Reliability based Assessment of Quay Walls

Abstract

Uncertainties in the soil parameters play a major role in the design of quay walls. In the current design approach, partial factors are prescribed to account for uncertainties in the soil, as well as for other types of uncertainties. This semi-probabilistic (level I) design approach needs to result in a reliable design for a range of quay structures and for multiple soil stratifications. It is therefore expected that, in general, this method results in overdimensioning of the structure. Whether this assumption holds, is investigated in this thesis by carrying out a reliability analysis for two quay walls in the Port of Rotterdam. The first case study covers a simple double-anchored combi-wall, whereas in the second case study a quay wall with relieving platform is considered.
Only the most relevant failure mechanisms were considered, which are yielding of the combi-wall, yielding of the anchor bar, shear failure of the grout body and soil mechanical failure. These failure mechanisms are complex soil-structure interaction problems. Therefore, both the soil and the quay structure have been modelled with the finite element program Plaxis 2D, using the Hardening soil model. For performing the probabilistic calculations on this model, the probabilistic module ProbAna has been used. This is a package developed by Plaxis which couples several types of reliability methods to the finite element software of Plaxis 2D. As the computational effort is relatively large when using FEM, the First Order Reliability Method (FORM) was used over sampling methods like Directional Sampling and Crude Monte Carlo simulation.
The results for the simple quay wall showed that the reliability level was sufficient for all considered limit states. Hence, almost all partial factors derived on this quay wall were lower than currently prescribed by the Eurocode. In the second case study, a quay wall with relieving platform, monitoring data of multiple years was used for calibration of the Plaxis-model. Thereafter, the reliability for the limit states yielding of the wall and soil mechanical failure was evaluated. It turned out that for both limit states, the reliability index was too low compared to the target reliability.
Although each case study concerned a different type of quay wall, the results reveal that choices made in the design, either optimistic or pessimistic, can have large influence on the reliability. Perhaps just as important, are the assumptions made regarding the stochastic description of the soil. It is still under discussion up to what distance soil parameters are correlated in space and how spatial averaging should be applied. Reference calculations showed that choices regarding the amount of independent soil layers and the degree of spatial averaging have a large influence on the reliability. More fundamental research to these topics is therefore recommended.