to simulate the type o£ placing technique used in construction of the pro- 

 totype structure. Most armor units are randomly placed without attempting 

 to obtain an interlocking action between units. Before each series of 

 tests, each newly constructed test section is subjected to a few waves of 

 sufficient height to provide a slight shaking down and consolidation of 

 the armor units and underlayer material. This is done under the assun^- 

 tion that a prototype structure will be subjected to waves of progres- 

 sively greater height at the outset of a given storm, and that it will be 

 unusual for the design storm wave or one of greater height to occur imme- 

 diately after construction has been completed. 



The preparation of the material for construction of scale models of 

 rubble-mound structures, except for the core material, is tedious, time 

 consuming, and because of the labor involved, expensive. After the core 

 material has been crushed, it is sized by a power-shaker type of seive 

 assembly. Some of the smaller sizes of the underlayer material are also 

 sized by the power shaker, with the unusually long pieces removed by hand. 

 Model armor units that simulate full-scale quarrystone armor units are 

 usually crushed basalt or limestone and handpicked and sized with a chip- 

 ping hammer and weighed on a torsion balance with a sensitivity of one- 

 tenth of a gram. Model armor units that simulate full-scale concrete 

 armor units of special shape (tetrapods, tribars, dolosse, etc.) are 

 molded of aluminum-magnesium alloy, zinc-magnesium alloy, polyester fiber- 

 glass premix material, or a mixture composed of sulfur, silicon, and barite 

 or sand. A typical mixture is composed of 90 percent Cylcap (a commercial 

 name for a sulfur-silicon compound) and 10 percent barite, or 78 percent 

 Cylcap and 23 percent Ottawa sand. Units molded with this mixture have 

 a specific weight of about 140 pounds per cubic foot. This mixture must 

 be heated to a temperature between 110° and 116° Celsuis C230° and 240° 

 Fahrenheit), and special precautions must be taken to prevent the breath- 

 ing of fumes if overheating of the mixture occurs. Because of the dif- 

 ficulty and expense of obtaining and sizing stone armor units and the 

 molding of special armor-unit shapes, all the units prepared for model 

 studies and research projects are retained for future use. The sizes and 

 weights of the units on hand at a particular time, together with the sizes 

 and weights of prototype units estimated to be required for a proposed 

 project, determine to a considerable extent the model scale selected for 

 a proposed model study. 



(2) Vertical Wall Structures . Vertical-wall structures in the 

 model may be constructed of wood, with a special section for pressure 

 cells and wave rods constructed of steel (Fig. 6-12); the structures may 

 also be constructed partly of wood and filled with gravel with a cap 

 molded of concrete (Fig. 6-13), or of transparent plastic. Because the 

 structures are usually simple in cross section, fabrication is not diffi- 

 cult unless they are relatively small and do not provide enough space for 

 the required instrumentation. This situation can result in the use of 

 larger models. 



(3) Floating Breakwaters . Full-scale floating breakwaters may be 

 formed of log rafts, steel A-frame with steel pontoon outriggers, rectan- 

 gular pontoons constructed of wood, steel, or concrete, steel or concrete 



358 



