摘要
海上风机在运行过程中风荷载与波浪荷载频率耦合作用对土体动力特性的影响不容忽略,为了探究这种影响,采用多向循环动单剪系统(VDDCSS)对砂土进行了一系列试验研究,分析了双向剪切频率比fr对砂土动应变、动孔压比以及动强度的影响。试验结果表明:剪应变发展受双向频率耦合及应力组合效应的影响。循环应力比(CSR)为0.15时,双向频率耦合效应大于应力组合作用;随着应力水平的增加(CSR = 0.20或0.25),频率耦合效应和应力组合作用均得到了加强。在低应力水平时,破坏试样的孔压曲线呈“上凹型”且频率比对试样孔压发展影响不大。而随着应力水平增加(CSR = 0.20或0.25),孔压最终值umax随之增大。除fr = 1.00与10.00对应试样外,土体动强度随fr与CSR值增大而减少。fr= 5.00 对应试样应变大、孔压值高、强度低的原因与此比值下土体受双向频率耦合与应力组合的叠加效果增强有关。
对于海上风机而言,嵌入海床的基础直接关乎风机整体稳定性与安全性,是风机设计最重要的一环。目前,在近海40m以内的风机建设中,大直径单桩基础是应用最为广泛的基础形式。海上单桩的稳定性与桩周土体的动力响应特性紧密相关,准确分析复杂荷载环境下土体动力响应是风机设计的首要任务。海洋工程中研究土体动力特性的方法主要包括以下几种:理论解析
以往大多数学者多利用动三轴仪和空心圆柱扭剪仪等探究频率对土体动力特性的影响。许成顺
采用英国GDS公司生产的伺服电机控制动态循环单剪仪(VDDCSS),如

图1 试验仪器
Fig. 1 Testing apparatus
使用重塑砂土试样,
D50 /mm | ||||
---|---|---|---|---|
0.22 | 4.17 | 2.66 | 2.09 | 1.56 |

图2 土的级配曲线
Fig. 2 Particle size distribution (PSD)
土样的制备方法为干沉积法:使用漏斗将固定质量的干砂分5次沉积到模具中,然后用橡胶棒在模具周边均匀地进行击打,以获得要求的制备密实度(

图3 土样制备过程
Fig. 3 Process of sample preparation

图4 制备土样的仪器
Fig. 4 Sample preparation instrument
装好试样后,在竖向施加100kPa的竖向应力,变形稳定后(竖向位移变化量小于 0.001 mm·

图5 试样受力模式
Fig. 5 Stress mode of soil sample
试验中x、y向剪切均采用应力控制,波形为不同频率的正弦波,具体的试验方案见
编号 | fx/Hz | fy/Hz | σ0 /kPa | CSR | ||
---|---|---|---|---|---|---|
A1 | 10.6 | 10.6 | 0.01 | 0.10 | 100 | 0.15 |
A2 | 10.6 | 10.6 | 0.02 | 0.10 | 100 | 0.15 |
A3 | 10.6 | 10.6 | 0.04 | 0.10 | 100 | 0.15 |
A4 | 10.6 | 10.6 | 0.06 | 0.10 | 100 | 0.15 |
A5 | 10.6 | 10.6 | 0.08 | 0.10 | 100 | 0.15 |
A6 | 10.6 | 10.6 | 0.10 | 0.10 | 100 | 0.15 |
B1 | 14.14 | 14.14 | 0.01 | 0.10 | 100 | 0.20 |
B2 | 14.14 | 14.14 | 0.02 | 0.10 | 100 | 0.20 |
B3 | 14.14 | 14.14 | 0.04 | 0.10 | 100 | 0.20 |
B4 | 14.14 | 14.14 | 0.06 | 0.10 | 100 | 0.20 |
B5 | 14.14 | 14.14 | 0.08 | 0.10 | 100 | 0.20 |
B6 | 14.14 | 14.14 | 0.10 | 0.10 | 100 | 0.20 |
C1 | 17.67 | 17.67 | 0.01 | 0.10 | 100 | 0.25 |
C2 | 17.67 | 17.67 | 0.02 | 0.10 | 100 | 0.25 |
C3 | 17.67 | 17.67 | 0.04 | 0.10 | 100 | 0.25 |
C4 | 17.67 | 17.67 | 0.06 | 0.10 | 100 | 0.25 |
C5 | 17.67 | 17.67 | 0.08 | 0.10 | 100 | 0.25 |
C6 | 17.67 | 17.67 | 0.10 | 0.10 | 100 | 0.25 |

