A reliability method (FOSM) has been used to assess the probability of sliding failure of the 33 years-old island caisson breakwater sheltering an oil terminal at Gela (Sicily). The vertically composite structure is placed in just 13 m depth upon a weak silty seabed. It has experienced settlements of about 1 m, toe scour around 2 m and degradation of the slender reinforced superstructure, especially damaged after a severe storm on 24 November 1991. On the basis of new detailed wave climate studies and field investigations a modern probabilistic analysis has been performed to verify the actual risk of failure, particularly against sliding over the rubble foundation due to wave loads. A new formulation of the failure function as been derived introducing new expressions for the pressure coefficients in the Goda formula. A detailed sensitivity analysis of the failure probability with the caisson geometry has also been performed.
Conventional breakwater design in coastal engineering is mostly based on a deterministic approach, i.e. the design load is compared with the resistance of the structure. The assessment of the probability of failure is only performed by studying the probability distribution of the most important loading variable, the wave height (Franco et al., 1986; Lamberti, 1992; Burcharth, 1992; Van der Meer et al., 1995). On the contrary, the real behaviour of the structure is much more complex, mainly because other uncertainties are also due to those random variables which express the mechanical characteristics of the structure, hence its reliability. Therefore both a PIANC working group (PTC II-n. 28) and a EU-funded research project (MAST III Proverbs)are now being devoted to the probabilistic analysis of vertical breakwaters. A first attempt was recently done by Christiani et al. (1996)
