Probabilistic design of breakwaters, the Ennore breakawater project: Failure mechanisms of a crested rubble mound breakwater

Breakwaters differ from each other by their functions or their type. The most important function of breakwaters is to allow the ships to be loaded and unloaded in calm water conditions and to provide dock or quay facilities. Several types of breakwaters can be distinguished, depending on their operation and design. The most common breakwater concepts are divided into two types: statically stable and dynamically stable breakwaters. The site variables, set by the situation at Ennore, concerne the geotechnical aspects, the construction material and the hydraulic boundary conditions. The hydraulic boundary conditions will be determined by the wave climate during extreme weather conditions, which is during cyclones. Cyclones arrive on the east coast of India usually with a return period of 2 years. Wave motions in harbours can, under extreme conditions, endanger the handling of vessels at berth. Therefore a certain degree of tranquillity of the harbour basin is required. The tranquillity of the harbour basin is essential for ships to load and unload in calm water conditions. Too high waves in the harbour basin will cause an unsafe handling of ships. Should the coal handling cease at Ennore, then the storage of coal will run out, leading to the breakdown of the power station. The breakwater structure has to reduce the wave heights in the harbour by adequate layouts. By analysing the failure mechanisms of all the different parts of the breakwater, reliability functions can be formed and the probability of failure, according to the various mechanisms, can be calculated. These reliability functions, described by each failure mechanism, are mainly based on hydraulic and geotechnical parameters. This study attempts to survey all imaginable failure mechanisms that may occur during the design, construction, and lifetime of the crested rubble mound breakwater at Ennore. In order to address the problems encountered systematically, a general faulttree has been given. The quantification of some failure mechanisms for the crested rubble mound breakwater at Ennore is effected for the following failure mechanisms: - instability of a rock armour slope - instability of an armour layer with ACCROPODE elements - slip circle of a rock armour layer - horizontal displacement of the crest element. The quantification of these failure mechanisms is a complex task which most important difficulty resides in the determination of the mean value and the standard deviation of the variables. After the calculation, an adjustment of these values may take place in order to improve the design.Hydraulic EngineeringCivil Engineering and Geoscience