Tunneling involves large uncertainties. Since 2009, design of rock tunnels in European countries should be performed in accordance with the Eurocodes. The main principle in the Eurocodes is that it must be shown in all design situations that no relevant limit state is exceeded. This can be achieved with a number of different methods, where the most common one is design by calculation. To account for uncertainties in design, the Eurocode states that design by calculation should primarily be performed using limit state design methods, i.e. the partial factor method or reliability-based methods. The basic principle of the former is that it shall be assured that a structure’s resisting capacity is larger than the load acting on the structure, with high enough 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 aim of this licentiate thesis is to provide a review of the practical applicability of using reliability-based methods and the partial factor method in design of rock tunnel support. The review and the following discussion are based on findings from the cases studied in the appended papers. The discussion focuses on the challenges of applying fixed partial factors, as suggested by Eurocode, in design of rock tunnel support and some of the practical difficulties the engineer is faced with when applying reliability-based methods to design rock tunnel support.

The main conclusions are that the partial factor method (as defined in Eurocode) is not suitable to use in design of rock tunnel support, but that reliability-based methods have the potential to account for uncertainties present in design, especially when used within the framework of the observational method. However, gathering of data for statistical quantification of input variables along with clarification of the necessary reliability levels and definition of “failure” are needed.

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.

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.