软黏土中张紧式吸力锚循环承载力简化计算方法
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摘要
张紧式吸力锚是一种重要的深水浮式平台基础。深水环境中,海底浅层沉积物多为饱和软黏土。软黏土中平均荷载与循环荷载共同作用下吸力锚的循环承载力对其设计至关重要。根据最佳系泊点受倾斜荷载作用下吸力锚的破坏模式,假设不同破坏区土体具有相同的平均剪应力,且平均剪应力与循环剪切强度比等于吸力锚所受静荷载与静荷载和循环荷载之和的比值。结合室内土性试验获得的饱和软黏土样不固结不排水归一化循环剪切强度随归一化平均剪应力的变化关系曲线,建立了破坏区土体不排水循环剪切强度的确定方法。对不同破坏区土体阻力进行分析,按照水平应力具有连续性这一特点,构建了上部滑动楔体破坏区与深部平面流动破坏区交界深度的计算方法,进而提出了计算吸力锚循环承载力的简化极限平衡方法。采用该方法对竖向和水平破坏模式吸力锚循环承载力模型试验结果进行了预测,预测与试验结果基本吻合,最小和最大偏差分别为0.79%和16.08%,平均偏差为5.74%,可较好地反映吸力锚循环承载力随循环破坏次数增加而减小的变化关系,验证了该方法的可行性。
Suction anchors with taut mooring system are important anchor foundations for the floating platforms in deep water where shallow sediments are mostly saturated soft clays. The cyclic bearing capacity of the suction anchor under a combination of average and cyclic loadings is vital factor for the design of the anchor in soft clay. The paper assumes that the average shear stresses of the soil for different failure zones are uniform and the ratio of the average shear stress to the cyclic shear strength is equal to the ratio of the static load to the sum of static and cyclic loads applied to the anchor based on the failure mode of the suction anchor under inclined loadings at the optimal load attachment point. Undrained cyclic shear strengths of the soil for failure zones are determined based on curves of normalized cyclic shear strength versus normalized average shear stress obtained from laboratory tests of the soft clay samples. Soil resistances for the different failure zones are analyzed. An approach for calculating the interface depth between upper sliding wedge failure zone and deep plane flow failure zone is developed according to the horizontal stress continuity condition.Further, a simplified limiting equilibrium method is proposed for evaluating the cyclic bearing capacity of the anchor. The proposed simplified calculation method is employed to predict the model tests of the cyclic bearing capacity of suction anchors with both vertical and horizontal failure modes. The predictions are found to be in agreement with the experimental results. The smallest and greatest deviations are 0.79 % and 16.08 % respectively and the average deviation is 5.74 %. The predicted results could preferably reflect the variation between the cyclic bearing capacity and the number of cycle to failure, which verifies the feasibility of the proposed simplified method for calculating the cyclic bearing capacity of the suction anchor.
Suction anchors with taut mooring system are important anchor foundations for the floating platforms in deep water where shallow sediments are mostly saturated soft clays. The cyclic bearing capacity of the suction anchor under a combination of average and cyclic loadings is vital factor for the design of the anchor in soft clay. The paper assumes that the average shear stresses of the soil for different failure zones are uniform and the ratio of the average shear stress to the cyclic shear strength is equal to the ratio of the static load to the sum of static and cyclic loads applied to the anchor based on the failure mode of the suction anchor under inclined loadings at the optimal load attachment point. Undrained cyclic shear strengths of the soil for failure zones are determined based on curves of normalized cyclic shear strength versus normalized average shear stress obtained from laboratory tests of the soft clay samples. Soil resistances for the different failure zones are analyzed. An approach for calculating the interface depth between upper sliding wedge failure zone and deep plane flow failure zone is developed according to the horizontal stress continuity condition.Further, a simplified limiting equilibrium method is proposed for evaluating the cyclic bearing capacity of the anchor. The proposed simplified calculation method is employed to predict the model tests of the cyclic bearing capacity of suction anchors with both vertical and horizontal failure modes. The predictions are found to be in agreement with the experimental results. The smallest and greatest deviations are 0.79 % and 16.08 % respectively and the average deviation is 5.74 %. The predicted results could preferably reflect the variation between the cyclic bearing capacity and the number of cycle to failure, which verifies the feasibility of the proposed simplified method for calculating the cyclic bearing capacity of the suction anchor.
