se:kth:diva-185144 (URN)978-91-7595-907-8 (ISBN)
Public defence
2016-05-04, Kollegiesalen, Brinellvägen 8, KTH campus, Stockholm, 13:00 (English)

QC 20160411

Available from: 2016-04-11 Created: 2016-04-11 Last updated: 2016-04-11Bibliographically approved

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