Development of a Practical Soil Constitutive Model Suitable for the Design of Offshore Wind Turbine Foundations in Sand Considering Cyclic Loading

海上风力涡轮机受到其环境即风和浪的加载和卸载循环的影响。这种循环荷载会随着时间的推移显着削弱地基，导致与相同量级的静态荷载相比，荷载能力降低。随着时间的推移，通过连续的加载循环，土体中的塑性应变逐渐积累，结构可能会失效。此外，还有液化引起的破坏机制(孔压增加，土壤摩擦力降低)。通过室内试验，本项目将寻求建立循环荷载作用下砂土的本构模型。这将有助于更好地了解循环荷载作用下砂性土的特性。因此，应该有可能改进海上风力机基础的设计。这对英国具有特别重要的意义，因为英国周围的大部分海底至少存在部分沙层，有些地区几乎完全由沙土组成，例如北海南部。海上风力涡轮机基础设计的改进应能节省材料和成本，有助于降低统一的能源成本。项目方法将集中于收集循环三轴试验的结果。三轴试验是一种标准的工业试验，用于测量土样对规定的应力和/或应变边界条件的机械响应。循环三轴试验将对具有不同初始密度和应力历史的一系列样品施加计算机控制的循环载荷。样品将在实验室制备，而不是从现场测试取心样品，因为沙质土壤在释放载荷时容易破裂。结果将被用来开发和校准本构土壤模型，这将寻求改善目前可用的模型在循环荷载方面的缺点。砂性土是复杂的，因此很难用本构模型完全定义。现有的模型优先考虑静态或循环载荷的建模，而代价是降低相对于其他载荷类型的精度。该项目在寻求将循环和静态载荷组合成单一本构模型方面具有创新性。该项目是风力和海洋能源系统与结构(WAMESS)中心博士培训的一部分，属于EPSRC风力发电、地面工程和能源研究领域。该项目将与辉固合作，利用他们在沃林福德(牛津附近)的大型商业土壤测试实验室。该项目将与另一个目标相似的博士项目协同工作，但更侧重于数学和计算建模方面。

Offshore wind turbines are subject to cycles of loading and unloading from their environment i.e. wind and waves. This cyclic loading can significantly weaken the foundations over time leading to a reduced loading capacity when compared with a static load of the same magnitude. Structures can fail due to the build-up of plastic strain in the soil over time through successive loading cycles. In addition, there are failure mechanisms caused by liquefaction (where pore water pressure builds up and soil friction decreases). Through laboratory experiments this project will seek to develop a constitutive model of sandy soil under cyclic loading. This will provide a better understanding of the behaviour of sandy soils with cyclic loads applied. Therefore, it should be possible to improve the design of offshore wind turbine foundations. This is of particular significance to the UK as much of the seabed surrounding Britain has at least partial presence of sandy layers, with some areas comprised almost entirely of sandy soil e.g. southern North Sea. Improvements in the design of offshore wind turbine foundations should lead to material and cost savings, helping to drive down the levelised cost of energy.The project methodology will be focussed around gathering results from cyclic triaxial tests. Triaxial tests are a standard industry test used to measure the mechanical response of a soil sample to prescribed stress and/or strain boundary conditions. The cyclic triaxial tests will apply computer controlled cyclic loads to a range of samples prepared with different initial densities and stress histories. The samples will be prepared in the lab, as opposed to testing cored samples from the field due to the tendency of sandy soils to break apart when loads are released. The results will be used to develop and calibrate a constitutive soil model, which will seek to improve the currently available models' shortcomings with respect to cyclic loading. Sandy soils are complex and therefore difficult to define fully with a constitutive model. Existing models prioritise modelling either static or cyclic loads at the expense of reducing accuracy with respect to the other load type. This project is novel in seeking to combine both cyclic and static loads into a single constitutive model.This project is part of the Wind and Marine Energy Systems and Structures (WAMESS) Centre for Doctoral Training and falls within the EPSRC Wind Power, Ground Engineering and Energy research areas. This project will be run in partnership with Fugro, making use of their large commercial soil testing laboratory in Wallingford (near Oxford). The project will work in tandem with another doctoral project of a similar aim, but more focussed on mathematical and computational modelling aspects.