引文
[1]ERBRICH C,HEFER P.Installation of the Laminaria suction piles-a case history[C]//Proceedings of the 34th Annual Offshore Technology Conference.Houston:[s.n.],2002.
[2]AUDIBERT J M E,CLUKEY E C,HUANG J.Suction caisson installation at Horn Mountain-a case history[C]//Proceedings of the 13th International Offshore and Polar Engineering Conference.Honolulu:[s.n.],2003,762-769.
[3]MAHMOUD R E.Performance of suction caisson anchors in normally consolidated clay[D].Texas:University of Texas at Austin,2005.
[4]WANG J H,LI S Z.Analysis procedure of the cyclic bearing capacity for suction anchors in soft clays[J].Marine Georesources&Geotechnology,2015,33:546-555.
[5]WANG J H,LI S Z.Model tests and analysis method on the bearing capacity for suction anchors subjected to average and cyclic loads[J].Ocean Engineering,2015,104:266-275.
[6]ANDRESEN L,ANDERSEN K H,JOSTAD H P,et al.Bearing capacity of offshore gravity platforms by 3DFEM[C]//Num.Mod.Geomech.-NUMOG X.Rhodes:[s.n.],2007.
[7]ANDRESEN L,JOSTAD H P,ANDERSEN K H,et al.Finite element analyses in offshore foundation design[J].Wood&Wood Products,2008.
[8]ANDRESEN L,JOSTAD H P,ANDERSEN K H.Finite element analyses applied in design of foundations and anchors for offshore structures[J].International Journal of Geomechanics,ASCE,2011,11(6):417-430.
[9]王建华,杨海明.软土中桶形基础水平循环承载力的模型试验[J].岩土力学,2008,29(10):2606-2612.WANG Jian-hua,YANG Hai-ming.Model tests on horizontal cyclic bearing capacity of bucket foundations in soft clays[J].Rock and Soil Mechanics,2008,29(10):2606-2612.
[10]王建华,李书兆.循环荷载作用下软土中吸力锚承载力拟静力有限元分析[J].岩土工程学报,2013,35(8):1404-1412.WANG Jian-hua,LI Shu-zhao.Pseudo static finite element analysis of bearing capacity of suction anchor subjected to cyclic loads in soft clay[J].Chinese Journal of Geotechnical Engineering,2013,35(8):1403-1412.
[11]李书兆.循环荷载作用下张紧式吸力锚基础变形与承载力分析方法研究[D].天津:天津大学,2014.LI Shu-zhao.Study of analysis methods of deformation and bearing capacity of suction anchor under cyclic loads[D].Tianjin:Tianjin University,2014.
[12]LI S Z,WANG J H.Constitutive model of saturated soft clay with cyclic loads under unconsolidated undrained condition[J].Transactions of Tianjin University,2013,19:260-266.
[13]王建华,李书兆.描述软土不排水循环应力应变响应的拟动力关系[J].地震工程学报,2014,36(3):421-428.WANG Jian-hua,LI Shu-zhao.A pseudo-dynamic relationship for describing the cyclic stress-strain response of soft clays in undrained conditions[J].China Earthquake Engineering Journal,2014,36(3):421-428.
[14]CASSIDY M J.Non-linear analysis of jack-up structures subjected to random waves[D].Oxford:University of Oxford,1999.
[15]CASSIDY M J,RANDOLPH M F,BYRNE B W.Aplasticity model describing caisson behaviour in clay[J].Applied Ocean Research,2006,28(5):345-358.
[16]李涛,MEISSNER H.循环荷载作用下饱和黏性土的弹塑性双面模型[J].土木工程学报,2006,39(1):92-97.LI Tao,MEISSNER H.Elasto-plastic two surface model for clays under undrained cyclic loading[J].China Civil Engineering Journal,2006,39(1):92-97.
