Mechanism And Quantitative Evaluation Of Wave-induced Seabed Instability In The Yellow River Delta

This dissertation is related to the scientific and technical problem of wave-induced seabed instability. The Yellow River delta with special ocean dynamic environments such as storm surge and special engineering geological conditions such as silt deposits is selected as the study area. The wave-induced pore pressure response of seabed, the wave-induced seabed instability modes and geo-hazards, and the effects of wave-induced seabed instability on offshore wind turbine are discussed based on the framework of soil mechanics, in which the physical and mechanical mechanisms and the quantitative evaluation methods are highlighted, in order to contribute to the assessment of submarine geo-hazards and to serve the development of wind resources. The main achievements can be drawn as follows.1. Mechanism and calculation of wave-induced pore pressure in the seabed.The importance of yielding under isotropic compression, shear expansion, as well as the dependence of stress path of geotechnical materials is highlighted based on the results of wave flume experiments. The stress-deformation-pore pressure is the main thread running through the elastic, the pore pressure development mode and the elasto-plastic models that are used to calculate wave-induced pore pressure in the seabed. The elastic model can only simulate the transient response and the pore pressure development mode can only simulate the residual response, while the elasto-plastic is able to consider the two mechanisms simultaneously, these characteristics are controlled by their physical and mechanical hypothesis. The transient mechanism tends to be decisive with smaller-height and longer-period waves, and finer-particle and larger-density soils. The residual mechanism is decisive with the combination of waves with smaller height and longer-period and soils with finer particle and larger density. Results indicate both transient and residual mechanisms are important in the wave-induced pore pressure response of silt seabed in the Yellow River delta. The excess pore pressure is larger and the influence depth is deeper if the waves are more severe. The wave-idncued seabed instabilities are more likely to happen within the surface layer because the excess pore pressure radio is larger among the seabed depth of 2 m.2. Wave-induced seabed instability modes and the evaluation of submarine geo-hazards.The wave-induced seabed instability modes such as scour, liquefaction, seepage instability and shear slide, may occur separately or simultaneously, in which the excess pore pressure plays an important role. The wave-induced seabed scour are significantly affected by the seepage force and the high-density oscillatory flow. The wave-induced shallow landslide is a progressive failure under low stress conditions, which can be estimated by comparing the wave-induced shear stress and the critical shear strength of seabed soil. The seepage instability is an important instability mode for silt seabed under wave loads. The criterions of liquefaction and seepage instability are equivalent, and their occurence depend on the distribution of soil particles and the amplitude of wave-induced seepage gradient. The storm wave reactivcation of landslides in the Yellow River delta is well explained based on the progressive failure, the critical state strength of wave-induced shear slide, and the calculated maximum slide depth of 2 m. The calculated wave-induced seepage gradient is the largest within 3 m under the surface layer, which agree well with the depth of pockmarks, so that the wave-induced seepage instability and the subsequent scour may contribute to the formation of pockmarks. The wave-induced scour and unidirectional flow induced-net transport may cause the silt flow and the gully landform. Both shallow slide and silt flow can lead to the landform of scarps. The re-consolidation of liquefied soil may cause the desification of seabed and the seepage instability may cause the coarsening of seabed. The disturbed layers may be formed if local seabed is densified and coarsed, while hard crust may be formed if large scale densification and coarsening of seabed occur.3. Influence of seabed instability on lateral deformation of monopiles of offshore wind turbine.The monopile is suitable foundation type for the offshore wind turbine in the Yellow River delta. The lateral deformation of monopile caused by wind, wave and current is critical to the security of offshore wind turbine. Taking the typical 5 MW offshore wind turbine at the water depth of 10 m for example, large bending moment at pile head is caused by the wind loads acting on the rotor and the structure, and large horizontal force is caused by the wave loads acting on the pile in the water, while the contribution of current loads is pretty small under the characteristic loads in the Yellow River delta. The integration method considering the nonuniform distribution of wind, wave and current is more appropriate to calculate the horizontal force and bending moment. The p-y curve method is used to calculate the deformation of monopile in the saturated silt seabed. Results indicate that the smallest insert depth of pile is 18～20 m in the silt seabed under the 50-year load condition. The insert depth should be increased to 30 m to avoid the effects of seabed scour, and an extra 3～5 m insert depth should be added to offset the influence of wave-induced excess pore pressure.