The precast concrete armor alternate also made it practical for a 

 contractor to build the island from an onshore quarry since the next lar- 

 gest rock size required, the B grade, could be hauled over public roads 

 with normal equipment. Required sizes of Class A tetrapods and other 

 armor rock classifications varied with specific gravity and are shown in 

 Table 3. All armor was randon-placed in two layers. 



f . Filter and Core Material - To avoid loss of core material through 

 the rock layers from "pumping" caused by wave action, an effective filter 

 is essential. The island's filters could not conform with the generally 

 accepted filter gradings recommended by Terzaghi and modified by the U.S. 

 Army, Corps of Engineers, Waterways Experiment Station at Vicksburg (Posey, 

 1957) . Because it was considered impractical to place the relatively thin 

 blankets assumed by the usual filter gradings in an exposed ocean location, 

 the size spread in each material was made considerably greater than recom- 

 mended. It was anticipated that there would be minor losses in the filters, 

 especially of the Class G material; however, as the fines were lost from 

 the outer layers it was expected that a stable grading would be achieved. 

 Most of this readjustment is believed to have occurred during construction 

 as the Class G material was normally the first material placed in each lift. 



The Class F material served a dual function: as the lightest class 

 of armor in the lower layers, and as the outer layer of filter elsewhere. 

 Class F material was a quarry-run material with an open gradation ranging 

 from 4 tons down to a minimum of 15 percent less than 5 pounds. Class G 

 material was an optional quarry-run gravel material with a dense gradation 

 ranging down to not less than 25 percent passing the No. 20 sieve. The 

 core was sand because it was less costly than even quarry waste. Bidders 

 were allowed to choose between placing the core by dredge from underwater 

 borrow areas or importing from shore borrow areas. 



g. Construction History - Sealed bids, on a unit price basis using 

 the engineer's quantity estimates for total cost comparison, were obtained 

 from selected contractors. The contract was awarded in August 1956 to the 

 low bidder, who elected to open his own onshore quarry about 6 miles from 

 his loading-out site, and to use precast concrete tetrapods for the Class 

 A armor. At the loading-out site, 4.5 miles upcoast from the island site, 

 the contractor built a temporary loading structure in about 22 feet of 

 water. It consisted of an L-shaped pier of eight 40-foot-diameter steel 

 caissons filled with rock and sand and connected to shore by a trestle. 

 The pier was sized to provide moderate protection for one flat barge. A 

 50-ton stiff- leg derrick was mounted on the pier to handle materials. 



Tetrapods were cast in a construction yard onshore near the loading- 

 out pier. Because locally available sand and aggregates are mildly reac- 

 tive, the cost of obtaining nonreactive sand was investigated. Also, 

 type II low-alkali cement had been specified, and recent production from 

 the selected cement mill had been averaging approximately 0.3 percent 

 alkali calculated as equivalent sodium oxide. It was decided to use 

 locally available sand and aggregates, but to maintain a close check on 

 the free alkali content of the cement. The job average was 0.29 percent. 



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