正常固结黏土中扭矩对负压沉箱承载力的影响分析

2020-04-17 14:45:26 《土木建筑与环境工程》 2020年1期

周松望 张艳 王栋

摘 要:当负压沉箱被用作深水管汇或管道终端基础时,除了受到上部结构传来的竖向力、水平力和弯矩,还会受到扭矩,扭矩可能降低沉箱的竖向承载力、水平承载力和抗弯能力。采用理论分析和有限元法研究复合加载条件下典型沉箱(长径比介于1~2之间)与正常固结黏性土的相互作用,考虑安装造成的沉箱侧壁周围土体的弱化,得到了不排水条件下沉箱单向最大扭矩,探索了扭矩与不同荷载分量联合作用时承载能力的改变。结果表明,当扭矩不超过20%的抗扭转能力时,可以忽略扭矩对竖向承载力、水平承载力或抗弯能力的影响;当扭矩介于抗扭转能力的20%~80%时,其他承载力分量最多降低20%。提出了能够用于工程设计的扭矩对其他荷载分量抗力的影响系数。

关键词:海底管线;负压沉箱;有限元;扭矩;承载力

中图分类号:TU449 文献标志码:A 文章编号:2096-6717(2020)01-0018-06

Abstract:When suction caissons are used as foundation of the manifold or pipe terminal in deep waters, they are subjected to a torsion except for the vertical force, horizontal force and moment applied. The torsion may reduce the vertical, horizontal and moment bearing capacities of caisson foundation. In this study, theoretical analyses and finite element simulations are conducted to study the interaction between normally consolidated clay and the typical caisson (with a length-to-diameter ratio between 1 and 2) subjected to combined loadings. The strength reduction of the soil around the caisson skirt induced by installation are considered in the theoretical and numerical analyses. For caissons under undrained conditions, the uniaxial torsion capacity and the influences of the torsion on the other capacities are obtained. The results show that when it is less than 20% of the torsional capacity, the torsion applied has slight effect on the vertical, horizontal or moment capacities. When torsion applied reaches 20%~80% of the torsional capacity, the other three capacity components can be reduced by as much as 20%. The torsion influence factors against three capacities are proposed for routine designs.

Keywords:on-bottom pipelines; suction caisson; finite element methods; torsion; capacity

負压沉箱常用作固定平台和海底管汇的支撑基础[1-4]。在海底管道的日常运行阶段,受管道内热应力和海底底流的影响,负压沉箱不仅要承受竖向荷载(V)、水平荷载(H)和弯矩(M)的作用,还会受到扭矩(T)的影响,扭矩可能引起沉箱其他承载力分量的降低。Finnie等[5]给出了考虑扭矩影响的浅基础和桩基础的水平与竖向承载力计算公式;针对扭矩和水平荷载联合作用下的浅埋矩形基础,Nouri等[6]提供了塑性极限分析和三维有限元解答。对于长径比L/D(L和D分别代表沉箱的入土长度和直径)大于1、作为锚泊基础的负压沉箱,扭矩会造成竖向抗拔承载力和水平承载力的降低,但当施加的扭矩不超过20%的扭转承载能力时,扭矩的影响可以忽略[7-10]。支撑管汇的负压沉箱承受的竖向荷载为压力,而不是拉力,长径比L/D又常介于1~2之间[4],目前,还不清楚该长径比范围内沉箱承受的扭矩对其他承载力分量(即V、H和M)的影响程度。

笔者采用有限元方法,模拟不同扭矩条件下L/D=1~2的沉箱与正常固结黏土的相互作用,探索扭矩对竖向承载力、水平承载力和抗弯能力的削弱机制。在分析大量变动参数的基础上,提出考虑扭矩影响的承载力计算公式。

1 有限元模型与参数设置

采用Abaqus软件建立沉箱与土相互作用的三维有限元模型。与已有的研究[11-12]类似,取沉箱顶面中心为讨论水平荷载和弯矩的参考点。图1给出了沉箱尺寸、荷载与位移的符号和方向规定:z向向下为正,u和w分别为水平和竖向位移;转角θ对应弯矩M,代表沉箱围绕参考点的转动角度;β为对应扭矩T的沉箱扭转角度。取典型长径比L/D=1、1.5和2,沉箱壁厚为0.01D。各承载力结果将采用归一化表达,试算表明,沉箱直径D的取值不影响归一化公式,因此,除特殊声明外,均以D=10 m进行讨论。为避免边界对承载力的影响,模拟的土体范围为:径向由沉箱侧壁向外延伸3.5D;深度方向由沉箱底部向下延伸3L。土体侧边界径向位移为零,土体底部为固定边界。以L/D=2为例,有限元网格如图2所示,土体剖分采用线性六面体单元,完全积分,沉箱附近土体采用细网格,侧壁和刃角下的典型单元大小为0.01D。负压沉箱刚度远大于土体,因此,将其简化为刚体。

5 结 论

正常固结黏土中支撑管汇的负压沉箱长径比大多在1~2之间,采用理论分析和有限元方法探索了扭矩对沉箱竖向承载力、水平承载力和抗弯能力的影响。结果表明,扭矩会造成其他承载力分量的降低,降低程度与扭矩大小有关:对于长径比介于1~2的沉箱,在常规土体强度分布情况下(sum=0~20 kPa,k=0~2.5 kPa/m),当扭矩不超过20%的单向最大扭矩时,扭矩对其他承载力分量的影响非常小,可以忽略;当扭矩介于20%~80%的单向最大扭矩时,其他承载力分量最大降低20%。总结大量的有限元变动参数分析结果,给出了扭矩不超过80%的单向最大扭矩时扭矩对其他荷载分量的影响系数表达式。参考文献:

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(編辑 王秀玲)

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