increasing in thickness as water depth increases. At the island site it 

 ranges from 14 to 25 feet in thickness. Shoreward from the lease area the 

 sand content increases, but the overburden thickness decreases. The aver- 

 age bottom slope at the island site is 3 percent. 



Table 2 indicates some of the materials encountered in the subsurface 

 investigation. Underlying the overburden is a geologically recent shale 

 or siltstone formation. Consolidation estimates indicated a probable 

 settlement of less than 6 inches at the ocean floor from the weight of a 

 solid island, and most settlement was expected to occur during construc- 

 tion. Bottom material shoreward of the island was not coarse enough to be 

 desirable for dredge fill, but studies indicated that proper control of 

 the discharge location could use ocean currents to separate and winnow out 

 the fine fractions, leaving a satisfactory granular core of dredged 

 material. 



d. Rock Sources - The operating quarry most convenient to marine 

 loading facilities was on Catalina Island. Because using this quarry would 

 have required a barge haul of about 90 miles to the island site, a signi- 

 ficant effort was devoted to locating alternate rock sources. Laboratory 

 tests of samples taken from near Prisoner's Harbor on Santa Cruz Island, 

 only 27 miles from the island site, indicated that this rock was suitable 

 for the island revetments, but it was estimated that few units weighing 

 more than 15 tons could be obtained. The igneous rock from Prisoner's 

 Harbor appeared similar to other Santa Cruz Island rock used in the con- 

 struction about 30 years previously of the Santa Barbara breakwater, which 

 has shown excellent weathering characteristics. Richfield arrived at an 

 agreement with the owner of this part of Santa Cruz Island whereby the new 

 quarry site was offered to all bidders as a royalty-free source of rock 

 and gravel. Exploratory blasting was financed by Richfield and witnessed 

 by all interested bidders. Other quarries on the mainland involved either 

 long hauls with transfer to barges or produced materials considered unsuit- 

 able for seawater and wave exposure. 



Bidding documents were prepared to allow any source of rock on a spec- 

 ification basis of quality, density, size, and gradation. Alternate sizes 

 of armor rock units were specified depending upon the specific gravity of 

 the rock. 



e. Precast Concrete Armor - Studies of precast concrete armor units 

 revealed several factors which made it desirable to allow for their use as 

 an atemative to the rocks of the largest (Class A) category. Research 

 [U.S. Army, Corps of Engineers, Waterways Experiment Station, 1955) shows 

 that, for a given weight and specific gravity, tetrapods are more stable 

 against wave action than quarried rocks. Practically, this meant that 

 lighter weight armor could be used; hence a smaller crane could place the 

 armor on the seaward face of the island. Equally important was the elimi- 

 nation of the necessity for quarrying and transporting heavy rocks. It is 

 usually difficult to predict the maximum rock size a quarry will economi- 

 cally produce until actual operations are underway. In addition to tetra- 

 pods, the design included tetrahedrons as optional precast concrete armor. 



12 



