近海风机吸力式沉箱基础复杂荷载作用下极限承载力的有限元极限分析

Finite element limit analyses of the undrained bearing capacity of suction caisson foundations under complex loading conditions

吸力沉箱作为近海大型风机新型基础形式具有诸多优点。极限承载力设计，作为该基础形式设计阶段的关键一环，在设计理论和验算方法方面还不够完善。利用有限元极限分析，对福建某待建海上风电场的5MW风机吸力式沉箱基础选型方案进行承载力分析。提出基于极限分析的土体-结构潜在浸水裂缝模型能够在程序优化过程中自动判断土体一结构界面的浸水裂缝开展。分别给出不同深径比以及是否允许裂缝开展时吸力沉箱不排水极限承载力的计算包络图和安全系数。将计算结果与经验公式、极限平衡解、塑性上限解和有限元解进行比较，验证了结果的准确性。分析表明，吸力式沉箱基础在该工程极限荷载作用下能够满足稳定性要求。允许存在潜在浸水裂缝时，沉箱极限承载力较不允许时低约3％～11％，二者失效机制和土体塑性区分布明显不同。极限状态下沉箱内部土体是否进入塑性与沉箱深径比有关；传统极限平衡法和极限分析多采用桶内土体刚性假定，将高估沉箱的极限承载力。

Suction caissons possess various advantages over traditional roman,u,, Determining the ultimate bearing capacity of suction caissons in response to combined vertical, horizontal and moment loading is essential for the design. Based on the theory of finite element limit analyses, the ultimate bearing capacity of suction caisson foundations for the 5MW wind turbine in an offshore wind farm to be built in Fujian province is investigated. The potential formation of a water-filled gap along the soil-structure interface at failure is considered in both the lower and upper bound formulations. The failure envelopes and the factors of safety are provided for caissons with different embedment ratios. The analyzing results are validated by comparison with empirical equations, limit equilibrium method, upper bound limit analysis and finite element analysis. Results suggest that the caissons are able to remain stable under the extreme loads of the engineering project. The analyses may overestimate the ultimate bearing capacity of caissons by 3% to 11% if water-filled gaps are not allowed, and significant difference in the failure mechanisms and plastic zones for the cases with or without the allowance of gaps can be observed. Due to the ＇ rigid soil＇ assumption, limit equilibrium methods or limit analyses may overestimate the ultimate bearing capacity of the suction caisson.