Test on muddy soil reinforcement by negative pressure and electro-osmosis inside cover-bearing-type bucket foundation for offshore wind turbines

Cover-bearing-type bucket foundation for offshore wind turbines has been paid more and more attention due to its low cost and great bearing capacity. In order to ensure the cover-bearing mode, the muddy soil inside the bucket foundation should be reinforced by some soil consolidation methods, such as negative pressure and electro-osmosis. Firstly, tests were conducted to obtain the reasonable current density. Meanwhile, to improve the electro-osmotic speed and effectiveness, other factors such as intermittent power and layout of electrode, were also studied in the tests. Then, the soil reinforcing tests by negative pressure combined with electro-osmosis were performed for the muddy soil consolidation inside the bucket foundation. The results showed that soil reinforcement by negative pressure was quicker and more obvious during the early phase, and electro-osmotic method can affect more range of soil by rational arrangement of electrodes. Compared with negative pressure, the electro-osmotic method was a continuous and relatively slow process of reinforcement, which was complementary to the negative pressure method. The voltage value of electro-osmosis had little effect on the muddy soil reinforcement inside the bucket foundation, and 1.5 A was chosen as the most reasonable current value for scale model testing in the electro-osmotic method.     

Negative Pressure Consolidation of Silt Inside Bucket Foundation

A series of model experiments of bucket foundations concerning suction installation and negative pressure consolidation in saturated silt were carried out in a cube steel bin at Tianjin University. The experimental results show that the silt inside the bucket has been strengthened by negative pressure, and the strengthening effect decreases with the increase of the distance from the bucket. A three-dimensional numerical model of the experiments was built by means of finite element software ABAQUS with fluid-solid coupling method. The results show that the bearing capacity of the silt inside the bucket foundation increases significantly at the former stage of negative pressure consolidation, while the increasing trend slows down over time. The rotation centers of the bucket foundation and the inner soil region tend to be closer to each other based on the consolidation. The bearing capacity of the bucket foundation is improved effectively with the increase of soil strength. The effects of negative pressure consolidation on the bearing capacity of bucket foundation were also illustrated by an actual offshore wind power project case.     

Seismic response of offshore wind structure supported by bucket foundation

An integrated finite element model(FEM)of offshore wind tower-foundation-soil is established by ABAQUS, where a large-scale composite bucket foundation with seven compartments inside is applied to supporting the upper wind tower. The dynamic response of the structure-foundation system is studied under three seismic waves with the same peak ground acceleration of 0.035g. It can be seen that the dynamic response increases at the beginning with the structure height, then it decreases because the structural damping increases due to the mass effect of the upper wind turbine generator system. It is shown that the anti-liquefaction capacity of the soil inside and underneath the foundation is improved owing to the high overburden pressure of the upper structure and the constraint effect of the bucket skirt and subdivisions. Moreover, the liquefaction resistance of the soil inside the middle compartment is improved to a higher degree than that inside the side compartments.     

真空预压技术加固海堤深厚软基原型试验研究

真空荷载提供了一种减少堤坝软土地基固结时间的选择. 本文开展了一个海堤全断面的试验及原型观测研究. 试验堤高5. 5 m, 22 m深且三角形布置的塑料排水板作为竖向排水通道, 现场布置了水平测斜和沉降仪等观测仪器, 埋设在不同地基深度的孔隙水压力计用来量测真空-堆载联合荷载下的地基中超静孔隙水压力的变化. 通过场地条件分析、高真空度维持和真空卸载, 得到了系统的沉降变形和孔隙水压力资料, 试验结果验证了真空-堆载联合预压加固海堤软土地基的效果.     

Test research on influence of draft on towing behaviors of 3-bucket foundation platform

To investigate the natural frequencies and towing behaviors of a 3-bucket foundation platform at different drafts, the decay and towing experiments were carried out in a towing tank on a scale of 1:20. The air pressure inside the bucket foundations, the water pressure at the bottom of the bucket foundations, the acceleration of the platform and the towing force were determined in the test process. The time-history curves of the measured parameters were obtained, and the frequency responses of the parameters at different drafts were analyzed by means of fast Fourier transform(FFT). The results showed that the platform natural frequency of heave decreased slightly with the rise of draft. The natural frequencies of roll and pitch are much lower than that of heave, and they increased slightly with the increase of draft. When towing in the following sea, the maximum acceleration of surge, sway and heave has downward trends with the increase of draft, but the change range decreased gradually with the increase of draft. When the draft is 5.0 m(the ratio of draft to bucket height is 0.56), the towing dynamic responses achieve the maximum, which is not conducive to the towing of the platform. When the draft is 6.0 m(the ratio of draft to bucket height is 0.67), the towing dynamic responses are the most stable.     

