The foundation soils of tension leg platforms (TLP), earth dams, and many other major structures are subject to cyclic loading with an initial locked-in shear stress; TLPs continuously from ocean waves and dams less frequently due to earthquakes. In order to investigate the effect of the frequency of the applied cyclic loading, thirty-one Norwegian Geotechnical Institute Direct Simple Shear tests were run. Twenty-seven cyclic tests were run; twenty- one with an lrutlal locked-in static shear stress of which sixteen were constant volume (undrained) and five were drained. Frequencies for the applied cyclic loading of 0.2 Hz, 0.05 Hz, and 0.025 Hz were used throughout this investigation. San Fernando Sandy Silt (SFSS) from the Lower San Fernando Dam In California was the material tested in this study. This hydraulic fill dam experienced a major flow slide as a consequence of the San Fernando Earthquake in 1971 and has been the focus of extensive earthquake engineering research. This Investigation determined to what extent the frequency of the applied loading affects accumulated deformation of SFSS WIth an Initial locked-in shear stress. The results show that the accumulated deformation is inversely proportional to the frequency of the applied loading, but is considerably less than expected by Newmark's inverse-square model. In certain cases, when comparing laboratory and field results, this could lead to an unsafe situation (unconservative).
The design and stability of offshore oil platforms is an Important area of study. The last major untapped source of oil and gas on this planet is located under the depths of the oceans. The best potential for major new supplies lies in some very deep areas of water. Tension Leg Platforms (TLP) are the best developed method to date for use in the deeper waters.