Dynamic Responses Of The Sheet-pile Groin Under Tidal Bore

The tidal bore of the Qiantang River has a fast flow and high level,which is extremely destructive to the coastal buildings due to its strong hydrodynamic effect.The seawalls and breakwaters are vital infrastructures for maintaining the safety of the plain on both sides of the river.The groins are often used as an important safeguard measure in engineering to ensure its stable operation.However,traditional rockfill groins have a shallow,loose body,weak structure and poor integrity.It’s very easy to produce destabilization damage under strong tidal bore while its maintenance period is short,frequent and expensive.To overcome these deficiencies,a sheet-pile groin composed of two rows of piles,along with a sheet connecting the pile tops,was proposed in engineering practice.Its dynamic stability calculation method under tidal bore is a prerequisite to ensure the long-term operation and maintenance safety of hydraulic structures.Compared to traditional groins,this newly proposed groin can protect the river banks and sea walls well to avoid bucking failures and provide a much longer servicing period with fewer maintenance costs.They are widely promoted in the field of river embankment construction,and the research work on their dynamic characteristics under the tidal bore is also developing rapidly.However,the influence of tidal bore on the sheet-pile groin needs to consider the coupling effect of foundation soil–piles–sheet–fluid,but the current research mainly focuses on the scattered soil–structure,structure–fluid and fluid–structure interaction problems.There are also apparent limitations and a lack of insight into the dynamic response mechanism of a sheet-pile groin with different hydraulic and pile–soil characteristics parameters.Meantime,the flow field characteristics,the spatial and temporal distribution pattern of tidal bore pressure,and the force mechanism of the piles during the tidal bore impact on the sheet-pile groin are not yet clear.The research results have not yet reached the level of guiding the actual engineering practice of sheet-pile groin.In view of this,this thesis adopts a combination of numerical simulations and model tests to study the flow field and tidal bore pressure characteristics at the moment of impact.It proposes a new and more applicable coupled soil–pile–sheet pile–water vibration calculation model that considers the hydrodynamics and pile-soil interaction model.The research results are expected to provide a theoretical basis for the engineering design of the sheet-pile groin.The main research work and innovations of this thesis are:1)Using indoor flume test and ANSYS finite element numerical simulation method to explore the free water surface,tidal bore velocity,and tidal bore pressure regularity in the whole process of impacting the sheet-pile groin.The distribution characteristics of tidal bore pressure on the headwater surface of the sheet-pile groin are discussed at different times,heights and locations.The relationship between the peak tidal bore pressure difference of the piles with the initial water depth and the corresponding position is analyzed.It deepens the understanding of the characteristics of the flow field movement and the law of the action of tidal bore pressure under tidal bore;2)A theoretical calculation model for horizontal vibration characteristics of a sheet–pile groin in an elastic foundation considering the wave force is proposed,which is based on the Winkler plane strain model and the diffraction wave theory.The coupled dynamic calculation model of the sheet–pile groin is established using the displacement attenuation function to express the pile–pile interaction.The response characteristics of the pile interaction factor and the horizontal dynamic impedance at the pile top under different pile–soil characteristic parameters are analyzed by employing the superposition principle and matrix transfer method.The influence of different wavelengths and wave heights on the dynamic characteristics of the sheet–pile groin is discussed,which enriches the dynamic response mechanism of the sheet–pile groin impacted by the tidal bore;3)A theoretical model for horizontal vibration characteristics of a sheet–pile groin in a saturated foundation considering hydrodynamic pressure is proposed.The model introduces Biot’s dynamic poroelastic theory to describe the saturated soil around the pile and uses the soil displacement attenuation function to represent the pile-pile interaction relationship.The radiation wave theory is used to derive an expression for the hydrodynamic pressure generated by the movement of the water body around the pile caused by the pile vibrations.The potential function and the variable separation method are introduced to derive the analytical expressions for the pile–pile interaction factor and the horizontal dynamic impedance on the pile top.The effects of different water depths,pile–soil characteristics and pile arrangement on the horizontal dynamic impedance at pile top and dynamic response of the sheet–pile groin are also discussed.It will provide guidance for avoiding resonance hazards in the sheet–pile groin;4)Based on the above model,it introduces the concept of the secondary wave effect to describe the influence of rear pile vibration on the dynamic response of the front pile.A coupled computational model of soil–piles–sheet–water considering the longitudinal vibration of the sheet under transient excitation on the pile side is established.The actual tidal bore conditions are simulated.The difference and superiority of considering the secondary wave effect compared with the traditional pile–soil interaction model are verified.Dynamic responses of the sheet–pile groin are explored in the time and frequency domain,and the influence on the horizontal dynamic impedance for different factors is summarized;5)The Biot two-phase medium theory is adopted to describe the saturated soil on the pile side,and the inside soil between the row piles is considered to participate in the vibration of the row pile in the form of additional mass connected to the dynamic Winkler model.It proposed a pile–soil interaction model based on the additional mass model of the inside soil between the row piles.A coupled theoretical model of saturated soil–piles–sheet–water is established considering the sheet’s longitudinal vibration and vertical bending vibration when transient excitation conditions impact the front pile side.The analytical solutions for the time–domain displacement and frequency–domain dynamic impedance of the pile top are derived.The differences in the time–domain displacement response of the sheet–pile groin at different locations,joint boundaries and pile–soil interaction models are compared.The effects of different water depths and pile–soil parameters on the dynamic complex impedance of the sheet–pile groin are discussed.