| Developing offshore wind energy is an important strategy to achieve the national energy transformation and carbon neutral,the cost of foundation can take up more than 1/3 in the total project,which plays an important role in the economic feasibility of the project.At the same time,with the development of offshore wind power construction to deeper sea areas,the lateral bearing capacity of offshore wind turbines is also more and more demanding.The bucket foundation is gradually becoming one of the main foundation forms of offshore wind power tower due to its advantages of convenient construction,low cost,recyclability and strong anti-overturning ability,which original is widely used in offshore oil and gas engineering.The successful installation of bucket foundation is the prerequisite for the foundation to provide lateral bearing capacity.However,during the installation and service of offshore fan,the foundation needs to resist not only the lateraling and bending moment loads caused by sea breeze,wave and current,but also the vertical loads transmitted by the upper fan and tower.The loading combination and bucket-soil interaction are extremely complex,so the current research on the jacked penetration mechanism and lateral bearing characteristics of the bucket foundation can not meet the needs of practical engineering.The existing researches mainly focus on the research on the penetration mode of foundation,the action of soil plugging and the calculation of lateral bearing capacity through the characteristics of load-displacement curve of the foundation,but there are few researches on the deformation mechanism and law of soil around the bucket,and the related researches on group buckets foundation are even less.In view of the problems existing in the above studies,this paper adopts the physical modelling tests and numerical analysis to carry out experimental studies on the formation mechanism of soil plugging in the bucket during the jacked penetration of a single and group buckets foundation,the ultimate bearing capacity of lateral monostatic loading and the cumulative deformation law of cyclic loading,In order to elucidate the deformation mechanism of soil around the bucket during jacked penetration and lareral loading process,it provides theoretical basis and engineering guidance for the calculation of penetration resistance,lateral ultimate bearing capacity and cumulative deformation prediction.This paper mainly studies in following four aspects:(1)Jacked penetration mechanism of bucket foundation.Based on the model tests,the characteristics of load-displacement curves are obtained,and the soil deformation field around the bucket is selected to study the soil plugging effect and the superposition law of group buckets during the process of penetration.The effects of sand densities,foundation length-diameter ratio and group spacings on the core area of soil plugging and the displacement superposition area of group buckest at the end of penetration are analyzed.(2)Lateral monotonic behaviour of bucket foundation.A single and group buckets case with double spacings in medium-dense sand are selected as the research objects to reveal the mode of motion and shear strain of the soil around the bucket during loading process.The effects of sand density,length-diameter ratio of foundation and bucket spacing on ultimate bearing capacity of foundation,surface displacement field range of single bucket and group effect are quantified.The concept of critical spacing of group buckets is proposed and predicted.(3)The accumulated deformation law of bucket foundation under lateral cyclic load.The number of cyclic loading,symmetry ratio,amplitude ratio and loading order were selected as control variables.The influence of different cyclic parameters on the accumulated deformation law of foundation was analyzed by combining load,displacement and image analysis,the variation of foundation stiffness-cycle number under different cyclic symmetry ratios and amplitude ratios are studied.(4)Numerical test analysis of lateral load characteristics of bucket foundation.The stiffness attenuation model of the bucket-soil system which can consider the cyclic weakening of sand is applied to the lateral cyclic load test of the bucket foundation,the characteristics of load-displacement curve,soil displacement field around the bucket and foundation stiffness during the process of lateral monotonic and cyclic load are compared with the results of laboratory model tests;The distribution law of stress field and plastic strain field of soil around the bucket is also revealed.According to the above studies and analysis,the research progress of this paper can be summarized into the following four aspects:(1)Formation of soil plugging into the bucket during jacked penetration: soil extrusion upward-transition section-wedge shaped earth plug core area-cylindrical earth plug core area are penetrated as a whole,it is accompanied by active-passive-active arch transition into the bucket,the formation of soil plugging in the bucket mainly includes two stages: soil displacement in the bucket wall shear and soil instability in the bucket end foundation.the form of shear action around the bucket is butterfly,and there are arch displacement superposition area and shear strain band loss between the group buckets.(2)In the monotone loading process,the soil around the bucket presents translational and rotational coupling coexistence.The displacement field is mainly divided into passive in front of the bucket,active behind the bucket,translational into the bucket and circular disturbance area at the end of the bucket.The displacement field is in the shape of "opening up parabola",and the shear strain field is in the form of "upward pressure" distribution in front of the bucket.Within the critical interval,the group coefficient is positively correlated with the ratio of distance to diameter,and decreases with the increase of sand density,while the critical interval of group bucket increases with the increase of sand density.(3)Under cyclic load,the cumulative deformation of single bucket increases first and then slowly,the cyclic stability effect exists in group buckets,the cyclic stability frequency is positively correlated with amplitude ratio and negatively correlated with symmetry ratio;The motion mode of soil around the bucket is rotation,The shear strain field of single bucket is in the compression mode of front and side walls,while that of group buckets is in the compression mode of rear and double bucket walls.The foundation stiffness increases first and then slightly decreases as the sand becomes densified and the plastic strain around the bucket increases.(4)Under cyclic load,the plastic strain zone of soil around the bucket is mainly concentrated in the active zone in front of the ducket,and the plastic zone of group buckets is less than that of single foundation.In summary of the above four aspects of research,the research progress of this paper can be summarized as: based on the deformation characteristics of soil around the bucket,the real failure mechanism and the law of group buckets superposition of soil around the bucket in the whole process of jacked penetration and lateral loading and were revealed.The calculation formula of the influence range of foundation rotation angle with surface displacement field and the concept of critical spacing of group bucket foundation are proposed,a stiffness attenuation model of the bucket-soil system considering cyclic weakening of sandy soil was constructed and introduced into the numerical test analysis of long-term lateral cyclic loading of large diameter bucket foundation,which has a important guiding significance to the design of offshore fan bucket foundation,foundation treatment and foundation accumulation deformation prediction. |
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