(1) Silt (M) . Silt is a fine-grained soil of low plasticity which 

 may exhibit an apparent cohesion due to capillary forces. Silts have 

 relatively poor strength characteristics, except when they are dry or in 

 the form of siltstones and are poor foundation materials in cold climates 

 due to frost heave. Confined, relatively dense silts may perform satis- 

 factorily as foundation soil, but must be evaluated on a case by case 

 basis. Most coastal silts are found in combination with some clay which 

 will increase cohesion and improve foundation characteristics. 



(2) Clay (C) . Clay is distinguished by its fine particle size and 

 cohesive strength which is inversely related to its water content. For 

 this reason, a clay's performance as a foundation material is strongly 

 influenced by its stress history. In situ overconsolidated clays, clays 

 which have been loaded to higher stresses than the present load may per- 

 form quite well in foundations. Normally consolidated or underconsolidated 

 clays typical of estuaries will generally experience large settlements when 

 loaded. The minerals included in the clay composition influence the 

 properties of the soil; e.g., montmorillonite is a highly active mineral, 

 and a soil containing such a mineral will present high swelling and shrink- 

 age characteristics. Two other commonly occurring minerals are illite 

 (less active than montmorillonite and commonly found in marine clays) and 

 kaolinite (the least active mineral) . 



c. Organic Materials (0) . Peat, organic mulch, and muskeg are highly 

 organic soils which usually have a spongy nature and a fibrous texture. 

 Organic materials come from the decay of vegetable matter. They are 

 recognized by their odor, which is intensified by heating, and by their 

 dark color (although some dark soils may be inorganic). Usually organic 

 soils have high moisture and gas contents and a relatively low specific 

 gravity. 



2. Properties and Characteristics of Soils . 



The major significant engineering properties of soil are shear strength, 

 compressibility, and permeability. The types of geotechnical problems 

 encountered in the design of coastal structures which utilize these charac- 

 teristics are slope stability, bearing capacity, settlement and erosion. A 

 detailed discussion of the properties and characteristics of soils and the 

 tests required to determine them can be found in Geotechnical Engineering 

 in the Coastal Zone (Callender and Eckert, in preparation, 1983). The 

 potential contaminants derived from industrial wastes, such as toxic heavy 

 metals (mercury, cadmium, lead, and arsenic), chlorinated organic chemicals 

 (DDT and PCB's) and pathogens (bacteria, viruses, and parasites) should 

 also be considered in the evaluation of the use of any soil in coastal 

 structures. In general "polluted" soils should not be used. 



a. Shear Strength . The three types of tests commonly performed to 

 determine soil strength are designated as 



(1) Unconsolidated-Undrained triaxial test, commonly known as 

 a UU-Test or Q-Test, 



(2) Consolidated-Undrained triaxial test, commonly known as a 

 CU-Test or R-Test, and 



54 



