Table 2 (Concluded) 



Type of Failure Displacemenb, % Description 



Moderate 3-5 No gaps in top layer larger than 6 units; 



only slight displacements of bottom units 



Major 5-30 Top layer removed over a large area; bottom 



layer over not more than 2 units 



Total Over 30 Primary armor and underlayers removed over 



a large area with exposure of core 

 material 



57. This classification of prototype damages is realistic as far as 

 field reconnaissance of a damaged breakwater is concerned, but it departs 

 somewhat from the convention of detecting incipient damage in model tests. It 

 does not take into account any concrete armor units which have broken in 

 place. This inadequacy is compensated for, in part, by the displacement of 

 intact primary armor units being accompanied, in most instances, by concurrent 

 displacement of broken pieces. It is the exposure and, ultimately, the ero- 

 sion of underlayers and core that spell the actual failure of a rubble-mound 

 breakwater in the functional sense, with the exception of the case when a 

 monolithic crest element has been rendered ineffective. Field investigators 

 should also search for evidence of other modes of failure besides hydraulic 

 displacement, including sliding due to toe failure, excessive foundation set- 

 tlement, and seismic displacements. Classification of damages as a function 

 of both cause and effect is discussed in detail in Bruun (1985). 



58. Laboratory investigations, as pioneered by Iribarren (1938) and 

 Hudson (1958), typically attempt to identify the point of incipient damage. 

 Kogami (1978) defined this criterion as "...the condition in which the number 

 of armor units clearly recognized to have been moved or rocked on the cover 

 layer surface by wave actions was less than }% of the total of the units on 

 the forward cover layer...." The account of rocking implies that a precise 

 method of measuring the extent of rocking is available. Another interpreta- 

 tion relates to the point at which displacement has reached a depth in the 

 armor layer equal to the equivalent cube dimension of the armor units (Losada 

 and Gimenez-Curto 1979). Techniques developed by WES in the 1950's for mea- 

 suring model breakwater displacements with before and after soundings have 

 been estimated to have a resolution (repeatability) of ±2 percent (Carver 

 1983). Identification of incipient damage with this commonly used method 



36 



