Study On Bearing Characteristics Of Pile Group Foundations Of Offshore Jacket Wind Turbine In Sand

Compared with the traditional energy,the offshore wind energy has higher advantages,and is of great significance for controlling the environment pollution and improving the energy structure.Due to the large structural rigidity of the offshore jacket wind turbine,the cyclic load of wind,wave and current has a low impact on it,so the bearing capacity of the offshore wind turbine for natural loads is higher than that of the large-diameter single pile foundation,and is suitable for being installed in the deep water area,so that the sea bottom geological requirement is low,Therefore,it will be widely used in offshore wind farm.In this paper,the self-developed large-scale indoor test system for pile-soil interaction are used to discuss the sinking pile,horizontal static load and cyclic dynamic response of the offshore jacket wind turbine model,and combined with the pile-soil interface cyclic shear test,the cyclic weakening mechanism of the pile side friction is studied.This paper systematically reveals the load-bearing characteristics of pile foundation of offshore jacket wind turbine group.The study includes the conclusions as follows:(1)The test of multi-pile foundation sinking piles in sand soil shows that the resistance of closed pipe pile increases with the increase of sinking pile depth and begins to play a role and the proportion of the side frictional resistance of the pile gradually increases;the pile end resistance increases with the sinking pile depth,there is a critical depth of about 11D;when the distance between piles of multi-pile foundation is constant,under the same sinking pile depth,the sinking pile resistance of the later piles is significantly larger than that of the former piles.(2)The horizontal static load test of the jacket wind turbine model in sand shows that the horizontal ultimate bearing capacity of the pile foundation with open pipe pile and closed pipe pile is 360 N and 380 N respectively;the maximum bending moment of the front and rear row piles is below the mud surface in different positions.The bending moment of the front row pile body is larger than the rear row pile body above the maximum bending moment point,and the pile surrounding soil reaction force of the shallow soil of the rear row pile is smaller than that of the front row pile.(3)The cyclic loading test of the jacket wind turbine model in sand shows that with the increase of the number of cycles,the extreme value of horizontal displacement of the top of the tower gradually increases;the lateral friction of the pile body shows a downward trend as a whole,and the variation of the front pile is greater than that of the rear pile.Both tend to be stable after 10000 cycles.After the circulation,the bearing capacity of the foundation shows a decreasing trend.The reduction amplitude is closely related to the load frequency,cyclic load ratio and pile end form.The greater the load frequency and cyclic load ratio,the greater the reduction;the reduction range of open pipe pile is larger than that of closed pipe pile.(4)The cyclic dynamic response analysis of the jacket wind turbine model in sand shows that when the cyclic load ratio and load frequency are small,natural vibration frequency of the structure increases with the increase of the number of cycles;when the cyclic load ratio and load frequency are large,natural vibration frequency of the structure has a significant downward trend.After a certain number of cycles,the natural vibration frequency fluctuates up and down within a fixed range.The damping ratio of the system as a whole shows a significant downward trend with the increase of the number of cycles.The maximum reduction occurs in the open pipe pile model test,and it can produce a 70% reduction in the test.(5)The cyclic shear test shows that the interface shear stress and horizontal displacement curve are “closed loop”,and the weakening of interface shear stress mainly occurs in the first 10 cycles.The trend of the dimensionless weakening factor shows that the pile body side frictional resistance decreases with the increase of the number of cycles,and the larger the displacement of the pile body,the greater the attenuation amplitude;the greater the lateral pressure of the soil around the pile,the greater the attenuation.