Evaluation and Implementation of Model Uncertainties in Geotechnical Design Presenter: Prof. Dr.-Ing. Kerstin Lesny HafenCity University  Hamburg  Germany Geo-Risk 2017, Denver C20a/ Reliability- and Risk-Based Code Developments, Part I

Members of the TC205/TC304 discussion group Sami Oguzhan Akbas Witold Bogusz Sébastien Burlon Kerstin Lesny (discussion leader) Giovanna Vessia Kok Kwang Phoon Chong Tang Limin Zhang Discussion report available on the ISSMGE website: Comments are welcome! http://140.112.12.21/issmge/TC205_304_reports/ (Quelle: www.kunterbuntich.de) Geo-Risk 2017  Denver/Colorado

Definition of Model Uncertainty Model: calculation model, not the whole geotechnical model including e.g. the ground model Type of model: analytical, empirical, semi-empirical closed-form solutions numerical (e.g. FE) model Deviation of: ideal situation real behavior calculated behavior calculated behavior real behavior Geo-Risk 2017  Denver/Colorado

Model Uncertainty Assessment Model factor approach: applicable only where a unique design quantity X exists Xmeas to be collected from a load test database which needs to cover the whole range of possible design situations and shall fulfill specific requirements to provide sufficient information. Appropriate methods for interpretation of load-displacement curves for ULS and SLS problems! MF = Xmeas/Xcal or 1/MF = Xcal/Xmeas   with: MF = model factor or bias X = load, a resistance, a displacement, etc. Geo-Risk 2017  Denver/Colorado

Model Uncertainty Assessment Representative quantity approach: Examples: head displacements of a laterally loaded flexible pile, top displace-ments of retaining walls, lateral strain in neighboring buildings due to excavation reduction of model uncertainty to a single quantity questionable in case of complex structures, e.g. retaining walls Two-step approach: Combination of numerical analyses and load test results to establish a generalized model factor for specific design model. MF = Xmeas/Xcal or 1/MF = Xcal/Xmeas   same as before, but: X = representative quantity characterizing the behavior of the structure Geo-Risk 2017  Denver/Colorado

Model Uncertainty Assessment Intrinsic uncertainties: ground conditions: spatial variabilty consideration of subsoil characteristics within design method (degree of simplification) determination of soil parameters (correlations to field tests or lab tests) load test execution: measurement errors, interpretation and evaluation of load test results, selection of load tests for database, lack of information, personal experience Cannot be eliminated! Implementation in design possible: as a random model factor in RBD as a deterministic model factor in LRFD (but not necessarily a constant value!) Geo-Risk 2017  Denver/Colorado

Our Wish List on the Path to an Ideal Code to further close the gap between theoretically based, complex approaches and the daily design practice of 'normal' geotechnical engineers, i.e. to provide a bridge between different 'languages speaking' or different 'ways of thinking’ to promote “ease of use” to promote gathering and to provide later access to data of successes and failures of geotechnical structures to learn and to verify our design models Geo-Risk 2017  Denver/Colorado

Conclusions model uncertainties to be evaluated using extensive databases covering all possible design situations under natural, but also under laboratory conditions intrinsic uncertainties (e.g. soil variability, measurement errors etc. ) can hardly be seperated use of deterministic (not constant) model factors in LRFD formats is possible, at least for standard design situations Proper transformation and acceptance in the engineering practice important! Geo-Risk 2017  Denver/Colorado