elevation of structures, the slopes of revetments to reduce runup, and the 

 size of armor stone to dissipate energy. 



The use of stone in coastal structures is based primarily on experience. 

 Continued research is needed on the development of testing procedures, 

 criteria, and methods of quarrying to determine and produce rock character- 

 istics that are desirable and suitable for use in coastal structures. 

 Current research includes various tests for shrink and swell behavior, 

 wetting and drying effects, mineralogic composition, specific gravity, and 

 other physical and chemical properties. 



b. Earth . Current investigations and studies relating to the use of 

 earth materials in the coastal and marine environment deal primarily with the 

 behavior of soils under various nearshore conditions and their use in connec- 

 tion with coastal structures. These investigations include numerous programs 

 that are in progress to develop field techniques and criteria for use in 

 design, construction, and maintenance of effective beach and dune protection. 

 The programs seek to describe and predict the interactions between the 

 materials that make up the coasts and the forces that act upon them. Studies 

 include the development of mathematical models that designers can use to 

 determine how much sandfill is required to adequately protect a segment of 

 shore for a certain timespan and how often additional fill will be required. 

 Studies are in progress to determine the effective use of earth materials in 

 low-cost shore protection. New dredge disposal techniques are being studied 

 to aid in beach nourishment projects and sand bypassing across coastal 

 inlets. Several field research facilities and projects have been established 

 to study coastal processes and their long-term effects on the erosion of and 

 protection of the natural coastline materials. 



Research is also continuing on the engineering properties of the various 

 soils. These studies include the determination of density and porosity of 

 sea floor sediments, the grain-size distribution of beach materials, and the 

 shear strength and consolidation characteristics of estuarine deposits. 



c. Portland Cement Concrete . Concrete structures are being increasingly 

 utilized for a wide variety of applications in the marine environment. 

 Structures are becoming more sophisticated and are being located in areas of 

 more severe exposure (e.g., ice and open sea), and subjected to cyclic and 

 impact loads. Consequently, their performance requirements become increas- 

 ingly severe and critical. Investigations are being performed relating to 

 internal response of structural elements, environmental conditions in which 

 the structure must serve, new materials and configurations, construction 

 practices and repairs, and new uses in the ocean. 



Existing problems relating to concrete design include cracking, spalling, 

 and corrosion of reinforcing steel, as well as the purposeful overdesign and 

 over-reinforcement of structures in an effort to cover the range of uncer- 

 tainty. These problems indicate the need for additional investigation to 

 better understand the properties of concrete. Investigations include: 



(1) Corrosion of reinforcement in submerged structures with 

 varying widths of cracks. Also where cracks are repeatedly opened 

 and closed under a large number of cycles there appear to be an 



387 



