Evaluation of new Eurocode ‘prEN1991-1-8’ on wave and current actions

Abstract

The Eurocodes are design codes, widely accepted throughout Europe, that provide guidelines on how to deal with loads, resistance, and uncertainty. For most type of structures, loads and materials, the Eurocode has been well developed. However, for the loads on coastal structures, caused by wave, water level and current actions, no such Eurocode exists. Therefore, a new Eurocode is in progress. The preliminary version of this Eurocode is titled prEN1991-1-8, and will describe how to deal with hydraulic loads on coastal structures.
The Eurocode has a safety philosophy based upon partial factors and consequence classes, but the extension of this method to hydraulic engineering is not so straightforward. A premature introduction of the new Eurocode could mean that important design aspects related to sea condition parameters, such as extreme value analysis, dependence and wave transformation, are underexposed. In turn, this might give way to unsafe structures. Because of this, it is important to investigate how the semi-probabilistic approach (proposed by prEN1991-1-8) compares to existing design methods, both in terms of safety and in terms of ease of application. The goal of this thesis is to acquire knowledge on how the introduction of the new Eurocode influences the design of coastal structures, and address any inconsistencies and issues in prEN1991-1-8 before the preliminary version is adopted as the definitive version.

The main focus of the research is the re-design of a coastal structure following the instructions in prEN1991-1-8. The breakwaters of IJmuiden are used as a case study for this. Three different approaches are adopted for the design: a deterministic approach, a semi-probabilistic approach and a full probabilistic approach. The breakwater elements that have been extensively examined in this thesis are the armour layer, consisting of either rock or artificial units, the crest height and the crown wall.
Conclusions have been drawn based on any unclarities that are encountered in the design process, and based on comparisons between the semi-probabilistic approach and the two existing design methods, respectively.

The introduction of the new document prEN1991-1-8 should result in more conformity regarding the treatment of sea condition parameters and their accompanying uncertainties in design calculations. However, the new (draft) Eurocode in its current form does not seem to achieve this objective. This conclusion has been drawn based on the practical issues (i.e. room for interpretation in the document and poor descriptions of key concepts) that were encountered in the case study, and because the breakwater design that was arrived at by following the semi-probabilistic approach described in prEN1991-1-8, did not resemble the design outcome obtained with the deterministic and full probabilistic approach.

In addition, the semi-probabilistic approach is still a time-consuming method when applied to breakwater elements, while it is relatively easy to set up a full probabilistic calculation. It is recommended to shift (at least part of) the attention towards the full probabilistic approach, and explore the possibilities of making it the default approach for coastal structures, as it raises fewer questions than the semi-probabilistic approach and deals with uncertainties more extensively.

Nevertheless, it can be viewed as a positive development that there will be one general document to consult for the design of coastal structures. Most importantly, standardised levels of safety and return periods are now available with the introduction of prEN1991-1-8, even though their interpretations are not always straightforward. It is of course also still possible to improve the semi-probabilistic approach as it is currently proposed by prEN1991-1-8. When this is preferred, it is recommended to include a more systematic explanation of the DA-1 format in EN1991-1-8, describing the characteristic values, partial factors and safety margins to be adopted in design without any ambiguity.