Geologic Controls 



The introductory paragraphs to this section discuss some of the 

 parameters affecting the strengths of cohesive and cohesionless soils. 

 Although the various parameters owe their existence to a number of 

 independent processes, the environment of a region generally defines 

 the conditions at a site. This dependency is even more pronounced in 

 the oceans of the world. Air and water currents generally control the 

 distribution of terrigenous materials, while temperature, depth, and 

 geomorphology determine the location of pelagic material. Since the 

 vane shear strengths depend upon the degree of soil cohesiveness ( <t -*0), 

 the following discussion considers the soil as either cohesive or co- 

 hesionless. The cohesionless category therefore includes both terri- 

 genous and pelagic materials. The cohesive subdivision follows a 

 similar procedure by considering fine silts and clays of terrigenous 

 origin and oozes and red clays of pelagic origin. 



Cohesionless materials abound in shallow-water areas. Ocean and 

 long-shore currents transport these materials from river deltas, coral 

 reefs, etc. to a variety of locations. However, the size of cohesion- 

 less material is such that a large amount of energy is necessary for 

 transportation. This requirement restricts most movement to near-shore 

 regions and to constricted channels with high current velocities. Cohe- 

 sionless materials occur in the deep ocean as turbidities (that have 

 been transported from shallow-water coastal zones) and authigenic accre- 

 tions. The turbidities are particularly prevalent near the base of the 

 continental slopes. Whether the cohesionless material is in shallow or 

 deep water, the soil strength depends upon basically the same parameters. 

 Unfortunately, the vane shear device cannot adequately correlate these 

 parameters to some form of strength measurement. 



Cohesive materials are found on the continental shelf near river 

 deltas and areas of low current activity and on the floor of the deep 

 ocean. Since the strength of the cohesive material varies with 

 mineralogy, particle size, void ratio, stress distribution, and stress 

 history, a wide range of strengths might be expected on the seafloor. 

 As currents change, the depositional pattern changes. Periods of low 

 deposition may result in soils which exhibit higher strengths (because 

 of secondary time effects) or lower strengths (remolding by benthic 

 organisms for example) than might be anticipated. These alterations, 

 in turn, cause variations in the strength-versus-depth profile. The 

 type of material deposited may also change. As the continental environ- 

 ment changes, the amount of illite, kaolinite, montmorillonite volcanic 

 debris, or marine life varies. In final form, the change is again 

 reflected by variations in the shear strength profile. The environmental 

 change may also alter the deposition of pelagic material. Some pelagic 

 materials exhibit unusual strength characteristics believed to be 

 related to their composition. These characteristics often result in 

 unique strength profiles. In some cases, the stress history of the soil 

 changes. If bottom currents erode the surface sediments, a severe 



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