[17]胡存.适用于饱和软土循环动力分析的边界面塑性模型及应用[D].天津:天津大学,2012.HU Cun.Anisotropic bounding-surface plasticity model for cyclic behaviors of saturated clay and its application[D].Tianjin:Tianjin University,2012.
[18]ANDERSEN K H,LAURITZSEN R,DYVIK R,et al.Cyclic bearing capacity analysis for gravity platforms:calculation procedure,verification by model tests,and application for the Gullfaks C platform[J].International Journal of Rock Mechanics&Mining Sciences&Geomechanics Abstracts,1988,27(2):A110-A111.
[19]ANDERSEN K H,LAURITZSEN R.Bearing capacity for foundation with cyclic loads[J].Journal of Geotechnical Engineering,ASCE,1988,114(5):540-555.
[20]ANDERSEN K H.Bearing capacity under cyclic loading-offshore,along the coast,and on land[J].Canada Geotechnical Journal,2009,46(3):513-535.
[21]ANDERSEN K H,MURFF J D,RANDOLPH M F,et al,Suction anchors for deepwater applications[C]//Proceedings of the International Symposium on Frontiers in Offshore Geotechniques(ISFOG).Perth:[s.n.],2005:1-30.
[22]DET NORSKE VERITAS.DNV Recommended Practice RP-E303 Geotechnical design and installation of suction anchors in clay[S].H?vik:[s.n.],2005.
[23]王建华,李书兆.利用循环强度分析张紧式吸力锚循环承载力的方法[J].岩土力学,2012,33(12):3521-3528.WANG Jian-hua,LI Shu-zhao.Method for analyzing cyclic bearing capacity of suction anchor with taut mooring system using cyclic strengths[J].Rock and Soil Mechanics,2012,33(12):3521-3528.
[24]陈文强.软黏土中张紧式吸力锚承载力理论分析与模型试验[D].天津:天津大学,2010.CHEN Wen-qiang.Theoretical analysis and model test of bearing capacity of suction caissons with taut wires in soft clay[D].Tianjin:Tianjin University,2010.
[25]刘晶磊.循环荷载作用下软黏土中张紧式吸力锚承载力研究[D].天津:天津大学,2011.LIU Jing-lei.Study on bearing capacity of suction anchors with taut mooring systems in soft clay under cyclic loading[D].Tianjin:Tianjin University,2011.
[2]AUDIBERT J M E,CLUKEY E C,HUANG J.Suction caisson installation at Horn Mountain-a case history[C]//Proceedings of the 13th International Offshore and Polar Engineering Conference.Honolulu:[s.n.],2003,762-769.
[3]MAHMOUD R E.Performance of suction caisson anchors in normally consolidated clay[D].Texas:University of Texas at Austin,2005.
[4]WANG J H,LI S Z.Analysis procedure of the cyclic bearing capacity for suction anchors in soft clays[J].Marine Georesources&Geotechnology,2015,33:546-555.
[5]WANG J H,LI S Z.Model tests and analysis method on the bearing capacity for suction anchors subjected to average and cyclic loads[J].Ocean Engineering,2015,104:266-275.
[6]ANDRESEN L,ANDERSEN K H,JOSTAD H P,et al.Bearing capacity of offshore gravity platforms by 3DFEM[C]//Num.Mod.Geomech.-NUMOG X.Rhodes:[s.n.],2007.
[7]ANDRESEN L,JOSTAD H P,ANDERSEN K H,et al.Finite element analyses in offshore foundation design[J].Wood&Wood Products,2008.
[8]ANDRESEN L,JOSTAD H P,ANDERSEN K H.Finite element analyses applied in design of foundations and anchors for offshore structures[J].International Journal of Geomechanics,ASCE,2011,11(6):417-430.
[9]王建华,杨海明.软土中桶形基础水平循环承载力的模型试验[J].岩土力学,2008,29(10):2606-2612.WANG Jian-hua,YANG Hai-ming.Model tests on horizontal cyclic bearing capacity of bucket foundations in soft clays[J].Rock and Soil Mechanics,2008,29(10):2606-2612.
