of the four component types of soil (gravel, sand, silt, clay); however, 

 not all soil mixtures are equally effective in a given situation. Organic 

 materials are usually considered detrimental and are not used, since they 

 tend to be more compressible and have lower shear strengths. Highly 

 expansive clay should also generally not be used for backfill of coastal 

 structures . 



A filter layer of gravel or crushed stone is often used (with or with- 

 out a geotextile filter) under and behind walls to provide for relief of 

 hydrostatic pressure and to prevent piping. These materials should meet 

 filter design criteria. 



(4) Wharves and Piers . The use of earth for wharves and piers is 

 generally as fill behind or for slopes underneath the facility. The natural 

 in-place soils generally provide support for the structures, and foundations 

 are designed in accordance with conventional geotechnical procedures. The 

 purpose of the fill is to help provide stability and rigidity to the struc- 

 ture and to provide useful working areas behind the structure. Most soils 

 may be used for this purpose, but they generally need some type of protec- 

 tion, such as rock riprap and a filter, to mitigate erosion. Coarse- 

 grained granular soils are preferable for use as backfill materials, since 

 they are typically stronger and less compressible. The properties of the 

 backfill soils of use in the design include dry density, water content, 

 shear strength, and compressibility. Other important properties may be the 

 compaction characteristics, permeability, and corrosive characteristics. 



(5) Land Reclamation. Land reclamation may include dredging for 

 marinas, construction of fills for water-oriented land developments, 

 enlargement of streams, and other related activities in waters and wetlands. 

 These activities generally involve discharge of fill material onto the 

 adjacent shoreline or into waters or wetlands for construction of struc- 

 tures; site development fills for recreational, industrial, commercial, 

 residential, and other uses; causeways or road fills; dams and dikes; 

 artificial islands, property protection; groins and beach restoration; 

 levees and artificial reefs. These fills may be obtained from land sources 

 and dumped by land methods or from water sources and placed hydraulically 



by dredging. The properties of soils most useful to know in land reclamation 

 projects are the strength characteristics, consolidation characteristics, 

 and chemical properties after placement of the fill. 



(6) Dikes. Dikes can be constructed of sand, clay, or a combina- 

 tion of both. Earth dikes are usually utilized as containment structures 

 for dredged materials, but may also be used as protective devices such as 

 hurricane barriers. They generally require some type of protection when 

 subjected to wave action. Clay dikes and dikes with a sand core and clay 

 cover have also been built with seaward slopes of 1:6 to 1:10 with a grass 

 cover. 



(7) Protective Beach and Dune Restoration . The placement of 

 sandfill along a part of shore front is a nonstructural erosion control 

 technique, referred to as beach nourishment, that is utilized for the 

 protection of beach areas or for the creation of protective beaches in 

 areas where none exist. Artificial restoration projects should generally 

 define the source of material, the method of placement, and the grain-size 



68 



