Field Observation and Analysis of Wave-Induced Liquefaction in Seabed
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Keywords:
Seabed
Wave loading
Trough (economics)
Effective stress
Overburden pressure
The analysis of wave-induced liquefaction of seabeds is of profound importance for offshore structures such as wind turbines and oil platforms. The seabed around pile foundations is subjected to two effects, vertical wave loads and the lateral pile–soil interactions induced by the rocking of super structures. This paper numerically studies the excess pore pressure responses of the seabed around a pile when both effects are considered. Numerical analysis is conducted using an in-house developed, effective stress-based, three-dimensional finite element code. The mechanical behaviors of the seabed are simulated by a kinematic hardening constitutive model that can describe the cyclic mobility of soils. Comparing the results of the seabed around a pile and the seabed with no pile foundation shows that the excess pore pressures accumulate faster when both of the effects are considered. Water depth plays an essential role in determining whether the effect of pile–soil interaction on excess pore pressures can be neglected. In deep waters, the excess pore pressure response mainly derives from the effect of the pile rocking, while the influence of vertical wave pressure on the seabed is relatively small. However, in shallow waters, both of the effects are significant.
Seabed
Wave loading
Effective stress
Soil liquefaction
Foundation (evidence)
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Abstract Wave-induced liquefaction in seabed may adversely impact the stability and bearing capacity of the foundation elements of coastal structures. The interaction of wave, seabed, and structure has been studied mostly for only mildly sloping seabed (<5deg) using a decoupled approach. However, some of the marine hydrokinetic devices (MHKs) may be built on or anchored to the seabed with significant steepness. The wave-induced response and instantaneous liquefaction within sloping seabed supporting a small structure (representing a small MHK device) are evaluated herein by developing an almost fully coupled finite element model. The effects of coupling approach on the stress response and liquefaction of the seabed soils are investigated. Subsequently, post-liquefaction deformation of seabed soils around the structure is assessed. The poroelasticity equations governing the seabed response coupled with those for other domains are solved simultaneously. For post-liquefaction analysis, the soil is modeled as elastic-perfectly plastic material. The development of instantaneously liquefied zones near the foundation is studied in terms of seabed steepness and wave parameters. The changes in the effective stress paths due to the development of liquefied zones are evaluated in view of the soil's critical state. The results indicate that the decoupled solution yields significantly larger stresses and liquefaction zones around the structure. The seabed response and the liquefaction zones become smaller for steeper slopes. The presence of liquefied zones brings the stress state closer to the failure envelope, reduces the confining stresses, and induces larger plastic strains around the foundation element.
Seabed
Wave loading
Effective stress
Soil liquefaction
Foundation (evidence)
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Seabed
Wave loading
Soil liquefaction
Effective stress
Offshore geotechnical engineering
Poromechanics
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The design of marine structures includes the structure design and the foundation design. While the calculation of wave force is quite crucial for the former, the evaluation of seabed stability is essential for the latter. The passage of a wave will impose wave pressures and shear forces on the seabed, and the wave-induced changes of pore water pressures and effective stresses within the seabed will make the seabed deform, soften and liquefy etc. Therefore, the evaluation of wave-induced seabed stability is important to the foundation design of marine structures, as well as to the superstructure design. Inspired from the mechanical properties of seabed, this article firstly suggests one simplified mechanical model, and then derives the soil displacement distribution of the two-dimensional seabed under wave action. In addition, two-dimensional finite element models are established, the results from which are compared well with those from the mechanical model. Such simplified model can be used in the evaluation of seabed stability and the response analysis of passive piles in the marine environment.
Seabed
Wave loading
Foundation (evidence)
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Zhang, J.; Song, S.; Zhai, Y.; Tong, L., and Guo, Y., 2019. Numerical study on the wave-induced seabed response around a trenched pipeline. Journal of Coastal Research, 35(4), 896–906. Coconut Creek (Florida), ISSN 0749-0208.The investigation of wave-induced seabed dynamic response in the vicinity of an offshore trenched pipeline is particularly important for analyzing the stability of a pipeline. In this study, an improved two-dimensional (2D) numerical model is used to investigate the wave-induced dynamic seabed response for manifold backfilled depths and the associated residual liquefaction under the wave loading. To calculate the accumulated pore pressure, the superstatic pore pressure accumulation Sassa model is improved by (1) extending 1D to 2D and (2) expressing the shear modulus using the soil plastic parameters. The improved model is first validated by comparing the simulation with the experimental data without a pipeline. The effects of wave and pipeline characteristics, such as wave length and height, pipeline diameter, and stiffness, on the wave-induced dynamic seabed response are simulated. The effects of backfill sand properties and backfill depth on pore pressure accumulation around a pipeline are examined. The results indicate that the influence of pipeline parameters on the dynamic response is only obvious in a certain scope and the possibility of pipeline instability due to soil liquefaction decreases with the increase of the backfill depth.
Seabed
Wave loading
Offshore geotechnical engineering
Wave height
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