[10]王建华,李书兆.循环荷载作用下软土中吸力锚承载力拟静力有限元分析[J].岩土工程学报,2013,35(8):1404-1412.WANG Jian-hua,LI Shu-zhao.Pseudo static finite element analysis of bearing capacity of suction anchor subjected to cyclic loads in soft clay[J].Chinese Journal of Geotechnical Engineering,2013,35(8):1403-1412.
[11]李书兆.循环荷载作用下张紧式吸力锚基础变形与承载力分析方法研究[D].天津:天津大学,2014.LI Shu-zhao.Study of analysis methods of deformation and bearing capacity of suction anchor under cyclic loads[D].Tianjin:Tianjin University,2014.
[12]LI S Z,WANG J H.Constitutive model of saturated soft clay with cyclic loads under unconsolidated undrained condition[J].Transactions of Tianjin University,2013,19:260-266.
[13]王建华,李书兆.描述软土不排水循环应力应变响应的拟动力关系[J].地震工程学报,2014,36(3):421-428.WANG Jian-hua,LI Shu-zhao.A pseudo-dynamic relationship for describing the cyclic stress-strain response of soft clays in undrained conditions[J].China Earthquake Engineering Journal,2014,36(3):421-428.
[14]CASSIDY M J.Non-linear analysis of jack-up structures subjected to random waves[D].Oxford:University of Oxford,1999.
[15]CASSIDY M J,RANDOLPH M F,BYRNE B W.Aplasticity model describing caisson behaviour in clay[J].Applied Ocean Research,2006,28(5):345-358.
[16]李涛,MEISSNER H.循环荷载作用下饱和黏性土的弹塑性双面模型[J].土木工程学报,2006,39(1):92-97.LI Tao,MEISSNER H.Elasto-plastic two surface model for clays under undrained cyclic loading[J].China Civil Engineering Journal,2006,39(1):92-97.
[17]胡存.适用于饱和软土循环动力分析的边界面塑性模型及应用[D].天津:天津大学,2012.HU Cun.Anisotropic bounding-surface plasticity model for cyclic behaviors of saturated clay and its application[D].Tianjin:Tianjin University,2012.
[18]ANDERSEN K H,LAURITZSEN R,DYVIK R,et al.Cyclic bearing capacity analysis for gravity platforms:calculation procedure,verification by model tests,and application for the Gullfaks C platform[J].International Journal of Rock Mechanics&Mining Sciences&Geomechanics Abstracts,1988,27(2):A110-A111.
[19]ANDERSEN K H,LAURITZSEN R.Bearing capacity for foundation with cyclic loads[J].Journal of Geotechnical Engineering,ASCE,1988,114(5):540-555.
[20]ANDERSEN K H.Bearing capacity under cyclic loading-offshore,along the coast,and on land[J].Canada Geotechnical Journal,2009,46(3):513-535.
[21]ANDERSEN K H,MURFF J D,RANDOLPH M F,et al,Suction anchors for deepwater applications[C]//Proceedings of the International Symposium on Frontiers in Offshore Geotechniques(ISFOG).Perth:[s.n.],2005:1-30.
[22]DET NORSKE VERITAS.DNV Recommended Practice RP-E303 Geotechnical design and installation of suction anchors in clay[S].H?vik:[s.n.],2005.
[23]王建华,李书兆.利用循环强度分析张紧式吸力锚循环承载力的方法[J].岩土力学,2012,33(12):3521-3528.WANG Jian-hua,LI Shu-zhao.Method for analyzing cyclic bearing capacity of suction anchor with taut mooring system using cyclic strengths[J].Rock and Soil Mechanics,2012,33(12):3521-3528.
[24]陈文强.软黏土中张紧式吸力锚承载力理论分析与模型试验[D].天津:天津大学,2010.CHEN Wen-qiang.Theoretical analysis and model test of bearing capacity of suction caissons with taut wires in soft clay[D].Tianjin:Tianjin University,2010.
[25]刘晶磊.循环荷载作用下软黏土中张紧式吸力锚承载力研究[D].天津:天津大学,2011.LIU Jing-lei.Study on bearing capacity of suction anchors with taut mooring systems in soft clay under cyclic loading[D].Tianjin:Tianjin University,2011.