Load Transferring Mechanism on Transition Section of Composite Bucket Foundation for Offshore Wind Turbines

The composite bucket foundation(CBF) is a new kind of foundation which has been applied in the offshore wind industry. A reasonable connection pattern between the tower and the CBF top cover is crucial for load transmissions from the superstructure. Therefore, it is essential to choose an optimum structure type for the transition section. The line type and the arc transition section models were established by ABAQUS, and the internal forces of cross section were extracted along the height direction. Specifically, the force transfer mechanism for different types of the transition sections was investigated comparatively with monotonic as well as composite loadings. The results show that the curved transition structure exhibits the better mechanical characteristics under the monotonic and composite loadings, and the reason can be illustrated that its specific arc-shape structure can effectively convert the tremendous bending moment from the turbine tower into the limited tensile and compressive stresses downwards, without the occurrence of force concentration.     

柔性浅基础下变刚度垫层设置的有限元分析

柔性浅基础不具备调整地基变形的能力,针对这一缺点,可在基底应力较大的部位设置强度较大的垫层,而在应力较小的部位设置强度较小的垫层,即通过在基底设置中心刚度大、边缘刚度小的变刚度垫层,以有效调整地基土的应力分布,改善地基土工作性状。采用PLAXIS2D有限元软件,分析比较了柔性浅基础下设置不同工况的变刚度垫层对地基土中应力分布及基础差异沉降的影响．发现在基底中部0．5～0．6倍基宽范围内设置变刚度垫层时,差异沉降控制效果最好。     

One-step-installation of offshore wind turbine on large-scale bucket-top-bearing bucket foundation

In 2010,the first offshore wind turbine with integrated installation was established in Qidong sea area of Jiangsu Province,China,which led to the implementation phase of one-step-installation technique based on the design and construction of large-scale bucket-top-bearing (LSBTB) bucket foundation.The critical technique of LSBTB bucket foundation included self-floating towing,penetration with adjustment of horizontal levelness,removability and one-step-installation.The process of one-step-installation included the prefabrication of LSBTB bucket foundation in onshore construction base,installation and debugging of wind power,overall water transportation of foundation and wind power system,and installation of foundation and offshore wind turbine on the appointed sea area.The cost of one-step-installation technique was about 5 000 Yuan/kW,which was 30%-50% lower than that of the existing technique.The prefabrication of LSBTB bucket foundation took about two months.During the one-step-installation process,the installation and debugging of wind power and overall water transportation need about one to two days in sea area within 35 m depth.After the proposed technique is industrialized,the cost will be further reduced,and the installation capacity is expected to be up to 500 wind turbines per year.     

散体材料桩和水泥桩复合地基固结理论研究综述

复合地基在地基处理中具有十分重要的地位,其用于加固地基,增强地基承载力和减小地基沉降具有良好的效果,因而在工程中被广泛采用。由于研究复合地基的固结是研究复合地基加固机理的主要内容之一,土体的固结直接影响复合地基的变形和承载能力。因此,有必要对复合地基的固结理论进行全面、系统地认识。本文主要对散体材料桩和水泥桩复合地基的固结理论进行综述,阐述了复合地基固结理论研究的现状和取得的理论成果。     

软弱地基换填处理及抛石辗压夯实加固施工技术

软弱地基换填是指将原地基下臣软弱层挖去，回填碎石砂等进行地基处理的一种施工方法。茂名市新中成衣批发市场综合楼为6层框架结构，建筑面积5200m^2，局部地基存在软弱下卧层，采取了快速水下换填法；其它基础承载力不够部分，采用抛石辗压夯实相结合法加固地基，取得成功。实践证明，使用该方法处理地基是一种快速经济的施工方法。     

Estimation of Heights of Soil Plug Inside Bucket Foundations During Suction Penetration by Deformable Discrete Element Modeling

Bucket foundations[1 ,2]are a relatively newtype offoundation usedto support offshore structures .Each bucketfoundation consists of a thin cylindrical steel shell ,cappedwithalid of steel or concrete .Thefoundationcan penetrateinto sea floor when pumping …     

强夯法在高速公路软土地基中的加固效果研究

强夯法是我国软土地基加固处理的常用方法,但其加固效果如何需要进一步的实践和研究。对具体加固实例的实测结果和试验数据分析表明,实施强夯后的软土地基在提高地基承载力、减少土体压缩性、降低湿陷性等方面效果明显。     

Bearing capacity and load transfer mechanism of a static drill rooted nodular pile in soft soil areas

The static drill rooted nodular pile is a new type of pile foundation consisting of precast nodular pile and the surrounding cemented soil. This composite pile has a relatively high bearing capacity and the mud pollution will be largely reduced during the construction process by using this type of pile. In order to investigate the bearing capacity and load transfer mechanism of this pile, a group of experiments were conducted to provide a comparison between this new pile and the bored pile. The axial force ofa precast nodular pile was also measured by the strain gauges installed on the pile to analyze the distribution of the axial force of the nodular pile and the skin friction supported by the surrounding soil, then 3D models were built by using the ABAQUS finite element program to investigate the load transfer mechanism of this composite pile in detail. By combining the results of field tests and the finite element method, the outcome showed that the bearing capacity of a static drill rooted nodular pile is higher than the bored pile, and that this composite pile will form a double stress dispersion system which will not only confirm the strength of the pile, but also make the skin friction to be fully mobilized. The settlement of this composite pile is mainly controlled by the precast nodular pile; meanwhile, the nodular pile and the surrounding cemented soil can be considered as deformation compatibility during the loading process. The nodes on the nodular pile play an important role during the load transfer process, the shear strength of the interface between the cemented soil and the soil of the static drill rooted pile is larger than that of the bored pile.     

