100-ton cubes with 4-inch-diameter nylon rope, determining by model tests 

 whether the rope would withstand the forces induced during construction 

 and by wave action after construction was desired. To obtain elastic 

 similarity between model and prototype linking media, the ratio between 

 elastic and gravity forces must be equal in model and prototype (eq. 6- lib) 

 If it is assumed that (y^)™ = ^"^w^n* which is accurate enough for this 

 study, equation (6-llb) reduces to 





C6-11C) 



and, since for this model study (iL^/Lp)^ = CVSO)^, 



(EnA) 



where E-^ is the modulus of elasticity of nylon in pounds per square 

 inch and A the cross-sectional area of the rope in square feet. A 

 4-inch nylon rope has a breaking strength of about 360,000 pounds, and 

 the working load was assumed to be about 100,000 pounds with the elonga- 

 tion about 33 percent. To reproduce an equivalent model linkage at a 

 linear scale of 1:50, the medium should elongate 33 percent at a load of 

 about 1.5 pounds and break at a load of about 3 pounds. Tests of differ- 

 ent material found that the only material to have the required elongation 

 and breaking characteristics (a combination of neoprene and rubber-rayon 

 cords) was too stiff; i.e., the bending qualities were not adequate. 

 Because the breaking characteristics were the most important for these 

 tests, a size 000 silk surgical thread with a breaking strength of 3 

 pounds and an elongation of 3 percent was selected for the linkage tests. 

 Preliminary launching tests were made for several clusters of 100- ton 

 model cubes with the surgical thread as the linkage medium, and each 

 time a cluster was launched several of the lines were broken. Since 

 the 4-inch nylon rope proposed for use in the prototype was inadequate, 

 it was decided to perform all linkage tests with No. 15 nylon twine which 

 had a breaking strength of 41 pounds and was sufficiently flexible. In 

 this way the advantages of using a linking medium for different types of 

 repair sections could be evaluated, although the rope strength required 

 to prevent breaking could not be determined. 



(i) Summary of Tests and Results . The existing rock mound 

 and the broken monolithic head of the north jetty were reproduced in the 

 model down to the -36- foot contour (MLLW) , and the proposed rehabilitated 

 concrete monolith was constructed around the existing part of the monolith 

 from the plans furnished by the U.S. Army Engineer District, San Francisco 

 (Fig. 6-29) . The crest elevation of the monolith was +25 feet MLLW and 

 the radius was 55 feet. The protective cover layer for each plan for 

 repair tested was placed on the existing rubble slope [about 1 on 5) 

 around the rehabilitated monolith and as far shoreward and seaward as 

 was considered necessary to provide the required stability of the armor 

 units and protection of the monolithic head. A series of 10 tests was 



392 