图6 CSR = 0.15时x和y向加载方式
Fig. 6 Loading modes of x and y position at a CSR of 0.15

图7 不同频率比下的动剪应力路径示意
Fig. 7 Dynamic shear stress paths at different bi-directional shear frequencies
定义剪切应变,其中D为土样最大水平位移,H为土样初始高度。参照文献[





图8 不同循环应力比下应变路径变化
Fig. 8 Strain paths versus different CSRs values


图9 不同循环应力比下应变与循环次数的关系
Fig. 9 Bidirectional strain versus cycles at different cycle stress ratios
当CSR值增加至0.20时,双向应变速率对应的从快到慢为:1.00、5.00、2.50、1.70、1.25、10.00。对比CSR = 0.15时应变发展模式发现:在低应力比时,双向应变主要受频率耦合效应的影响,受应力组合作用影响较小;随着CSR值增加,双向应力组合逐步起作用: = 1.00时,x向与y向应力组合作用大(幅值点重合),应变发展迅速, = 10.00时(= 0.01)过低的频率使得双向应力组合作用弱。值得注意的是= 5.00对应试样双向应变值仍很大,主要原因为此频率比下试样不止受到应力组合作用(由于双向幅值点重合,应力组合作用最大),还受到双向频率耦合效应影响。CSR = 0.25时,所有试样在N = 25内达到破坏标准且其双向应变累积速率对应的频率比从快到慢仍为 :1.00、5.00、2.50、1.70、1.25、10.00。由于导致的双向应变差异值随之增大,其中为5.00、10.00、2.50、1.70时甚至出现单向(x向)破坏的现象。不同CSR值下双向应变发展模式不同,是由于双向应变发展受频率比导致的双向频率耦合及x、y向应力组合的影响。CSR = 0.15时,双向频率耦合效应大于应力组合作用;CSR = 0.20时,频率比为1.00对应双向应力组合作用大,对应双向应变累积速率加快;CSR = 0.25时,随着应力水平的增加,频率耦合效应和应力组合作用均得到了加强,使试样快速发展至破坏。

图10 不同循环应力比下总应变与循环圈数的关系
Fig. 10 Total shear strain versus cycles at different cycle stress ratios
常体积剪切试验通过竖向应力的变化控制剪切过程中试样高度保持不变,因此剪切过程中产生的孔隙水压力与有效应力的变化值相同,方向相反,即。
式中: u0为土样初始孔压值;t1和t2是双向频率比变化导致的时间偏移量参数;A1和A2是与频率比导致的双向幅值偏移量有关的参数。另外,值得注意的是CSR = 0.15时破坏试样的孔压累积值与循环圈数不满足指数型关系的原因与Nt的存在有关,Nt使得孔压累积速率分2段:在达到Nt前,孔压累积速率与未破坏试样基本一致;在达到Nt后,孔压快速增长直至接近液化,导致孔压与循环圈数不再呈指数关系。而其他应力比下的孔压增长模式为单调性增长或先增大后平稳的模式。

图11 不同循环应力比下孔压与循环圈数的关系
Fig. 11 Pore water pressure versus cycles at different cycle stress ratios
试验采用应变破坏标准,即应变达到10 % 时作为土体的破坏标准,而土样的动强度则为达到该应变时所需的破坏圈

