Deformable Seabed

Simultaneous records of the wave-induced bottom pressure and the vertical displacement. (From Suhayda, J.N., Marine Geotech., 2,135-146,1977. Reprinted with permission of Taylor Francis Group.) 

A relatively greater amount of wave energy is lost on a muddy coast at intermediate water depths than is dissipated on a sandy coast (Suhayda, 1977). 

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The pressures calculated using the above approaches generally use the linear (Airy) theory and expect the pore pressure response to be in phase with the surface waves. This is in agreement with what has been measured for sandy bottoms. In contrast, measurements made by Hirst and Richards (1978) showed that bottom pressures can be much larger than predicted by linear theory for soft clayey bottoms. In Sections 9.6.3.1 and 9.6.S.2 the case of a rigid seabed (i.e., sandy bottom) will be considered first followed by the case of a deformable seabed (i.e., soft clayey bottom). 

Compaction of chalky reservoirs during oil extraction and other important problems like the "casing collapse" or the "chalk production" are related to the mechanical properties of chalk. Controlling compaction is very important because reservoir deformations imply seabed subsidence that endangers the offshore stations. 

The deformations of the seabed under load have not been measured with sufficient accuracy for determination of either compressibility or coefficient of consolidation because of the limited number of tests that have been carried out. However, the results of tests in a dense sand and using bearing capacity theory have been used to back-calculate a friction angle of about 42 as shown in Figure 4.32 (Andresen et al., 1979). 

Source Chaney, R.C., Methods of predicting the deformation of the seabed due to cyclic loading. Proceedings of the Symposium on Seabed Mechanics. International Union of Theoretical and Applied Mechanics, University of Newcastle, Callaghan, NSW, Australia, 159-167, 1984. Republished with kind permission from Springer Science + Business Media. 

The expected vessel s movements were relevant to deformation of the superstructure however, displacements associated with sliding of the hull on the seabed could not be a-priori excluded, especially during extreme weather and marine conditions. Thus, a set of markers was placed on the sea floor in contact with the hull, periodically controlled by divers of the Italian Coast Guard and State Police. 

The same adverse international considerations apply to disposal in a subduction zone as apply to deep seabed disposal [22]. Moreover, subduction zones are associated with the most unstable regions of the planet s surface, where earthquakes and volcanoes are common, and they are generally associated with coastal areas where biological resources are concentrated. Furthermore, not all sediments on a descending plate are carried to depth in some cases they are scraped off and deformed near the contact of the plates. 

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