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Title: Experimental and numerical studies on wave-induced liquefaction to soil around marine structures founded in the seabed
Author: Vun, Pui Lee.
ISNI:       0000 0001 3549 4029
Awarding Body: University of Birmingham
Current Institution: University of Birmingham
Date of Award: 2005
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The main objective of this research is to investigate the problem of wave-induced liquefaction around two marine structures, which are suction caisson foundation and buried marine pipeline, using experimental and numerical approaches. Both experimental and numerical approaches were used to investigate the instability of suction caissons foundation founded in an unstable and liquefied seabed. Only finite element analysis was performed to examine the liquefaction potential of soil around a buried pipeline in a seabed subjected to progressive wave loading. The Finite Element computer program, namely SWANDYNE II (HR version), was used for all the numerical analyses in plane strain (two-dimensional) case; and its capability in simulating the wave-induced seabed responses was successfully verified using the analytical solution proposed by Hsu and Jeng (1994). In this study, a total of 82 model scale wave flume tests were conducted to investigate the instabilities of suction caisson foundations in loose, medium dense and dense silt beds subjected to various progressive wave loadings. Pore water pressure responses around the skirt tip and beneath the top cap were measured by pore pressure transducers during the tests. Residual liquefaction was observed to occur to the silt bed with a relative density of less than 80%. A valve located at the top cap of the caisson was used to control the direct inflow and outflow of water inside of the caisson. The opening on the top cap could affect the pore water pressure behaviour of soil around the caisson significantly. The pressure difference between the top and the bottom of the top cap increases when the top cap is totally impermeable (shut valve), and this can be affected by the wavelength and the compressibility of the fluid. The suction caisson was idealised as a two-dimensional plane strain problem based on its bending stiffness. The dense seabed was modelled by either Linear Elastic model or General Elastic model with Mohr-Coulomb cap and the numerical solution has a good agreement with the experimental result. The parametric studies on the wave conditions, as well as on the soil parameters, were conducted under consolidation condition in prototype scale (full scale) to investigate (i) the liquefaction potential around the outside of the skirt, (ii) the oscillatory variation in skin friction acting on the skirt, and (iii) the pressure difference across the top cap of the caisson. It was found that only momentary liquefaction occurred to the top layer of soil around the caisson foundation when the seabed was subjected to severe wave loading. Finite element study on the instability of a buried marine pipeline founded in a seabed subjected to wave loading was performed and an elasto-plastic constitutive model of Pastor-Zienkiewicz Mark-III Model (PZ3) was adopted to model the seabed. The applicability of this model to the wave-induced liquefaction to seabed was successfully validated by the experimental data from Teh (2003). The parametric studies on the pipe and trench geometries, the wave conditions and the soil parameters, were conducted under the consolidation condition in prototype scale. Most of the experimental findings from Sumer et al. (2004) and Teh et at. (2004) were successfully reproduced by the numerical analyses.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available