图12 不同循环应力比下动强度与频率比的关系
Fig. 12 Dynamic strength versus frequency ratio at different cycle stress ratios
(1)砂土应变路径发展模式受到频率比的影响且这种影响随循环剪应力比CSR的增大而增强。
(2)双向频率耦合效应及应力组合效应主要受频率比与循环剪应力比的影响。频率比控制双向耦合效应,而应力组合作用既受频率比影响也受循环剪应力比影响;在低应力水平时,双向频率耦合作用起主导作用,在高应力水平时,应力组合与双向频率耦合效应均会增强。
(3)在低应力水平时,除破坏试样,其余试样孔压与循环圈数呈指数关系,破坏试样不满足此关系与破坏时主要受频率耦合效应有关。在高应力水平下(CSR 为 0.20 或0.25),孔压与循环圈数满足指数关系是因为这种破坏模式同时受频率耦合和应力组合效应的影响。
(4)除 为1.00与10.00对应试样外,土体动强度随增大而增大,随CSR增大而减少; = 5.00 对应试样应变大、孔压值高、动强度低的原因与此比值下土体受到双向频率耦合与应力组合的叠加效果强相关。
作者贡献声明
张 艳:试验完成、数据处理、论文撰写。
贾敏才:研究思路指导、论文修改。
蒋明镜:项目负责人,论文修改。
谢志伟:辅助完成试验、论文修改。
参考文献
ARANY L, BHATTACHARYA S, JOHN H G, et al. Closed form solution of eigen frequency of monopile supported offshore wind turbines in deeper waters incorporating stiffness of substructure and SSI [J]. Soil Dynamics & Earthquake Engineering, 2016, (83): 18. [百度学术]
ARANY L, BHATTACHARYA S, JOHN H G, et al. Design of monopiles for offshore wind turbines in 10 steps[J]. Soil Dynamics & Earthquake Engineering, 2017(92): 126. [百度学术]
SHADLOU M and BHATTACHARYA S. Dynamic stiffness of monopiles supporting offshore wind turbine generators [J]. Soil Dynamics & Earthquake Engineering, 2016(88): 15. [百度学术]
ARANY L and BHATTACHARYA S. Simplified load estimation and sizing of suction anchors for spar buoy type floating offshore wind turbines [J]. Ocean Engineering, 2018(159): 348. [百度学术]
张毅,马永亮,曲先强,等.冰区海上风机的动力响应及疲劳分析[J]. 舰船科学技术, 2018, 40(1): 81. [百度学术]
ZHANG Y, MA Y, QU X Q, et al. Dynamic response and fatigue analysis of offshore wind turbine in ice region [J]. Ship Science and Technology, 2018, 40(1): 81. [百度学术]
HETTLER A, GUDEHUS G. Estimation of shakedown displacement in sand bodies with the aid of model tests[C]//International Symposium on Soils Under Cyclic and Transient Loading. Swansea: [S.n.], 1980, (1): 3–8. [百度学术]
GUDEHUS G, HETTLER A. Cyclic and monotonous model tests in sand[C]//Proceeding of the International Conference On Soil Mechanics and Foundation Engineering. Stockholm: [S.n.], 1981, (3): 211–214. [百度学术]
ACHMUS M, YU S K, KHALID A R. Behavior of monopile foundations under cyclic lateral load [J]. Computers and Geotechnics, 2009, 36(5): 725. [百度学术]
王俊岭, 闫澍旺, 霍知亮. 复合加载模式下海上风机桩基础破坏机制研究[J]. 勘察科学技术, 2013, (1): 4. [百度学术]
WANG Junling, YAN Shuwang, HUO Zhiliang. Study of failure patterns of monopile foundation for offshore wind turbines under combined loading [J]. Site Investigation Science and Technology, 2013, (1):4. [百度学术]
ARANY L, BHATTACHARYA S, HOGAN S J. et al. An analytical model to predict the natural frequency of offshore wind turbines on three-spring flexible foundations using two different beam models [J]. Soil Dynamic and Earthquake Engineering, 2015, (74): 40. [百度学术]
牛壮壮, 俞剑, 黄茂松.大直径单桩水平循环弱化有限元分析[J]. 防灾减灾工程学报, 2009, 39(1): 5. [百度学术]
NIU Zhuangzhuang, YU Jian, HUANG Maosong. Finite element modelling on degradation of a monopile subjected to cyclic lateral loads[J]. Journal of Disaster Prevention and Mitigation Engineering, 2009, 39(1): 5. [百度学术]
LONG J H and VANNESTE G. Effects of cyclic lateral loads on piles in sand [J]. Journal of Geotechnical Engineering, 1994, 120(1): 225. [百度学术]
LOUKIDIS D and SALGADO R. Analysis of the shaft resistance of non-displacement piles in sand [J]. Geotechnique, 2008, 58(4): 283. [百度学术]
BHATTACHARYA S and ADHIKARI S, Experimental validation of soil–structure interaction of offshore wind turbines [J]. Soil Dynamics & Earthquake Engineering, 2011, 31(5/6): 805. [百度学术]
BHATTACHARYA S, NIKITAS N J, GARNSEY N A, et al. Observed dynamic soil–structure interaction in scale testing of offshore wind turbine foundations [J]. Soil Dynamics & Earthquake Engineering, 2013(54):47. [百度学术]
