On Methods of Establishing Design Diagrams for Structural Integrity of Slender Complex Types of Breakwater Armour Units

Many of the recent dramatic failures of a number of large rubble mound breakwaters armoured with Dolosse and Tetrapods were caused by breakage of the concrete armour units. Breakage took place before the hydraulic stability of intact units in the armour layers expired. Thus there was not a balance between the strength (structural integrity) of the units and the hydraulic stability (resistance to displacements) of the armour layer. Although the relative strength of the units decreases with increasing size (Burcharth 1980) the shape of the units was kept constant and not related to the size of the units and the size was not increased beyond the demand dictated by the hydraulic stability.While the hydraulic stability can be roughly estimated by formulae and further evaluated in conventional hydraulic model tests, it is much more complicated to assess the structural integrity of the armour units.The first methods for the estimation of the structural integrity of the slender type of armour units appeared few years ago. A method based on similarity consideration and full scale impact tests was developed for units mainly exposed to impact loads (Burcharth 1981). Instrumented small scale model armour units to measure bending movements in a cross section of the units and the accelerations (impact speed) were first used by DHL in 1980. Recent developments were presented by Scott et al. 1986. Correct scaling of the most important material properties of concrete in small scale hydraulic model tests was first presented by NRC (Timco 1981 ). Work based on direct strain-gauge measurement of stresses on large concrete model armour units was presented by Nishigori et al. in 1986.So far, none of the mentioned methods have produced general applicable tools for the design of armour layers. This is mainly because the first mentioned method is related only to impact dominated units and the other methods have been used only for checking specific designs and not yet for the development or support of general design methods.Consequently there is a need for an approach by which more general information and guidelines for the design of slender armour units can be developed. This paper deals with a general method for the development of design tools for slender type of armour units. The first part of the paper presents a general explanation of the method. It is planned to present a second part showing an application of the method based on the results obtained from the extensive prototype research program with 42 t Dolosse at Crescent City performed by the U.S. Army Corps of Engineers WES Vicksburg (Howell 1986). The reason for and the object of this early presentation of the method is that, if accepted by other researchers, it might be a frame for a uniform presentation of results. Without a uniformity it will take much longer before the oncoming data on structural integrity can be made useful through contributions to the future data bank which will be needed as a design tool.