Probabilistic design of Core-loc armoured breakwaters

Deterministic design methods are commonly used to determine preliminary breakwater designs. Partial safety factors take into account previous experiences and provide a robust preliminary design. However, local circumstances can prove to differ considerably compared to average design conditions and stochastic variations in breakwater strength parameters are commonly neglected. With new armouring techniques, such as Core-loc armouring, the uncertainties about the armour strength are relatively large. Design guidelines include a safety factor, but often an additional safety margin is applied in the final design of the armouring to ensure stability. This can result in structure strengths more, or less, than locally required. The economic optimum geometry with the lowest costs is possibly not achieved. These costs consist of the initial construction cost, the collapse damage cost and the economic damage cost due to downtime. To include the damage cost or risk (= failure probability x economic consequence) of breakwater collapse and functional failure, a probabilistic approach can be used to determine the failure probabilities. In Veracruz, Mexico, the port authority of the Port of Veracruz investigates the feasibility of a large port extension next to the existing port of Veracruz. In the preliminary layout a Core-loc armoured breakwater is anticipated to provide shelter at a container terminal and quay location. Deterministic design methods result in an element weight of 18.7t (8.5m3). Two construction methods are evaluated: a water-based and a land-based construction method, with crest heights of 3m +SWL and 11m +SWL respectively. In this deterministic evaluation the economic consequences of functional failure are not taken into account, but both alternatives fulfilled the harbour tranquillity restrictions by the port authority: a maximum downtime of 5%. The water-based construction method is elected as the best construction method, due to lower construction costs of 110.7 $ million. The deterministic breakwater design is optimised with a probabilistic method for the most important parameters: the weight of the Core-loc elements and the breakwater crest height. And a progressive deterioration over time of the strength of the Core-loc armour is taken into account. The probabilities of collapse and functional failure of the breakwater and the economic consequences of failure are determined for 56 combinations of element weight and crest height. The probability of collapse is composed of two failure mechanisms: the Core-loc armour and the toe structure. The probabilities of failure and economic consequences are time dependent, due to the sea level rise, the deterioration of the breakwater armour and the economic development of the port over the lifetime of 50 years. Therefore, all alternatives have different probabilities of failure for each year. Discounting all costs to a single year the economic optimal design geometry over the total lifetime proves to have a Core-loc element weight of 30.8t (14m3) and a crest height of 7 m +SWL. The construction costs of this geometry are 153 $ million. A crest height of 7m +SWL complies with an allowable downtime of approximately 0.2%. The downtime costs are of considerable more influence than estimated by the port authority. Also the consequences of a breakwater collapse result in a 65% heavier element weight. The discounted total costs over the lifetime of the breakwater are 219 $ million for the probabilistic design and 468 $ million for the deterministic design. The collapse costs and downtime costs have a significant influence on the total costs over the lifetime and therefore on the economic optimal geometry of the breakwater. A more robust design than deterministically derived can reduce the total cost over the lifetime by almost 50%.Civil Engineering and Geoscience