风机荷载作用下频率比对砂土动力响应的影响

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f_r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（C_SR）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（C_SR = 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（C_SR = 0.20或0.25），孔压最终值u_max随之增大。除f_r = 1.00与10.00对应试样外，土体动强度随f_r与C_SR值增大而减少。f_r= 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。

Effect of Frequency Ratio on Dynamic Responses of Sand Under Wind Turbine Loading

The soil sample is subjected to a coupling effect of wind and wave action with various frequencies during the operation of offshore wind turbines. A series of tests were conducted to investigate the influence of the frequency ratio between wind and wave loads on sand using a multi-directional cyclic simple system （VDDCSS）. The effect of frequency ratio on the dynamic strain，pore water pressure，and dynamic strength of sand was analyzed. The test results show that the development of shear strain is affected by the bidirectional frequency coupling and the combination of x and y-direction stress caused by the frequency ratio. At a C_SR of 0.15， the bidirectional frequency coupling effect is greater than the stress combination effect. With the increase of the stress level （at a C_SR of 0.20 or 0.25）， both the frequency coupling effect and the stress combination effect are strengthened. At a low-stress level， the pore pressure curve of the failed specimen is "upward concave" and the frequency ratio has little effect on the pore pressure development of other specimens. As the stress level increases （at a C_SR of 0.20 or 0.25）， the final value of pore pressure u_max increases with increasing C_SR. Except for the samples corresponding to a f_r of 1.00 and 10.00， the dynamic strength of others soil samples decreases with the increase of f_r and C_SR. At a f_r of 5.00， the sample has a large strain，a high pore pressure value， and a low dynamic strength， which is related to the superposition effect of the two-way frequency coupling and stress combination.