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    正常固結黏土中扭矩對負壓沉箱承載力的影響分析

    2020-04-17 14:45:26 《土木建筑與環境工程》 2020年1期

    周松望 張艷 王棟

    摘 要:當負壓沉箱被用作深水管匯或管道終端基礎時,除了受到上部結構傳來的豎向力、水平力和彎矩,還會受到扭矩,扭矩可能降低沉箱的豎向承載力、水平承載力和抗彎能力。采用理論分析和有限元法研究復合加載條件下典型沉箱(長徑比介于1~2之間)與正常固結黏性土的相互作用,考慮安裝造成的沉箱側壁周圍土體的弱化,得到了不排水條件下沉箱單向最大扭矩,探索了扭矩與不同荷載分量聯合作用時承載能力的改變。結果表明,當扭矩不超過20%的抗扭轉能力時,可以忽略扭矩對豎向承載力、水平承載力或抗彎能力的影響;當扭矩介于抗扭轉能力的20%~80%時,其他承載力分量最多降低20%。提出了能夠用于工程設計的扭矩對其他荷載分量抗力的影響系數。

    關鍵詞:海底管線;負壓沉箱;有限元;扭矩;承載力

    中圖分類號:TU449 文獻標志碼:A 文章編號: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

    負壓沉箱常用作固定平臺和海底管匯的支撐基礎[1-4]。在海底管道的日常運行階段,受管道內熱應力和海底底流的影響,負壓沉箱不僅要承受豎向荷載(V)、水平荷載(H)和彎矩(M)的作用,還會受到扭矩(T)的影響,扭矩可能引起沉箱其他承載力分量的降低。Finnie等[5]給出了考慮扭矩影響的淺基礎和樁基礎的水平與豎向承載力計算公式;針對扭矩和水平荷載聯合作用下的淺埋矩形基礎,Nouri等[6]提供了塑性極限分析和三維有限元解答。對于長徑比L/D(L和D分別代表沉箱的入土長度和直徑)大于1、作為錨泊基礎的負壓沉箱,扭矩會造成豎向抗拔承載力和水平承載力的降低,但當施加的扭矩不超過20%的扭轉承載能力時,扭矩的影響可以忽略[7-10]。支撐管匯的負壓沉箱承受的豎向荷載為壓力,而不是拉力,長徑比L/D又常介于1~2之間[4],目前,還不清楚該長徑比范圍內沉箱承受的扭矩對其他承載力分量(即V、H和M)的影響程度。

    筆者采用有限元方法,模擬不同扭矩條件下L/D=1~2的沉箱與正常固結黏土的相互作用,探索扭矩對豎向承載力、水平承載力和抗彎能力的削弱機制。在分析大量變動參數的基礎上,提出考慮扭矩影響的承載力計算公式。

    1 有限元模型與參數設置

    采用Abaqus軟件建立沉箱與土相互作用的三維有限元模型。與已有的研究[11-12]類似,取沉箱頂面中心為討論水平荷載和彎矩的參考點。圖1給出了沉箱尺寸、荷載與位移的符號和方向規定:z向向下為正,u和w分別為水平和豎向位移;轉角θ對應彎矩M,代表沉箱圍繞參考點的轉動角度;β為對應扭矩T的沉箱扭轉角度。取典型長徑比L/D=1、1.5和2,沉箱壁厚為0.01D。各承載力結果將采用歸一化表達,試算表明,沉箱直徑D的取值不影響歸一化公式,因此,除特殊聲明外,均以D=10 m進行討論。為避免邊界對承載力的影響,模擬的土體范圍為:徑向由沉箱側壁向外延伸3.5D;深度方向由沉箱底部向下延伸3L。土體側邊界徑向位移為零,土體底部為固定邊界。以L/D=2為例,有限元網格如圖2所示,土體剖分采用線性六面體單元,完全積分,沉箱附近土體采用細網格,側壁和刃角下的典型單元大小為0.01D。負壓沉箱剛度遠大于土體,因此,將其簡化為剛體。

    5 結 論

    正常固結黏土中支撐管匯的負壓沉箱長徑比大多在1~2之間,采用理論分析和有限元方法探索了扭矩對沉箱豎向承載力、水平承載力和抗彎能力的影響。結果表明,扭矩會造成其他承載力分量的降低,降低程度與扭矩大小有關:對于長徑比介于1~2的沉箱,在常規土體強度分布情況下(sum=0~20 kPa,k=0~2.5 kPa/m),當扭矩不超過20%的單向最大扭矩時,扭矩對其他承載力分量的影響非常小,可以忽略;當扭矩介于20%~80%的單向最大扭矩時,其他承載力分量最大降低20%。總結大量的有限元變動參數分析結果,給出了扭矩不超過80%的單向最大扭矩時扭矩對其他荷載分量的影響系數表達式。參考文獻:

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    (編輯 王秀玲)

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