A Critical Review of the Observational Method
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A Critical Review of the Observational Method Johan Spross Licentiate Thesis Department of Civil and Architectural Engineering Division of Soil and Rock Mechanics KTH Royal Institute of Technology Stockholm, 2014 TRITA-JOB LIC 2024 ISSN 1650-951X PREFACE The research presented in this licentiate thesis was carried out during 2012 and 2013 at the Division of Soil and Rock Mechanics, Department of Civil and Ar- chitectural Engineering, at KTH Royal Institute of Technology in Stockholm, Sweden. The work was supervised by Prof Stefan Larsson and Dr Fredrik Johansson. I owe them much gratitude for their constant support and encouragement, not forgetting their valuable comments on my work. The input from my reference group is also gratefully acknowledged. In addition, I would like to thank my col- leagues at the Division of Soil and Rock Mechanics for many rewarding discus- sions, in particular Prof Em Håkan Stille, who initiated my Ph.D. project, and Dr Rasmus Müller, who made me dive far deeper into statistics than I ever in- tended. Last, but not least, our wonderful administrative staff must be acknowl- edged for guiding through the university bureaucracy and helping out with all practical issues. Stockholm, March 2014 Johan Spross iii FUNDING ACKNOWLEDGEMENT The research presented in this thesis was co-funded by the Swedish Hydropower Centre (SVC), the BeFo Foundation and the Swedish Research Council for Envi- ronment, Agricultural Sciences, and Spatial Planning (FORMAS). Their support is gratefully acknowledged. SVC has been established by the Swedish Energy Agency, Elforsk, and Svenska Kraftnät, together with Luleå University of Technology, KTH Royal Institute of Technology, Chalmers University of Technology, and Uppsala University. Parti- cipating companies and industry associations are: Alstom Hydro Sweden, Andritz Hydro, E.ON Vattenkraft Sverige, Falu Energi & Vatten, Fortum Generation, Holmen Energi, Jämtkraft, Jönköping Energi, Karlstads Energi, Mälarenergi, Norconsult, Pöyry SwedPower, Skellefteå Kraft, Sollefteåforsens, Statkraft Sve- rige, Sweco Energuide, Sweco Infrastructure, SveMin, Umeå Energi, Vattenfall Research and Development, Vattenfall Vattenkraft, Voith Hydro, WSP Sverige, and ÅF Industry. Lastly, Philips stiftelse is acknowledged for supporting with a travelling grant. iv ABSTRACT Building a sustainable structure in soil or rock that satisfies all predefined tech- nical requirements implies choosing a rational and effective construction method. An important aspect is how the performance of the structure is verified. For cas- es when the geotechnical behaviour is hard to predict, the existing design code for geotechnical structures, Eurocode 7, suggests the so-called “observational method” to verify that the performance is acceptable. The basic principle of the method is to accept predefined changes in the design during construction, in or- der to accommodate the actual ground conditions, if the current design is found unsuitable. Even though this in theory should ensure an effective design solution, formal application of the observational method is rare. It is therefore not clear which prerequisites and circumstances that must be present for the observational method to be applicable and be the more suitable method. This licentiate thesis gives a critical review of the observational method, based on, and therefore limited by, the outcome of the performed case studies. The aim is to identify and highlight the crucial aspects that make the observational method difficult to apply, thereby providing a basis for research towards a more applica- ble definition of the method. The main topics of discussion are (1) the apparent contradiction between the preference for advanced probabilistic calculation methods to solve complex design problems and sound, qualitative engineering judgement, (2) the limitations of measurement data in assessing the safety of a structure, (3) the fact that currently, no safety margin is required for the complet- ed structure when the observational method is applied, and (4) the rigidity of the current definition of the observational method and the implications of deviations from its principles. Based on the review, it is argued that the observational method can be improved by linking it to a probabilistic framework. To be applicable, the method should be supported by guidelines that explain and exemplify how to make the best use of it. The engineering judgement is however not lost; no matter how elaborate probabilistic methods are used, sound judgement is still needed to define the problem correctly. How to define such a probabilistic framework is an urgent topic for future research, because this also addresses