筒型基础筒-土耦合承载机理及计算方法研究

Bucket foundation is widely used on harbor and ocean engineering. In recent years, with the development of offshore wind power engineering, the maximum diameter of bucket foundation reach 40 meters. Significantly reducing of height-diameter ratio changes the bearing mechanism of soil in the bucket, and bearing mechanism has been changed from conventional skirt bearing model to cylinder bearing model. Limit analysis is Classic method of analysis bearing capacity. Establishing velocity field and stress field is very difficult because of certain depth of skirt of bucket foundation in soil. The research is developed based on finite element limit analysis method，which is presented combining finite element method, limit analysis and mathematical programming approach and can effectively solve establishing three-dimensional velocity field and stress field under irregular shape and space foundation condition, thereby, bearing capacity of bucket foundation can be obtained by limit analysis method. The research content including: bucket and soil coupling bearing mechanism is clarified under different height-diameter ratios, and decision criteria of skirt bearing model and cylinder bearing model is proposed. The velocity field and stress field of two type bearing mechanism is established under unidirection and combine loading, and upper and low bound solution of bucket foundation is obtained. The influence between bearing capacity of bucket foundation and soil stress state is explored, and the foundation stability evaluation method of bucket foundation is established considering combined loadings, bucket and soil coupling relation, skirt material strength and soil stress level.

筒型基础在港口与海洋工程领域应用广泛。近年来，随着海上风电的发展，筒型基础直径可达40m，高径比明显减小，导致筒内土体参与承载的模式由传统的筒壁承载向筒顶盖承载转变。极限分析法是推求地基承载力的经典方法，但应用于具有一定入土深度的筒型基础时，其机动场或应力场的构建难度巨大。本项研究将基于有限元极限分析法开展，该方法将有限元法、极限分析法与数学规划法相结合有效解决不规则形状基础空间三维机动场或应力场的构建，从而实现极限分析法求解筒型基础承载力。研究内容包括：揭示不同高径比筒型基础筒-土耦合地基承载模式，提出筒裙承载模式与筒顶盖承载模式的划分标准；构建单向加载与复合加载条件下，筒型基础两类承载模式的空间机动场与应力场，推导筒型基础地基承载力上、下限解；研究土体应力状态对筒型基础地基承载力的影响，建立考虑多维荷载作用、筒-土耦合、筒裙材料强度、地基应力水平的筒型基础地基稳定性评价方法。

在海洋资源的开发过程中，海洋工程结构物基础是海洋资源开发的重要保障。在众多的海洋工程基础型式中，筒型基础具有施工方便，承载力高且建造成本低等优点，是未来海洋工程基础发展的重要方向。海上环境多变复杂，作为海洋工程结构物基础，不但承受着上部结构所传来的竖向荷载、风浪流等引起的水平荷载、还包括受由上部水平荷载引起的大弯矩荷载。由于筒型基础内部是与基础刚度不同的地基土体，将其简化为实体基础进行设计必然会引起计算上的差异，基础内部土体应如何考虑，对不同类型的筒型基础地基稳定性设计也没有明确的规定。在这种情况下，在基础设计时为了保守起见，设置了较大的安全系数，由此在经济上产生了不必要的浪费。针对以上问题，提出了采用筒-土协同度来评估多维荷载作用下筒型基础地基承载力的方法。基于极限分析方法，求解了筒型基础地基竖直向、水平向及抗弯极限承载力的上限解。分析了筒-土部分协同对筒型基础承载力的影响。研究表明：竖向或水平向荷载的单向或多维作用下，筒型基础与筒内土体的协同作用较强，筒-土可作为一个受力单元分析；倾覆力矩作用下，筒型基础与筒内土体协同作用较弱。基于归一化H-M包络线受竖向荷载影响较小的规律，提出了考虑筒-土部分协同的筒型基础V-H-M承载力校核方法。建立了基于筒-土部分协同承载模式的筒型基础抗倾覆承载力理论计算方法。