Finite element numerical simulation of single pile under horizontal cyclic loading considering softening effect of soft soil
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摘要: 海洋环境中的单桩基础受到风、波浪、海流等循环荷载长期作用，在循环荷载作用下桩和软土之间的接触面上的剪应力和桩周软土的剪切强度均会降低，从而影响桩基的承载特性。结合试验手段，通过界面剪切试验获得了Q235钢与天津滨海淤泥质软黏土之间的循环界面摩擦系数，通过动三轴试验获得了天津滨海淤泥质软黏土的循环软化参数，为量化桩土界面循环摩擦特性和桩周软土循环软化特性提供了一种参数获取方法。在此基础上，结合可考虑软土应变软化效应的软土本构模型，建立了不排水软基上单桩基础受水平循环荷载作用的有限元模型，研究了单桩在受到水平循环荷载作用下的桩头位移-反力响应与桩身截面弯矩的发展规律，并获得了循环荷载作用后桩周土体软化区域的发展规律。计算结果表明：随着循环振次的增加，桩基周围土体软化区域不断发展，高水平循环位移作用下土体软化区域沿深度发展程度高，对应桩身最大弯矩出现位置下移。Abstract: The single pile foundation in marine environment is subjected to long-term cyclic loading due to wind, waves and currents. Under the cyclic loading, the interface shear stress between pile and soft soil and the shear strength of soft soil around the pile will decrease, thus affecting the bearing characteristics of pile foundation. The cyclic interface friction coefficient between Q235 steel and Tianjin coastal soft clay is obtained through the interface shear tests, and the cyclic softening parameters of Tianjin coastal soft clay are obtained through the dynamic triaxial tests, which provides a method to obtain the parameters for quantifying the cyclic friction characteristics of pile-soil interface and the cyclic softening characteristics of soft soil around piles. On this basis, a finite element model for single pile foundation in undrained soft foundation under horizontal cyclic loading is established with the combination of the constitutive model for soft soil, which can consider the strain softening effect of soft soil. The development laws of pile head displacement and reaction response and pile section bending moment under horizontal cyclic loading are studied while the development law of softening area of soil around the pile under cyclic loading is obtained. The calculated results show that the softening area of soil around the pile foundation develops continuously with the increase of cyclic number. Meanwhile, the softening zone of soil has a high degree of development along the depth under the action of high horizontal cyclic displacement. The position corresponding to the maximum bending moment of pile body moves downward.
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图 3 固结试验中试样轴向变形与试验时间关系曲线
Figure 3. Relation curves between axial deformation of specimen and test time in consolidation tests
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图 4 固结试验开始前与结束后试样形状对比
Figure 4. Comparison of specimen shape before and after consolidation test
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图 5 界面剪切位移与界面剪应力关系曲线
Figure 5. Relationship between interfacial shear displacement and interfacial shear stress
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图 6 界面剪应力随时间的变化曲线
Figure 6. Relationship between test time and interfacial shear stress
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图 7 三轴固结过程中孔隙水压力与试验时间关系曲线
Figure 7. Relation curve between pore water pressure and test time during triaxial consolidation
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图 8 动三轴试验参数确定示意图
Figure 8. Diagram of parameter determination for dynamic triaxial tests
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图 9 轴向应变与最大剪应力关系曲线
Figure 9. Relationship between axial strain and maximum shear stress
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图 12 动三轴试验与数值计算结果对比
Figure 12. Comparison between dynamic triaxial test and numerical results
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图 13 单桩离心机试验与数值模拟结果对比
Figure 13. Comparison between single pile centrifuge test and numerical results
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图 14 低水平位移循环工况下参考点水平位移与反力关系曲线
Figure 14. Relationship between horizontal displacement and reaction force of reference point under low horizontal displacement cyclic loading
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图 15 低水平位移循环工况下桩身截面弯矩分布曲线
Figure 15. Distribution curves of bending moment of pile section under low horizontal displacement cyclic loading
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图 16 单桩在低水平位移循环工况下土体软化系数
Figure 16. Soil softening coefficient of single pile under low horizontal displacement cyclic loading
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图 17 第100次循环时土体的位移场与应力场
Figure 17. Displacement field and stress field of soil at the 100th cycle
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图 18 高水平位移循环工况下参考点水平位移与反力关系曲线
Figure 18. Relationship between horizontal displacement and reaction force of reference point under high horizontal displacement cycle
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图 19 高水平位移循环工况下桩身截面弯矩分布曲线
Figure 19. Distribution curves of bending moment of pile section under high horizontal displacement cycle
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图 20 单桩在高水平循环位移工况下土体软化系数
Figure 20. Soil softening coefficient of single pile under high horizontal displacement cyclic loading
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图 21 水平循环力工况下参考点水平位移与反力关系曲线
Figure 21. Relationship between horizontal displacement and cyclic loading of reference point under horizontal cyclic force
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图 22 水平循环力工况下桩身截面弯矩分布曲线
Figure 22. Distribution curves of bending moment of pile section under horizontal cyclic force
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图 23 单桩在水平循环力工况下土体软化系数
Figure 23. Soil softening coefficient of single pile under horizontal cyclic force
表 1 试验所用黏土的物理力学性质
Table 1 Physical and mechanical properties of clay
含水率w/% 塑限wL/% 液限wP/% 相对质量密度Gs 密度ρ
/(g·cm-3)孔隙比e 不排水抗剪强度cu/kPa 17 18.6 34.2 2.72 1.89 0.665 15 
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