Centrifugal Model Tests Of Ship Impact On A Pile Group At Different Positions And Directions

Pile groups are the typical foundations adopted by offshore structures such as bridges,offshore wind turbines etc.These structures are usually closed to the busy waterway so they are vulnerable to be impacted by navigable ships.During the impact,the impact force caused by ships is partially or entirely sustained by the pile foundation.Nowadays,researches on ship impact focus on the response of pile groups subjected to ship impact without eccentricity.The spatial random characteristics of the impact position on pile cap means that ship impact loads with certain eccentricity on pile groups are common condition.It's well known that the failure mode of pile groups subjected to lateral eccentric loads has a great difference with that under lateral loads.Thus,the engineering design based on traditional knowledge has huge safety hazards.Centrifugal modeling has become a powerful tool in geotechnical research because of its advantages in the simulation of real stress state of soil,stable test environment and relatively low cost.In this study,centrifugal model tests were carried out to investigate the dynamic response of pile groups subjected to lateral eccentric impact of ship.The objectives of this study include(1)investigating the dynamic response of pile groups under eccentric lateral impact of ship,(2)investigating the influence of impact position and impact directions on impact force and dynamic response of pile groups.Firsyly,an innovative impact pendulum-type device,which can adjust the impact position and direction flexibly,was manufactured.To design the pendulum,the pendulum dynamic equation under centrifugal environment was established to predict the equation the relationship between the lifting angle of pendulum and impact velocities.An equivalent mass of the pendulum is deduced by making the impact energy of the pendulum equal to that of a futitious block.Then the equivalent mass is used to calculate the tonnage of ship in prototype.Two comparative centrifugal model tests(TG1)of pile groups subjected to lateral impact and lateral eccentric impact.Eighteen centrifugal model tests(TG2)of pile groups subjected to ship impact with different impact positions and impact directions were carried out in this study.It is found from the TG1 tests that most of the impact energy was abosorbed by the cap of the pile group firstly,which caused the inertial force,inertia torque and the kinetic energy of the cap increased sharply;then the kinetic energy of the cap was gradually transmitted to the soil by the free vibration of the pile group.The pile-head bending moments and shear forces of the group piles were similar in the lateral impact test,but in the lateral eccentric impact test,the pile-head bending moments and shear forces of the piles were quite different each other.Although impact force measured from the lateral impact test was larger than that from the lateral eccentric test,the maximum value of the bending moments and the shear forces in the lateral eccentric impact test were larger than those in the lateral impact test.The result of TG2 tests show that the peak value of impact force decreased with both eccentricity and impact direction angle.The peak value of the lateral displacement also decreased with eccentricity,but the peak of the torsion angle of the cap increased with eccentricity.In addition,the maximum value of the pile-head bending moments and the peak of the average pile-head torque of the group piles were both increased with eccentricity but decreased with impact direction angle.