Stone sizes are given by weight in Figures 7-116 and 7-117 since the armor 



in the cover layers is selected by weight at the quarry, but the smaller stone 



sizes are selected by dimension using a sieve or a grizzly. Thomsen, Wohlt, 



and Harrison (1972) found that the sieve size of stone corresponds approxi- 



W 1/3 

 mately to 1.15 — , where W is the stone weight and w„ is the stone 



■' Wp ' , & 30 



unit weight, both in the same units of mass or force. As an aid to under- 

 standing the stone sizes referenced in Figures 7-116 and 7-117, Table 7-12 

 lists weights and approximate dimensions of stones of 25.9 kilonewtons per 

 cubic meter (165 pounds per cubic foot) unit weight. The dimension given for 

 stone weighing several tons is approximately the size the stone appears to 

 visual inspection. Multiples of these dimensions should not be used to 

 determine structure geometry since the stone intermeshes when placed. 



A logic diagram for the preliminary design of a rubble structure is shown 

 in Figure 7-118. The design can be considered in three phases: (1) structure 

 geometry, (2) evaluation of construction technique, and (3) evaluation of 

 design materials. A logic diagram for evaluation of the preliminary design is 

 shown in Figure 7-119. 



As part of the design analysis indicated in the logic diagram (Fig. 7- 

 118), the following structure geometry should be investigated: 



(1) Crest elevation and width. 



(2) Concrete cap for rubble-mound structures. 



(3) Thickness of armor layer and underlayers and number of armor 

 units. 



(4) Bottom elevation of primary cover layer. 



(5) Toe berm for cover layer stability. 



(6) Structure head and lee side cover layer. 



(7) Secondary cover layer. 



(8) Underlayers. 



(9) Bedding layer and filter blanket layer. 



(10) Scour protection at toe. 



(11) Toe berm for foundation stability. 



(1) Crest Elevation and Width . Overtopping of a rubble structure 

 such as a breakwater or jetty usually can be tolerated only if it does not 

 cause damaging waves behind the structure. Whether overtopping will occur 

 depends on the height of the wave runup R . Wave runup depends on wave 

 characteristics, structure slope, porosity, and roughness of the cover 

 layer. If the armor layer is chinked, or in other ways made smoother or less 

 permeable — as a graded riprap slope — the limit of maximum riprap will be 



7-229 



