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Reliability-based design of rock tunnel support
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Soil and Rock Mechanics.ORCID iD: 0000-0002-9835-7053
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Since 2009, design of rock tunnels can be performed in accordance with the Eurocodes, which allows that different design methodologies are applied, such as design by calculation or design using the observational method. To account for uncertainties in design, the Eurocode states that design by calculation should primarily be performed using the partial factor method or reliability-based methods. The basic principle of both of these methods is that it shall be assured that a structure’s resisting capacity is larger than the load acting on the structure, with sufficiently high probability. Even if this might seem straightforward, the practical application of limit state design to rock tunnel support has only been studied to a limited extent.The overall aim of this project has been to develop reliability-based methods for environmental and economic optimization of rock tunnel support, with a special focus on shotcrete support. To achieve this, this thesis aims to: (1) assess the applicability of the partial factor method and reliability-based methods for design of shotcrete support, exclusively or in combination with the observational method, (2) quantify the magnitude and uncertainty of the shotcrete’s input parameters, and (3) assess the influence from spatial variability on shotcrete’s load-bearing capacity and judge the correctness of the assumption that the load-bearing capacity of the support is governed by the mean values of its input parameters.The thesis shows that the partial factor method is not suitable, and in some cases not applicable, to use in design of rock tunnel support. Instead, the thesis presents a reliability-based design methodology for shotcrete in rock tunnels with respect to loose blocks between rockbolts and a design methodology for shotcrete lining based on a combination of the observational method and reliability-based methods. The presented design methodologies enable optimization of the shotcrete support and shotcrete lining by stringently accounting for uncertainties related to input data throughout the design process. The thesis also discusses the limited knowledge that we as an industry sometimes have in our calculation models and the clarifications that should be made in future revisions of the Eurocode related to target reliability and the definition of failure.

Abstract [sv]

Sedan 2009 kan dimensionering av bergtunnlar utföras i enlighet med Eurokoderna, vilka tillåter att olika dimensioneringsmetoder tillämpas, såsom dimensionering genom beräkning eller dimensionering med observationsmetoden. För att ta hänsyn till osäkerheter föreskriver Eurokoderna att dimensionering genom beräkning primärt skall utföras med hjälp av partialkoefficientmetoden eller tillförlitlighetsbaserade metoder. Grundprincipen i båda dessa metoder är att det skall säkerställas att en konstruktions bärförmåga, med tillräckligt hög sannolikhet, är större än lasten som verkar mot konstruktionen. Även om detta kan förefalla enkelt så har den praktiska användningen av framförallt tillförlitlighetsbaserade metoder inom bergbyggande endast studerats i begränsad utsträckning.Målet med detta projekt har varit att utveckla tillförlitlighetsbaserade metoder för miljömässig och ekonomisk optimering av förstärkning i tunnlar med fokus på sprutbetongförstärkning. För att uppnå detta, syftar denna avhandling till att (1) utvärdera tillämpbarheten av partialkoefficient metoden och tillförlitlighetsbaserade metoder för dimensionering av sprutbetongförstärkning, (2) kvantifiera storleken och osäkerheten i sprutbetongförstärkningens indata parametrar och (3) utvärdera effekten från rumslig spridning på sprutbetongens bärförmåga.Avhandlingen visar att partialkoefficientmetoden inte är lämplig att använda vid dimensionering av förstärkning i tunnlar. En tillförlitlighetsbaserad dimensioneringsmetodik för sprutbetong med avseende på blockutfall mellan bultar samt en dimensioneringsmetodik för tunnel-lining av sprutbetong baserad på observationsmetoden och tillförlitlighetsbaserade metoder har utvecklats inom ramen av denna avhandling. De utvecklade metodikerna möjliggör optimering av förstärkning och tunnel-lining av sprutbetong genom att stringent ta hänsyn till osäkerheter kopplade till indata kontinuerligt genom hela designprocessen. Avhandlingen diskuterar även den begränsade kunskap vi har om våra beräkningsmodeller samt vilka förtydliganden som bör göras i framtida revideringar av Eurokoderna kopplade till riktvärden för kravställda brottsannolikheter och definitionen av brott.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2020. , p. 61
Series
TRITA-ABE-DLT ; 208
Keywords [en]
Rock engineering, reliability-based design, Eurocode 7, observational method, tunnel engineering
Keywords [sv]
Bergmekanik, sannolikhetsbaserad dimensionering, Eurokod 7, observationsmetoden, tunnelbyggnad
National Category
Geotechnical Engineering
Research subject
Civil and Architectural Engineering, Soil and Rock Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-272722ISBN: 978-91-7873-522-8 (print)OAI: oai:DiVA.org:kth-272722DiVA, id: diva2:1426755
Public defence
2020-05-28, Via Zoom - https://kth-se.zoom.us/j/490988607, Du som saknar dator/datorvana kan kontakta fredrik.johansson@byv.kth.se för information / Use the e-mail address if you need technical assistance, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20200506