Bond strength improvement of GFRP rebars with different rib geometries

Based on the Canadian Standards Association (CSA) criteria,105 pullout specimens were tested to investigate the effect of different rib geometries on bond strength of glass fiber reinforced polymer (GFRP) rebars embedded in concrete. Two kinds of conventional reinforcing rebars were also studied for comparison. Each rebar was embedded in a 150 mm concrete cube,with the embedded length being four times the rebar diameter. The experimental parameters were the rebar type,rebar component,rebar diameter,rebar surface texture,rib height,rib spacing and rib width. Theoretical analysis was also carried out to explain the experimental phenomena and results. The experimental and theoretical results indicated that the bond strength of GFRP rebars was about 13%～35% lower than that of steel rebars. The bond strength and bond-slip behavior of the specially machined rebars varied with the rebar type,rebar diameter,rebar surface texture,rib height,rib spacing and rib width. Using the results,design recom-mendations were made concerning optimum rib geometries of GFRP ribbed rebars with superior bond-slip characteristics,which concluded that the optimal rib spacing of ribbed rebars is the same as the rebar diameter,and that the optimal rib height is 6% of the rebar diameter.     

土钉与槽钢组合支护在软土深基坑中的应用

在高层建筑深基坑支护各种方案中,通常土钉支护费用最低,当深基坑土质条件较差,如沿海地区淤泥质土层,含水量较大,基坑垂直支护面难以成型,难以采用土钉支护方案,引用钢板桩支护原理,本文采用25号槽钢与土钉组合深基坑支护方案,解决土钉在软土中应用不足和发挥槽钢封面的特点,成功解决软土地区土钉支护问题,由于槽钢可以全部回收重复使用,该支护方案造价较低,经济效益较好。     

真空预压技术对软粘土物理与力学属性影响的试验研究

研制了一套新三轴试验仪,该仪器能将堆载和真空加载在土样上,在固结进行中能同时监控超孔隙水压力、轴向应变(沉降)与体积应变.并根据浙江省温州市原状软粘土进行了固结试验,软粘土的天然含水率是72.5%.试验结果发现,真空预压、堆载预压或真空堆载联合预压3类加固方式中,粘土的固结特性没有明显差异.研究表明3种加固方法改变了软土的一些物理属性并改善了它的力学属性.     

土壤含水量对浑水入渗能力的影响

土壤含水量对浑水入渗能力产生明显影响,土壤含水量由小变大,浑水入渗能力减小;3个入渗参数随土壤含水量的增大而减小。研究结果对于指导浑水入渗及引洪补源具有重要意义。     

真空堆载联合预压在浙江舟山武港码头工程的应用

采用真空联合堆载预压技术能通过排水固结提高地基承载力,和其他处理方法相比,能降低工程造价。结合工程实例,对加固原理、施工工艺、施工质量控制要点进行阐述,并通过监测和检测结果分析,为同类工程提供参考借鉴。     

Bearing Capacity and Technical Advantages of Composite Bucket Foundation of Offshore Wind Turbines

Based on mechanical characteristics such as large vertical load,large horizontal load,large bending moment and complex geological conditions,a large scale composite bucket foundation (CBF) is put forward.Both the theoretical analysis and numerical simulation are employed to study the bearing capacity of CBF and the relationship between loads and ground deformation.Furthermore,monopile,high-rise pile cap,tripod and CBF designs are compared to analyze the bearing capacity and ground deformation,with a 3-MW wind generator as an example.The results indicate that CBF can effectively bear horizontal load and large bending moment resulting from upper structures and environmental load.     

复合载体夯扩桩和传统桩对比试验研究

复合载体夯扩桩是近几年新兴的一种桩型,在地基处理中已经得到大量应用。通过对湖北某教学楼工程加固后复合载体夯扩桩进行试验和监测,来评价分析复合载体夯扩桩加固效果,从而对复合载体夯扩桩以下几个方面做了研究：夯扩桩加固机理、桩土一起作用特性、夯扩桩破坏模式和工程性质等方面。从而在探究中发现,通过采用复合载体夯扩桩加固后与传统桩地基处理效果对比分析表明,前者具有更高的操作性与良好的经济效益性。