Empirical Formulae for Breakage of Dolosse and Tetrapods

The slender, complex types of armour units, such as Tetrapods and Dolosse are widely used for rubble mound breakwaters. Many failures of such breakwaters were caused by unforeseen early breakage of the units, thus revealing an inbalance between the strength (structural integrity) of the units and the hydraulic stability (resistance to displacements) of the armour layers. Breakage occurs when the stresses from the static, pulsating and impact loads exceeds the tensile strength of the concrete. While the hydraulic stability can be studied in Froude-scale hydraulic model tests, it is not possible to study armour unit stresses in small scale models. This is partly because the strain in model armour units are too small to be recorded, and partly because the scaling law for impactload generated stresses is nonlinear. The paper discusses the scaling laws related to type of stresses and presents a method which allows studies of armour unit stresses by means of a load-cell technique. The technique necessitates impact load response calibration of the load-cell mounted model armour units against the equivalent response of prototype or large scale armour units. The procedure followed was presented by Burcharth and Liu Burcharth, H.F., Liu, Z., 1992. Design of Dolos armour units. In: Proceedings of the 23rd International Conference on Coastal Engineering, Venice, Italy.. and Burcharth Burcharth, H.F., 1993. Structural integrity and hydraulic stability of Dolos armour layers. Series Paper 9, published by the Department of Civil Engineering, Aalborg University, Denmark, 1993.., who also presented design diagram for determination of breakage of Dolosse in trunk sections. The paper presentes an expansion of this work to include breakage of Dolosse in round-heads and Tetrapods in trunk sections. The paper presents a simple dimensional empirical formula instead of diagrams for the estimation of the number of broken Dolosse and Tetrapods in prototype situations, because probabilistic design of breakwaters requires failure mode formulae with the associated uncertainties.