The sediment type and composition are required for each site to gain 

 a more complete picture of the stability of the site. The classification of the 

 sediment as sand, silt, clay, or a combination of these indicates, to a degree, 

 the probable behavior of the sediment under the proposed structural loading. 

 In sands, settlement will occur with little or no lag time as the structural load 

 is applied. In clays, settlement is a function of the consolidation characteristics, 

 and lag time may therefore be considerable. In sands, depending on relative 

 densities, spontaneous liquefaction could occur and result in excessive settle- 

 ments which could cause structural failures. In clays, thixotropic properties 

 may be significant if sediments are disturbed during installation or operation 

 of the structure. 



The constituents of the sediment are also very important to stability, 

 as is illustrated by the following examples. In coarse-grain sediments with 

 shells from plankton and other biological growths, the crushing of the tiny 

 cells in these growths will cause settlements which may be excessive. In clays, 

 the presence of expansive clays such as montmorillonite or nonexpansive 

 clays such as illite may be significant. Also, the presence of montmorillonite 

 in as small a quantity as 5 to 10% changes the behavior of a clay radically. 

 Thus, the constituents of the sediment must be determined to make a proper 

 evaluation of the site. 



Since earthquakes are usually associated with faults and their behavior, 

 the local and regional fault zones of each site should be established. I n addi- 

 tion, the frequency and magnitude of past earthquakes occurring in the area 

 should be noted and accounted for in design. Research should also be con- 

 ducted on the effects of earthquake forces, particularly at the interfaces of 

 density changes (that is, at sediment— water, rock— sediment, and rock— water 

 interfaces). This research may reveal new parameters that may require mea- 

 surement in site surveys. 



Sediment transport, as referred to in this report, includes transport 

 of sediments by gravity forces and by currents. Sediment transport by gravity 

 forces in the seafloor environment may include landslides, creep movement, 

 and flow slides. Sediment may be moved in the water in several ways including 

 by scour and fill-type water currents and by turbidity currents. All of these 

 sediment movements may cause destruction or serious damage to the structure 

 and interfere with the operational objective of the installation. Thus, site 

 surveys must be of sufficient accuracy and scope to adequately collect data 

 to assess the presence or possible occurrence of these sediment movement 

 phenomena. An example of the necessity of a precise comprehensive survey 

 is suggested by the far-reaching influence of turbidity currents as indicated by 

 the deposits along submarine canyons. 



