饱和砂土和黏性土中裙式吸力桶形基础抗拔及拔出机理

To save cost, suction caissons instead of offshore piles and gravity bases are strongly recommended. Essentially, the removal or extraction of suction caissons on decommission, a major advantage of suction caissons is their ability to be retrieved by applying over-pressure at the underside of the caisson lid, simply by pumping water into the caisson. Suction caissons can be easily installed in a wide range of seabed soils such as fine sand, silt and clay. The project includes two main contents: uplift bearing capacity during the servable commission, and extraction mechanism on decommission. The approaches used are related to the laboratory tests combined with the theoretical and numerical methods.. To explore the uplift bearing, some factors such as sizes of skirted suction caissons, uplift rate, passive pressures, reverse end bearing capacity, soil properties and gapping between walls and soils will be studied in detail. In particular, passive pressures resulted from uplift loading are investigated in the aspects of causes and dissipation. Seepage influenced by gapping and passive pressures will be also dealt with. Consequently, uplift bearing will be put forward in a formula form that can be adopted conveniently by designers.. For extraction of skirted suction caissons, pumping water into the caisson is a valid way. However, suction caisson extraction in clay is an essentially undrained process with insignificant seepage. The application of an over-pressure leads to a corresponding increase in pore-water pressure within the clay with little change in effective stress and internal side friction. Thus, there is somewhat difficult to extract the caissons with pumping water only. The combination of pumping water and oscillation should be an available method. The mechanism of the combination of pumping water and oscillation for the extraction will be proposed provided the frequency an oscillation force should be reasonably determined. The output obtained in this study will guide the construction of offshore wind turbines, and will meet their needs of the development after decommission.

海上风电场升级改造而采用更大容量发电设备，风机叶片直径、塔架高度随之增加，原基础难以满足新的设计要求。从可持续发展角度，塔架基础的拔出、回收、重复利用等课题正逐渐引起人们的高度重视。裙式吸力桶形基础具有较好抗拔能力，采取适当措施也易于拔出，是一种新型的、非常适合这种情况下的基础形式。针对我国典型海上风电的饱和砂土、黏性土海床地基，以模型试验为主要研究手段，结合理论和数值分析方法，研究服役期中的裙式吸力桶形基础桶壁与土体的间隙、被动吸力、反向端部承载力等因素的产生发展机制，及对其竖向抗拔承载力的影响；阐明服役期结束后注水压力、高频振动与注水压力联合作用下的裙式吸力桶形基础的拔出机理；明确它们在饱和砂土和黏性土中的特征区别。建立裙式吸力桶形基础的竖向抗拔承载力计算理论及方法，确定基础拔出的注水压力及振动频率及振动力。吸力沉贯、基础抗拔、拔出作为一个连续过程进行系统研究是本课题的研究特色。

裙式吸力桶形基础，非常适合作为砂土、黏性土海床地基的海上风电塔架的基础。海上风电场寿命一般20年，由于风电场不是随意选择的，在原有场地基础上进行升级改造成为必然。随着科技进步，此时往往采用比先前更大容量发电设备，导致风机叶片直径、塔架高度随之增加，原基础难以满足新的设计要求。塔架基础的拔出和回收过程中基础与土体的相互作用问题是关键性的科学问题。本项目以模型试验为主要研究手段，结合理论和数值分析方法，针对吸力基础的全寿命周期内沉贯、服役及服役结束后，基础拔出三个连续阶段进行系统研究，探究土与基础之间的力学作用机理具有较高的科学意义，这也是该项目的研究特色。主要取得如下研究成果：（1）提出了桶壁贯入阻力计算公式，得到了黏土中沉贯吸力值范围，对指导吸力基础沉贯施工至关重要。分析了吸力基础拔出过程中基础底部塑性区土体的极限应力状态，提出了新的反向地基承载力失效机制。所提出的由吸力基础上拔引起的泥面沉陷范围和沉陷量的计算公式，为精确确定多桶吸力基础的桶间距提供理论支撑，具有重要的工程应用价值。（2）研究了服役期内的被动吸力、反向端部承载力的产生发展机制，及其对竖向抗拔承载力的影响。发现，裙式吸力基础上拔破坏形式为底部局部拉伸破坏，增大裙高和裙宽均可增大裙式吸力基础的极限抗拔承载力。（3）提出了吸力基础注水拔出阻力计算公式，能够比较好地预测吸力基础拔出过程中的阻力变化趋势和桶内压力变化。施加高频振动，模拟土-桶壁结构界面处发生的强度软化。得到了吸力基础在不同强度黏土中的注水拔出过程，上拔阻力与桶内压力之间的关系。发现了桶内土-桶壁结构界面处黏土的软化效应强于桶外侧，土体强度越大时，吸力基础在黏土中的拔出阻力峰值也越高。研究成果对海上风电场的升级改造提供有力的理论依据和指导。