桩–筏–土体系的地震软化行为及ABAQUS模拟研究
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摘要
通过开展离心机动力试验,充分揭示在地震荷载的持续作用下,土体将发生一定程度的软化,表现为周期的持续增大;然而基础(筏板)周期和桩身弯矩包络图几乎不发生改变,说明整个桩–筏结构在地震作用下的行为不会类似于土体发生软化(恶化的现象),而是能持续保持稳定,其动力行为主要取决于自身特性如桩身刚度和上部结构传递给筏板的荷载水平(惯性)等,同时土体的动力软化效应对桩–筏结构的影响很小。为了验证试验结果的正确性和合理性,采用ABAQUS 6.9程序对试验结果进行计算分析,土体模型包括软件内嵌的Hypoelastic非线性模型和所开发的考虑动力降强效应的HyperMas用户子程序模型(UMAT)。计算结果符合试验实测结果,尤其能准确地捕捉桩身最大弯矩值和周期等重要的工程设计参数。且有侧重于基础单元的地震响应分析,非降强模型和降强模型的计算差异很小,再次说明土体软化对既有桩–筏结构影响很小,但采用非降强模型计算更为快捷、高效。
By conducting dynamic centrifuge tests on the pile-raft-clay system,that soil softening will be taken place in both near field and far field clay in a great degree is investigated,which are manifested as an increase in the resonance periods of clay layers with the level of shaking and successive earthquakes,while this is not the case for the pile-raft foundation since resonance periods of raft and bending moment envelopes of the pile are hardly affected by earthquake loadings.Furthermore,pile-raft dynamic behavior is hardly affected by the stiffness degradation of surrounding clay and could keep stable in successive earthquakes process.Then centrifuge tests are back-analyzed by using ABAQUS 6.9.The results show that,which are conducted using Hypoelastic model embedded in ABAQUS and a developed constitutive relationship,gave reasonably good agreement with the experimental observations.The ability of the numerical model to reasonably replicate the centrifuge tests suggested that ABAQUS simulation can be a powerful method to analyze the dynamic behavior of pile-raft-clay system under different conditions which can not be considered in the centrifuge experiments.Moreover,in ABAQUS analysis,the soil model with or without considering degradation can give almost similar results,but calculation using Hypoelastic soil model showed timesaving and speedy advantages.
By conducting dynamic centrifuge tests on the pile-raft-clay system,that soil softening will be taken place in both near field and far field clay in a great degree is investigated,which are manifested as an increase in the resonance periods of clay layers with the level of shaking and successive earthquakes,while this is not the case for the pile-raft foundation since resonance periods of raft and bending moment envelopes of the pile are hardly affected by earthquake loadings.Furthermore,pile-raft dynamic behavior is hardly affected by the stiffness degradation of surrounding clay and could keep stable in successive earthquakes process.Then centrifuge tests are back-analyzed by using ABAQUS 6.9.The results show that,which are conducted using Hypoelastic model embedded in ABAQUS and a developed constitutive relationship,gave reasonably good agreement with the experimental observations.The ability of the numerical model to reasonably replicate the centrifuge tests suggested that ABAQUS simulation can be a powerful method to analyze the dynamic behavior of pile-raft-clay system under different conditions which can not be considered in the centrifuge experiments.Moreover,in ABAQUS analysis,the soil model with or without considering degradation can give almost similar results,but calculation using Hypoelastic soil model showed timesaving and speedy advantages.
引文
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[13]HUANG Y,ZHANG F,YASHIMA A,et al.Three-dimensionalnumerical simulation of pile-soil seismic interaction in saturateddeposits with liquefiable sand and soft clay[C]//Proceeding ofComputational Mechanics WCCM VI in Conjunction with APCOM.Beijing:[s.n.],2004:5–10.
[2]马亢,裴建良.桩筏基础–土动力相互作用的离心机模型试验研究[J].岩石力学与工程学报,2011,30(7):1488–1495.(MAKang,PEI Jianliang.Study of dynamic interactions between pile-raftfoundation and soft clay by centrifuge tests[J].Chinese Journal ofRock Mechanics and Engineering,2011,30(7):1 488–1 495.(inChinese))
[3]马亢,裴建良.桩–土动力相互作用问题中桩身刚度的影响效应研究[J].岩石力学与工程学报,2011,30(增2):3626–3632.(MA Kang,PEI Jianliang.Effect of pile stiffness on the pile-soildynamic interaction problem[J].Chinese Journal of Rock Mechanicsand Engineering,2011,30(Supp.2):3 626–3 632.(in Chinese)).
[4]SNYDER J L.Full scale test lateral load tests of a 3×5 pile group insoft clays and silts[M.S.Thesis][D].Brigham,USA:Brigham YoungUniversity,2004.
[5]VUCETIC M.Normalised behaviour of offshore clay under uniformcyclic loading[J].Canadian Geotechnical Journal,1988,25(1):33–41.
[6]VIGGIANI G,ATKINSON J H.Stiffness of fine-grained soils at verysmall strains[J].Geotechnique,1995,45(2):249–265.
[7]马亢.地震条件下桩筏基础与土的动力相互作用机制研究[博士学位论文][D].成都:四川大学,2010.(MA Kang.Dynamicpile-raft-soil interaction in soft clay condition during earthquakes[Ph.D.Thesis][D].Chengdu:Sichuan University,2010.
[8]SUBHADEEP B.Centrifuge and numerical modeling of softclay-pile-raft foundations to seismic shaking[Ph.D.Thesis][D].Singapore:National University of Singapore,2009.
[9]SUBHADEEP B,LEE F H,MA K.Far-field seismic soil-pile-raftinteraction in normally consolidated kaolin clay[C]//Proceedings ofthe 7th International Conference on Physical Modelling in Geotechnics.[S.l.]:[s.n.],2010:34–39.
[10]MASING G.Eigenspannungen und verfestigung beim messing[C]//Proceedings of the 2nd International Congress of Applied Mechanics.[S.l.]:[s.n.],1926:332–335.
[11]DASARI G R.Modeling of the variation of soil stiffness duringsequential construction[Ph.D.Thesis][D].United Kingdom:Cambridge University,1996.
[12]IDRISS I M,MORIWAKI Y,WRIGHT S G,et al.Behaviour ofnormally consolidated clay under simulated earthquake and oceanwave loading conditions[C]//Proceedsing of InternationalSymposium on Soils under Cyclic and Transient Loading.[S.l.]:[s.n.],1980:437–445.
[13]HUANG Y,ZHANG F,YASHIMA A,et al.Three-dimensionalnumerical simulation of pile-soil seismic interaction in saturateddeposits with liquefiable sand and soft clay[C]//Proceeding ofComputational Mechanics WCCM VI in Conjunction with APCOM.Beijing:[s.n.],2004:5–10.
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