Seabed Response Around Breakwaters Under Combined Wave And Current Loadings

Marine structures would lose its instability under long-term wave and current loadings.To have better understanding of the mechanism of this problem,both analytic approximation and numerical simulations were conducted to evaluate seabed soil response and instability around marine structure(breakwater)under combined waves and currents.Unlike previous research,the effects of current on fluid field and seabed response were investigated in this study,together with the liquefaction depth development caused by oscillatory and accumulated pore pressure.The major findings of this dissertation include:(1)An analytical approximation for the evaluation of seabed response(soil replacement and pore pressure distribution)in marine sediments under waves and currents was derived,and the results were verified by comparing with experimental data.Based on the proposed analytical solution,both coarse sand and fine sand were considered to investigate the seabed response under different types of marine sediment,the effect of current and wave steepness on oscillatory liquefaction depth was also concerned.It shown the inclusion of following currents and wave non-linearity will enlarge the amplitude of pore pressures and vertical effective normal stress,the maximum liquefaction depth increases as following current velocity and wave steepness increase,and decreases as opposite current velocity increases.(2)A two-dimensional numerical model was developed to investigate the wave propagation with the presence of a steady current flow.The model is based on RANS equations and k-? turbulence model,the VOF method is applied to capture the water free surface.Simulated results agreed well with experimental measurements,which indicated that this model has a good ability in simulating wave-current interaction.Results show that when waves travel with following current,the induced wave length increases and wave height diminishes,wave steepness decreases.(3)An integrated model was developed to investigate the seabed response under combined waves and currents,in which the in-elastic seabed porous medium model was conducted to learn the pore pressure accumulation among seabed sediment.By applying pore pressure based liquefaction criterion,the liquefaction zone near seabed surface transferred from a two dimensional pattern in the initial stage to one dimensional pattern under progressive waves.In addition,differences in current velocities lead to great disparities of liquefaction depth,waves with following currents have deeper liquefaction depth than wave-only case,while waves with opposite currents have shallowest liquefaction depth.(4)By appling elasto-plastic porous model,a two dimensional integrated wave-current-seabed model was built to investigate the accumulation and dissipation of pore pressure among marine sediment.Parametric studies shows the effect of different wave conditions and soil parameters as well as model characters on pore pressure accumulation and dissipation,of which the tendency is similar to that simulated by in-elastic porous medium model.Although this model simulated both accumulation and dissipation process of pore pressure,it has too many empirical coefficients which increase the difficulty of practical application.(5)An integrated two-dimensional wave(current)– structure – seabed model was proposed to study the fluid field and seabed response around marine structure(breakwater).The pre-consolidation process was taken into consideration in this model,and the effect of which on initial effective normal stress and pore pressure among seabed sediment was investigated.Simulation results shown that the breakwater dimension has a great influence on the residual liquefaction depth around breakwater,for increasing the width of breakwater or decreasing the height of breakwater all lead to shallower liquefaction depth,diminishing the gaps between breakwaters contributes to smaller liquefaction depth.In application,the residual liquefaction development speed would be decreased by applying more breakwaters under combined waves and currents.