Empirical formula for breakage of Dolosse and Tetrapods

The slender, complex types of armour units, such as Tetrapods and Dolosse a re widely used for rubble mound breakwaters. Many failures of such breakwat ers were caused by unforeseen early breakage of the units, thus revealing a n inbalance between the strength (structural integrity) of the units and th e hydraulic stability (resistance to displacements) of the armour layers. B reakage occurs when the stresses from the static, pulsating and impact load s exceeds the tensile strength of the concrete. While the hydraulic stability can be studied in Froude-scale hydraulic mode l tests, it is not possible to study armour unit stresses in small scale mo dels. This is partly because the strain in model armour units are too small to be recorded, and partly because the scaling law for impact load generat ed stresses is nonlinear. The paper discusses the scaling laws related to type of stresses and presen ts a method which allows studies of armour unit stresses by means of a load -cell technique. The technique necessitates impact load response calibratio n of the load-cell mounted model armour units against the equivalent respon se of prototype or large scale armour units. The procedure followed was pre sented by Burcharth and Liu (Burcharth, H.F., Liu, Z., 1992. Design of Dole s armour units. In: Proceedings of the 23rd International Conference on Coa stal Engineering, Venice, Italy.) and Burcharth (Burcharth, H.F., 1993. Str uctural integrity and hydraulic stability of Doles armour layers. Series Pa per 9, published by the Department of Civil Engineering, Aalborg University , Denmark, 1993.), who also presented design diagram for determination of b reakage of Dolosse in trunk sections. The paper presentes an expansion of t his work to include breakage of Dolosse in round-heads and Tetrapods in tru nk sections. The paper presents a simple dimensional empirical formula instead of diagra ms for the estimation of the number of broken Dolosse and Tetrapods in prot otype situations, because probabilistic design of breakwaters requires fail ure mode formulae with the associated uncertainties. (C) 2000 Elsevier Scie nce B.V. All rights reserved.