the concerns regarding safe- ty that is raised in the other topics of discussion. v SAMMANFATTNING För att i berg eller jord kunna konstruera en anläggning, som uppfyller satta tek- niska krav, krävs det att man väljer en rationell och effektiv konstruktionsmetod. En viktig aspekt i detta val är hur man verifierar konstruktionens funktion avse- ende exempelvis bärförmåga eller stadga. För fall när konstruktionens beteende svårt att förutsäga, erbjuder gällande standard (Eurokod 7) den så kallade obser- vationsmetoden. Denna metod tillåter i förväg förberedda förändringar i desig- nen under konstruktionstiden, om observationer av konstruktionens beteende indikerar att så behövs. På så vis anpassas konstruktionen till de faktiska förhål- landena i marken. Trots att detta tillvägagångssätt i teorin borde ge en rationell design, används metoden sällan. Det råder därför oklarheter om vilka förutsätt- ningar och omständigheter som krävs för att observationsmetoden ska kunna användas och dessutom utgöra den bästa lösningen. I denna licentiatuppsats granskas observationsmetoden och dess användbarhet. Målet med licentiatuppsatsen är att belysa de aspekter som kan utgöra svårigheter när observationsmetoden används. Dessa identifierades under arbetet med några fallstudier. Licentiatuppsatsen ger därmed en utgångspunkt för fortsatt forskning för att ta fram en mer användbar definition av observationsmetoden. De viktig- aste aspekterna som diskuteras i uppsatsen är (1) den skenbara motsatsen mellan användandet av sannolikhetsbaserade beräkningsmetoder för att lösa komplexa dimensioneringsfrågor och kvalitativa ingenjörsmässiga bedömningar, (2) de be- gränsningar som finns när man använder mätdata för att utvärdera konstruktion- ers säkerhet, (3) att det för tillfället saknas krav på säkerhetsmarginal mot brott för konstruktioner som byggts med observationsmetoden, och (4) vad svårighet- en att uppfylla Eurokodens strikta definition innebär för metodens användbarhet. Utifrån resultatet av granskningen dras slutsatsen att observationsmetoden kan förbättras genom att ge den ett sannolikhetsbaserat ramverk. För att förenkla användningen bör riktlinjer och anvisningar utformas. Även om metoden utveck- las mot en högre grad av beräkningskomplexitet, kommer ingenjörsmässiga be- dömningar också framgent att vara viktiga, eftersom en avgörande aspekt är hur problemställningen formuleras. Med ett sannolikhetsbaserat ramverk ökar möj- ligheten att lösa de frågeställningar kring säkerhet som också diskuteras i uppsat- sen. vii LIST OF PUBLICATIONS Paper I Spross, J. & Larsson, S. In press. On the observational method for groundwater control in the Northern link tunnel project, Stockholm, Sweden. Bulletin of Engi- neering Geology and the Environment. DOI: 10.1007/s10064-013-0501-8. I made the case study and wrote the paper. Larsson initiated and supervised the work and as- sisted with comments on the writing. Paper II Spross, J., Johansson, F. & Larsson, S. 2013. Reducing uplift pressure uncertainty with measurements under concrete dams. In Proceedings of the ICOLD 2013 Interna- tional Symposium, Seattle, 14 August 2013, 2551-2560. Denver: US Society on Dams. I adjusted the methodology to dam safety applications, and wrote the paper, supervised and as- sisted by Johansson and Larsson. Paper III Spross, J., Johansson, F. & Larsson, S. 2014. On the use of pore pressure meas- urements in safety reassessments of concrete dams founded on rock. Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards, 8(2), 117- 128. DOI: 10.1080/17499518.2013.864172. I and Johansson developed the methodology together. I made the case study and wrote the paper, supervised and assisted by Johansson and Larsson. Paper IV Spross, J., Johansson, F., Stille, H. & Larsson, S. 2014. Towards an improved ob- servational method. Accepted by the European Rock Mechanics Symposium (EU- ROCK) 2014, Vigo, Spain, 27-29 May 2014. I and Johansson developed the methodology, assisted by Stille. I made the calculations and wrote the paper, supervised and assisted by Johansson and Larsson. ix Other publications Within the framework of this research project, I have also contributed to the fol- lowing publications. However, they are not part of this thesis. Müller, R., Larsson, S. & Spross, J. 2014. Extended multivariate approach for uncertainty reduction in the assessment of undrained shear strength in clays. Ca- nadian Geotechnical Journal, 51, 231-245. Spross, J., Johansson, F. & Larsson, S. 2013. Tillståndsbedömning av betong- dammar grundlagda på berg med observationsmetoden. [Safety reassessment of concrete dams founded on rock with the observational method]. Bygg & Teknik, 105(1), 83-87. x CONTENTS 1 INTRODUCTION ................................................................................................... 1