DOMENICO L, SUBHAMOY B, DAVID M W, Dynamic soil–structure interaction of monopile supported wind turbines in cohesive soil [J]. Soil Dynamics and Earthquake Engineering, 2013(49): 165. [百度学术]
TIWARI B and AI-ADHADH A R, Influence of relative density on static soil–structure frictional resistance of dry and saturated sand [J]. Geotechnical & Geological Engineering, 2014, 32(2): 411. [百度学术]
VICENTE N, GUTIERREZ M, ESTEBAN D, et al. Monopiles in offshore wind: preliminary estimate of main dimensions [J]. Ocean Engineering, 2017, 133(15): 253. [百度学术]
NANDA S, IAIN A, VINAYAGAMOORTHY S, et al. Monopiles subjected to uni- and multi-lateral cyclic loading[J]. Proceedings of the Institution of Civil Engineers, 2017, 170(3): 246. [百度学术]
许成顺,王冰,杜修力,等.循环加载频率对砂土液化模式的影响试验研究[J]. 土木工程学报, 2021,54(11): 10. [百度学术]
XU Chengshun, WANG Bing, DU Xiuliet al. Experimental Study on effect of cyclic loading frequency on liquefaction mode of sand[J]. China Civil Engineering Journal, 2021, 54(11):10. [百度学术]
曾垂青,张吾渝,高义婷,等.循环荷载作用下海北地区原状黄土动力特性试验研究[J].青海大学学报, 2021, 39(1): 6. [百度学术]
ZENG Chuiqing, ZHANG Wuyu, GAO Yitinget al. Study on the experiment of dynamic characteristics of undisturbed loess in Haibei area under the cyclic loading[J]. Journal of Qinghai University, 2021, 39(1): 6. [百度学术]
ZHU Z, ZHANG F, QING Y, et al. Effect of the loading frequency on the sand liquefaction behavior in cyclic triaxial tests [J]. Soil Dynamics and Earthquake Engineering, 2021, 147(2):106779. [百度学术]
ARAEI A A, RAZEGHI H S, TABATABAE S H, et al. Loading frequency effect on stiffness, damping and cyclic strength of modeled rockfill materials [J]. Soil Dynamics and Earthquake Engineering, 2012, 33(1): 1. [百度学术]
邓海峰, 刘振纹,祁磊,等. 波浪作用下饱和砂土孔压发展规律试验研究 [J].水利与建筑工程学报, 2017, 15(3): 5. [百度学术]
DENG Haifeng, LIU Zhenwen, QI Lei,et al., Experimental research on development pattern of pore -water pressure of saturated sand under wave loads[J]. Journal of Water Resources and Architectural Engineering, 2017, 15(3): 5. [百度学术]
李晶鑫,方祥位,申春妮,等. 波浪荷载作用下频率对饱和珊瑚砂动力特性影响研究[J].水利与建筑工程学报, 2018(5): 92. [百度学术]
LI Jingxin, FANG Xiangwei, SHEN Chunni,et al. Influences of frequency on the pore water pressure of saturated coral sand under wave loading[J]. Journal of Water Resources and Architectural Engineering, 2018(5): 92. [百度学术]
ARANY LASZLO, BHATTACHARYA S, MACDONALD JOHN H G. Closed form solution of Eigen frequency of monopile supported offshore wind turbines in deeper waters incorporating stiffness of substructure and SSI. [J] Soil Dynamics & Earthquake Engineering, 2016, (83): 18-32. DOI:10.1016/j.soildyn.2015.12.011. [百度学术]
SALEH J, SUBHAMOY B. Closed form solution for the first natural frequency of offshore wind turbine jackets supported on multiple foundations incorporating soil-structure interaction[J]. Soil Dynamics and Earthquake Engineering, 2018, 113:593.DOI:10.1016/j.soildyn.2018.06.011. [百度学术]
DEGROOT D J, LADD C C, GERMAINE J T, et al. Undrained multidirectional direct simple shear behavior of cohesive soil [J]. Journal of Geotechnical Engineering, 1996, 122(2): 91. [百度学术]
MATSUDA H, HENDRAWAN A P, ISHIKURA R, et al. Effective stress change and post-earthquake settlement properties of granular materials subjected to multi-directional cyclic simple shear [J]. Soil and Foundation, 2011, 51(5):873. [百度学术]
HU X, ZHANG Y, GUO L, et al. Cyclic behavior of saturated soft clay under stress path with bidirectional shear stresses[J]. Soil Dynamics & Earthquake Engineering, 2018(104):319. [百度学术]
YASUHARA K, HIRAO K, HYDE A F L. Effects of cyclic loading on undrained strength and compressibility of clay [J]. Soils and Foundations, 1992, 32(1): 100. [百度学术]