海洋平台桩腿周围局部冲刷及其对基础承载力的影响

Local Scour Around Pile Leg of Offshore Platform and Its Influence on Foundation Bearing Capacity

在海洋平台桩靴基础安装施工过程中,容易发生穿透现象而导致破坏。业界建议可通过在桩靴周边加设向上的袖结构来抑制贯入时周围土体的回流,从而缓解穿透破坏。但这种袖结构的存在会导致局部冲刷,形成的冲刷坑会减小桩靴有效埋深,进而影响其使用期的承载力。通过一系列水槽试验,探讨了不同形式桩腿的面积比、开孔率、来流角度等因素对于冲刷坑形状的影响,进而开展了考虑冲刷坑影响的桩靴基础承载力数值计算,并与忽略最大冲刷深度以上土体的传统模型、假设冲刷坑为倒圆台状的简化模型计算结果进行了对比。主要研究结果如下:(1)加设圆形袖结构的桩腿引起的冲刷深度要比方形袖结构桩腿小,建议工程中采用面积比较小的圆形袖结构来缓解穿透破坏;(2)桩腿的面积比、开孔率、来流角度等因素对冲刷坑形状都存在影响。最大冲刷深度随着开孔率的增大而减小,当开孔率较大时,冲刷现象近似于群桩冲刷,出现随着冲刷角度的增大冲刷深度先增大后减小的现象;(3)当考虑冲刷坑对于桩靴承载力的影响时,简化模型能够合理反映冲刷坑对于桩靴基础承载力的影响。

During the installation and construction of spudcan foundations of offshore platforms,it is easy to cause damage due to penetration phenomena.It is suggested by the industry that an upward sleeve structure can be added around the pile shoe to suppress the backflow of the surrounding soil dur-ing penetration,so as to alleviate penetration damage.However,the presence of such sleeve structure will lead to local scour,and the scour pit formed will reduce the effective burial depth of the spudcan foundation,thus affecting its bearing capacity during its service life.A series of flume tests were con-ducted to examine the influence of factors such as the area ratio of different pile legs,the opening rate,the inflow angle on the shape of the scour pit.Then,numerical calculations of the bearing capacity of the spudcan foundation considering the impact of the scour pit were conducted,and the results were compared with those derived from the traditional model neglecting the soil above the maximum scour depth,and those from the simplified model assuming that the scour pit is in the shape of an inverted cone.The main results are as follows:(1)The erosion depth caused by the pile leg with the circular sleeve structure is smaller than that of the square sleeve structure.It is suggested in the project to use the circular sleeve structure with a relatively small area ratio to alleviate the punch-through damage.(2)The area ratio of pile leg,and inflow angle affect the shape of scour pit.The maximum scour depth decreases with the increase of the opening rate.When the opening rate is large,the scour phe-nomenon is similar to that of pile group and the scour depth first increases and then decreases with the increase of the inflow angle.(3)The simplified model can reasonably reflect the impact of the scour pit on the bearing capacity of the spudcan foundation when considering the influence of scour pit.