Available from: 2020-05-06 Created: 2020-04-27 Last updated: 2020-05-06Bibliographically approved
List of papers
1. Challenges in applying fixed partial factors to rock engineering design
Open this publication in new window or tab >>Challenges in applying fixed partial factors to rock engineering design
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2017 (English)In: Geotechnical Special Publication, ISSN 0895-0563, no 283, p. 384-393Article in journal (Refereed) Published
Abstract [en]

The Swedish national guidelines for design of the main structural support system in road and railway rock tunnels have been adjusted to cohere with Eurocode 7. In the design guidelines, the limit states that the designer should consider are specified. The main method to account for uncertainties in the Swedish guidelines is similar to the method preferred in Eurocode 7: the partial factor method. For each limit state, fixed partial factors retrieved from different sections of the Eurocodes are specified. However, fixed partial factors may not correspond to the same structural reliability for all design situations. In this paper, we show for a common design situation in rock engineering design how partial factors in theory should vary with design geometries and uncertainties. The derived partial factors are compared to the Eurocodes’ fixed values. We find that using fixed partial factors to ensure structural safety in these limit states might not be suitable. The implications are discussed along with suggestions of other more suitable methods to account for uncertainties in rock engineering design.

Place, publisher, year, edition, pages
Reston: American Society of Civil Engineers (ASCE), 2017
National Category
Geotechnical Engineering
Research subject
Civil and Architectural Engineering
Identifiers
urn:nbn:se:kth:diva-204912 (URN)10.1061/9780784480700.037 (DOI)000406412100037 ()2-s2.0-85030456959 (Scopus ID)
Note

QC 20170419

Available from: 2017-04-04 Created: 2017-04-04 Last updated: 2020-04-27Bibliographically approved
2. Probability distributions of shotcrete parameters for reliability-based analyses of rock tunnel support
Open this publication in new window or tab >>Probability distributions of shotcrete parameters for reliability-based analyses of rock tunnel support
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2019 (English)In: Tunnelling and Underground Space Technology, ISSN 0886-7798, E-ISSN 1878-4364, Vol. 87, p. 15-26Article in journal (Refereed) Published
Abstract [en]

A common support measure for underground excavations in jointed rock masses to support loose blocks is to apply a thin shotcrete layer to the periphery of the excavation and systematically install rockbolts into the surrounding rock mass. In this support system, large blocks are carried by the rockbolts and small blocks are carried by the thin shotcrete layer. To verify the shotcrete layer's load-bearing capacity and to stringently account for the large uncertainties incorporated in the variables involved in determining its capacity, analytical calculations in combination with reliability-based methods can be used. However, a lack of knowledge exists regarding the magnitude and uncertainty of shotcrete characteristics (thickness, adhesion, flexural tensile strength, residual flexural tensile strength, and compressive strength), making it difficult to apply reliability-based methods. A statistical quantification of these characteristics is therefore important to facilitate reliability-based methods in design and verification of shotcrete support. In this paper, we illustrate how shotcrete support against small loose blocks can be viewed as a correlated conditional structural system and how this system can be analyzed using reliability-based methods. In addition, we present a unique amount of data for the aforementioned variables, which are all incorporated in the design and verification of a shotcrete layer's ability to sustain loads from small loose blocks. Based on the presented data, we statistically quantify and propose suitable probability distributions for each variable. Lastly, we illustrate how the proposed probability distributions can be used in the design process to calculate the probability of exceeding the shotcrete's load-bearing capacity. Both the probabilistic quantification and the defined correlated conditional structural system along with the illustrative calculation example are followed by a discussion of their implications.

Place, publisher, year, edition, pages
Elsevier, 2019
National Category
Geotechnical Engineering
Identifiers
urn:nbn:se:kth:diva-246439 (URN)10.1016/j.tust.2019.02.002 (DOI)000462421100002 ()2-s2.0-85061199710 (Scopus ID)
Note

QC 20190329

Available from: 2019-03-29 Created: 2019-03-29 Last updated: 2020-05-12Bibliographically approved
3. Spatial Variability of Shotcrete Thickness in Design of Rock Tunnel Support
Open this publication in new window or tab >>Spatial Variability of Shotcrete Thickness in Design of Rock Tunnel Support
(English)Manuscript (preprint) (Other academic)
National Category
Geotechnical Engineering
Identifiers
urn:nbn:se:kth:diva-249960 (URN)
Note

QC 20190617

Available from: 2019-04-24 Created: 2019-04-24 Last updated: 2020-04-27Bibliographically approved
4. Influence of spatially varying thickness on load-bearing capacity of shotcrete
Open this publication in new window or tab >>Influence of spatially varying thickness on load-bearing capacity of shotcrete
2020 (English)In: Tunnelling and Underground Space Technology, ISSN 0886-7798, E-ISSN 1878-4364, Vol. 98, article id 103336Article in journal (Refereed) Published
Abstract [en]

