This paper presents a state of the art of investigations to secure rock borrow for the breakwaters proposed to protect offshore nuclear power plants. Considerations relating to the geological, geotechnical, and design suitability of rock are discussed within the framework of economics, and the environmental impact of quarrying and transporting rock to potential sites on the East and Gulf coasts of the U.S. The methodology outlined has application to the construction of all large rockfill breakwaters.
The breakwaters proposed to shelter floating offshore nuclear power plants in the ocean environment promise to be among the largest structures ever built in the sea. At the present time, both economics and engineering state of the art dictate that the major portion of such breakwaters be constructed from natural materials.
Some of the advantages of rubble-mound structures relate to their relatively uncomplicated design, their mass and shape that offer the maximum resistance to wave action, and their bulk that results in relatively low foundation loading.
A closer look at the interaction between the structure and the foundation reveals still further advantages derived from the flexibility of such structures. Flexibility, combined with a responsible design, immunizes rockfill structures from catastrophic failure. Flexibility is the key to ensuring structural integrity in the event of the protective structure having to tolerate potentially damaging stresses, such as those associated with differential foundation settlement, toe scour, or earthquake-induced liquefaction.
Unfortunately, large rubble-mound structures require enormous quantities of high quality rock. Because of the substantial economic, logistic, and environmental considerations that such quantities of rock present, extensive investigations are necessary.
Preliminary studies should isolate the geological availability of sources of suitable rock in terms of its workability and transportation, imposing minimum impact on the environment. Existing quarries, in suitable areas, should be identified and evaluated as potential suppliers by comparing rock quality; in-site characteristics of the rock mass as these relate to breakage in optimum size ranges; production capability; haulage; and anticipated environmental and other difficulties.
Further investigations should establish, for a short list of quarries, representative petrographic descriptions and engineering index properties (e.g., specific gravity, compressive strength, and durability). A study should be made of the service records of rock that has been used in heavy ocean construction. In the final phase of investigation, large-scale blasting and sizing tests are essential to predict how the rock will break and to establish realistic material-modeling criteria for use in field density determinations and laboratory triaxial testing.
The timing of the over-all investigation is most important to (1) ensure that rock of suitable quality is used in the breakwater and that the sources of borrow selected are practically located in terms of economic haulage; (2) verify that realistic values of the density and the angle of internal friction of the rockfill are incorporated into the design; and (3) establish a responsible basis for construction bids and thereby permit the appointed contractor the maximum lead time to mobilize personnel and equipment, stockpile rock, and plan the complicated logistics of the operation.
