The design methodology proposed in the Recommendations for Maritime Works (ROM, Puertos del Estado, Spain) differentiates failure modes between hydraulic and geotechnical on one hand and between principal and non-principal on the other hand. In general, design of the scour protection in front of a breakwater is performed assuming a non-principal hydraulic failure mode, as it is possible to achieve negligible failure probabilities for this element under moderate costs.
The mid Río de la Plata estuary presents a combination of shallow waters, severe currents and moderate waves, together with very low bearing capacity soils. These conditions make that scour protection of a projected breakwater off the coast of Montevideo, Uruguay, should be designed assuming a principal hydraulic failure mode that in turn affects geotechnical failure modes.
This is a rare situation for which there are few references in the accumulated experience of breakwater design. Perhaps the most relevant precedent is the design and construction of Zeebrugge breakwaters (Van Damme et al. 2008). Moreover, local knowledge on the actual port of Montevideo is of little use given that current breakwaters were built more than 100 years ago (Nieto 2012).
Under these conditions the design of the scour protection layer presents several challenges, which are presented below and that will be discussed in detail in the final article.
The first challenge arises when defining the conceptual approach for design. As there is interaction between the hydraulic failure mode "loss of hydraulic stability of the scour protection layer" (which eventually leads to erosion in front of the structure) and the geotechnical failure mode “global stability of the breakwater”, both cannot be performed decoupled. The methodologies used for hydraulic and geotechnical verifications are fundamentally different, since the temporal scales in which the failures develop, the uncertainties involved in their approach and the analytical and numerical models used in each case are different. Probabilistic verification techniques are usually an appropriate tool for approaching complex problems of great economic impact and are quite well developed for the verification of hydraulic failure modes. However, they are not so well developed for the verification of geotechnical failure modes (see e.g. Phoon et al., 2016).
From the point of view of calculation and verification of the hydraulic failure modes there are at least three challenges. First, the multivariate characterization of all the random variables involved in the verification, namely: wave (incident wave height, direction, and period), currents (depth averaged intensity and direction), and sea level. Although usually assumed deterministically, two coefficients must be added to this list of random variables: the depth limited wave breaking coefficient and the breakwater reflection coefficient. The intensity of the bottom stresses in front of the structure, responsible for triggering the "loss of hydraulic stability of the scour protection layer", will depend on all these variables. Second, the determination of an equation that relates all these variables at the initiation of damage (i.e. a verification equation). Currently there are no accepted equations for the design verification of scour protection layers subjected to the combined action of currents and waves (incident and reflected, possibly depth limited, i.e. highly non-linear). The most similar situation for which there are accepted design equations is the start of the movement of the sand under combined wave-current flow. Obviously the use of these equations in the design of the scour protection involves great uncertainties, which must necessarily be taken into account in the final design. Third, given the uncertainties inherent in the design process, it is common practice to perform reduced scale model test of the breakwater in a hydraulic laboratory prior to its construction. Physically modeling the interaction of waves, currents and a partially reflecting structure, and its effect on the scour protection layer, presents great challenges for both its implementation in the laboratory and the interpretation of the obtained results, in the latter case mainly due to scale effects.
In dealing with the above challenges the following approaches were thoroughly discussed during the preparation of the project:
To reduce the probability of failure of both failure modes independently to very low levels, in order to move them away from the failure tree of the principal modes. It was proven that for the circumstances of this project this approach was not feasible, due to: incompatibilities with construction planning, and the high costs involved in the protection. The use of higher caliber material in the scour protection easily creates construction difficulties in view of the respect of the filter rules towards the soil material. In order to accommodate construction constraints, for the design of the filters, the application of open filter layers as well as the installation of geotextiles (fixed to willow matrasses) and classical filter layers have been discussed.
To artificially separate geotechnical and hydraulic failure analysis by introducing the concept of a minimum geometry in the geotechnical calculations, based on alarm and limit lines, as was the case for the breakwater in Zeebrugge. Such approach needs to be made consistent with the inspection and maintenance strategy applicable for the project. An alarm line defines a level at which the operator can start the mobilization process of the equipment that is required to safeguard the situation before the ultimate limit line is exceeded. This limit line should correspond with the level of safety that has been required for this geotechnical failure mode. From the operation point of view it is essential that clear alarm and limit line drawings are developed to support the inspection and maintenance strategy for a project.