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Reliability of marine clay foundations in cyclic loading
The safety of marine clay foundations against failure in cyclic loading is studied. This failure mode is associated with development of large cumulative shear strain amplitudes in the supporting clays. This applies when the foundations are subjected to cyclic shear loading caused by wave action in storms. A failure in cyclic loading may develop in the course of just one storm, or as the result of a sequence of many storms. Therefore, the dissertation deals with strain analysis in single storms, strain analysis in multiple storms, wave climate modelling, and reliability analysis over uncertain properties.
The strain accumulation method is used for strain prediction in single storms. A relationship between strain, pore pressure and stress level is introduced. With this relationship, the pore pressure ratio process in a clay is studied over all storms that occur during the lifetime of a foundation. This process is governed by the strain accumulation in these storms and by the consolidation of the clay in the calm periods in between. Under certain conditions, this process can be represented as a Markov chain. This is used to formulate a method for analysis of the safety against first-passage failure in cyclic loading in the course of these storms. This becomes a nested reliability analysis.
A stochastic storm profile is developed, based on Slepian model process theory, and gives the temporal evolution of the significant wave height in a storm. This is used to establish a history of shear stress amplitudes to represent the cyclic loading in the clay during the storm.
Reliability analyses are carried out for an example foundation. The results of a full first-passage failure analysis are compared with those of an analysis for the most severe storm in the lifetime of the foundation. For a relatively large foundation geometry with moderate pore pressure build-up in the long run, there is hardly any multiple-storm effect and thus no significant difference between the results of the two analyses. For a reduced geometry with stronger pore pressure build-up in the long run, multiple-storm effects come in force, and a lower reliability results from the first-passage failure analysis than from the extreme storm analysis. The conclusion from this is that for foundation design against failure in cyclic loading, it may be necessary to carry out a full first-passage failure analysis.