Bearing Mechanism And Analysis Method For Multi-Pile Foundations For Offshore Wind Turbines In Sand

As one of the most promising foundation types for offshore wind turbine s,multi-pile foundations have been paid more and more attention.However,the current design method for multi-pile foundations is on the basis of monopile tests and numerical analyses,which is not capable to consider either the influence of vertical-horizonal load combination,or the change of bearing characteristics of soil-pile system under long-term cyclic loading.By means of centrifuge model tests,finite element analyses and hydraulic gradient model tests,the load distribution pattern among multi-pile foundations was revealed,and the mechanism of soil-pile interactions under vertical-horizontal combined loading was analyzed in this study.Based on these researches,a practical analysis method for multi-pile foundations for offshore wind turbines wa s proposed.The main achievements obtained in the thesis are as follows:1.The load distribution pattern among multi-pile foundation with two levels,individual level and general level,was revealed by centrifuge model tests on tripod-pile and five-pile foundations.The influencing factors were discussed by numerical analyses.A calculation method for load boundary condition for every individual pile in the multi-pile foundation was set up.2.The mechanisms of the influence of vertical load on the horizon tal responses of soil-pile system were revealed by a series of sing pile centrifuge model tests under different vertical-horizontal load combinations,which include two opposite sides: the densification effect and the P-? effect.The densification effect plays a dominant role in the early horizontal loading and then decreases owing to the separation between soil and the pile.Meanwhile,the P-? effect is strengthened as the horizontal loads increases and causes larger deformation of the pile.These two opposite effects work together to make the bending moment of the pile decrease first then increase as the horizontal load increases.3.The numerical method was used to study the influencing factors of the densification effect.The relationship between the densification effect and the lateral soil-pile interaction was set up.Accordingly,an improved p-y curve model was established for vertical-horizontal combined loading conditions.The calibration method of the parameters in this improved p-y model was proposed.The validation of this new p-y moel was verified by comparing the results to those from finite element analyses.4.A new hydraulic gradient testing device was developed to simulate the long-term lateral cyclic loading conditions for pile foundations for offshore wind turbines.The water pressure within the sand was monitored to verify the hydraulic gradient field.The pile deflection in short-term cyclic loading test with this device was compared with those from a short-term centrifuge model to confirm the effectiveness of this new device.Comparing the results of 1g and 100 g long-term cyclic loading tests,it is indicated that the supergravity field has a significant influence on model tes t results.Therefore,1g model test is deviated from reality.5.A long-term cyclic loading test under 100 g level was conducted with the new device.The displacement accumulation of the pile and the change of natural frequency of the soil-pile system during long-term cyclic loading were analyzed.Based on the hydraulic test results,a modification of p-y curve was proposed to consider the change of the bearing capacity of soil-pile system under long-term cyclic loading.