A common approach to verify a shotcrete layer's ability to secure blocks that can exist between rockbolts in a tunnel is to use analytical calculations. For this situation, an attractive approach to account for variability in the shotcrete parameters is to use reliability-based methods. Variability can then be accounted for by assigning suitable probability distributions to all relevant input parameters. Structural safety can be ensured by verifying that the probability of limit exceedance is smaller than an acceptable target probability of failure. However, even though analytical calculations and reliability-based methods can be used to design shotcrete support, one of the commonly made basic assumptions is that the load-bearing capacity of the shotcrete is governed by the spatial average of the input parameters. Thus, the spatial variability of the parameters are neglected. As a result, if the capacity is governed by the lowest value of a certain parameter, this assumption is non-conservative. In this paper, we present a novel approach in which the minimum of either the spatial average of a shotcrete slab of varying thickness, or the spatial average along the periphery of a loose block of that same slab, is used to estimate the load-bearing capacity of the shotcrete in a tunnel. The approach is based on results from numerical simulations of a shotcrete slab that we perform to investigate the effect that a spatially varying thickness has on the flexural load-bearing capacity of the slab. The results from the simulations show that the shotcrete's flexural load-bearing capacity might be overestimated when using the spatial average of shotcrete thickness between four rockbolts in design. Using the presented approach, the spatial variability of shotcrete thickness can be accounted for in practical design of tunnels without complex and time-consuming numerical simulations.

Place, publisher, year, edition, pages
Elsevier, 2020
Keywords
Rock engineering, Tunnel, Shotcrete, Reliability-based methods
National Category
Civil Engineering
Identifiers
urn:nbn:se:kth:diva-271498 (URN)10.1016/j.tust.2020.103336 (DOI)000518873900023 ()2-s2.0-85079122118 (Scopus ID)
Note

QC 20200422

Available from: 2020-04-22 Created: 2020-04-22 Last updated: 2020-04-27Bibliographically approved
5. Reliability-based design principles of shotcrete support for tunnels in hard rock
Open this publication in new window or tab >>Reliability-based design principles of shotcrete support for tunnels in hard rock
(English)Manuscript (preprint) (Other academic)
National Category
Geotechnical Engineering
Identifiers
urn:nbn:se:kth:diva-272721 (URN)
Note

QC 20200506

Available from: 2020-04-27 Created: 2020-04-27 Last updated: 2020-05-06Bibliographically approved
6. Reliability aspects of rock tunnel design with the observational method
Open this publication in new window or tab >>Reliability aspects of rock tunnel design with the observational method
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2017 (English)In: International Journal of Rock Mechanics And Mining Sciences, ISSN 1365-1609, E-ISSN 1873-4545, Vol. 98, p. 102-110Article in journal (Refereed) Published
Abstract [en]

According to Eurocode 7, two accepted approaches for managing uncertainty in tunnel design are reliability based methods and the observational method. Reliability-based methods account for uncertainty by acknowledging the random variation of the input parameters; the observational method does this by verifying the expected behavior from an initial design during the course of construction. However, in the framework of the observational method, as defined in Eurocode 7, no guidance is given on the selection of suitable parameters for observation and how they can be linked to the limits of acceptable behavior and, at a sufficiently early stage, the decision for implementing contingency actions. Furthermore, no guidance is given on how to verify that the structure fulfills society's required safety level. In this paper, we present a design procedure for shotcrete-supported rock tunnels that combines reliability-based methods with the observational method. The design procedure applies a deformation-based limit state function for the shotcrete support that is based on the convergence confinement method. We suggest how the requirements in the observational method, as defined in Eurocode 7, may be satisfied for this application. In particular, we focus on the structural reliability aspects. The structural reliability of the preliminary design is assessed with Monte Carlo simulations by calculating the expected deformations of the tunnel. The appropriateness of the preliminary design is then verified by observing the actual deformations during the course of construction. The observed deformations are used to predict the future behavior of the tunnel and to update the assessed probability of unsatisfactory behavior. If the defined deformation-based alarm limit regarding the structural reliability is exceeded, predefined contingency actions are put into operation. The procedure is illustrated with a shotcrete-lined circular rock tunnel and practical aspects in satisfying the reliability requirements with the observational method are discussed.

Place, publisher, year, edition, pages
PERGAMON-ELSEVIER SCIENCE LTD, 2017
Keywords
Rock engineering, Tunnel, Observational method, Reliability-based methods, Eurocode 7
National Category
Geotechnical Engineering
Identifiers
urn:nbn:se:kth:diva-215786 (URN)10.1016/j.ijrmms.2017.07.004 (DOI)000412037200010 ()2-s2.0-85030526478 (Scopus ID)
Note

QC 20171018

QC 20191008

Available from: 2017-10-18 Created: 2017-10-18 Last updated: 2020-04-27Bibliographically approved

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