Centrifugal model tests on influences of overlying sea layer on basic period of free field
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摘要: 基于动态离心技术设计和构建了两组模型试验,还原并再现了上覆无水和有水的软土自由场仿真岩土台阵,利用白噪声扫频和强度不同的El Centro波作为振动台基底输入,采用考虑相消干涉的传统谱比法,获取了不同工况条件下的两组自由场模型的场地反应结果,通过对比分析有水和无水场地的振型反应特征以及基本周期差异,评价和总结上覆水在估算场地基本周期的作用及影响。研究结果表明：由于上覆水自重以及土水相互作用的影响,无水模型的地表峰值放大倍数要高于有水模型,同时无水模型的地表时程波形稀疏,表明地表地震波具有较为丰富的高频成分；无论是白噪声扫频还是El Centro波的基底加载方式,上覆无水和有水两组自由场模型的基本周期和振型放大系数均存在显著差异,表明上覆水对海底复杂介质体系的基本周期估算具有一定的影响,陆地模型和海域模型的基本周期最大偏差达35.5%。Abstract: Based on the dynamic centrifugation technique, two groups of model tests are designed and performed, and the geotechnical free field simulation arrays with and without water are restored and reproduced. The white noise sweep and El Centro waves with different intensities are used as the base input of the shaking table, and the traditional spectral ratio method considering cancellation interference is used to obtain the site response results of two groups of free field models under different working conditions. Through the comparative analysis of the modal response characteristics and basic period differences between watery and anhydrous sites, the function and influences of overlying water in estimating the basic period of the site are evaluated and summarized. The results show that due to the influences of the dead weight stress of overlying water, the surface peak magnification of the anhydrous model is higher than that of the watery model, and the surface time history waveform of the anhydrous model is sparse, indicating that the surface seismic waves have abundant high frequency components. Whether it is the white noise sweep or the base loading mode of El Centro wave, there are significant differences in the basic period and mode amplification factor of the two groups of free field models with and without water, indicating that the overlying water has a certain influence on the estimation of the basic period of the seafloor complex medium system, and the maximum deviation of the basic period of the land model and the sea model is 35.5%.
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本次离心机振动台试验得到了交通运输部天津水运工程科学研究院港口水工建筑技术国家工程实验室大力支持。感谢刘晓强博士、安晓宇博士、李建东工程师对本次试验付出了辛勤的工作和热情的支持。同时感谢匿名审稿人对本文提出了专业且富有建设性的修改意见。
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图 8 两组模型的地表加速度反应谱对比图
Figure 8. Comparison of ground acceleration response spectra of two models
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图 9 典型自由场地表处的场地反应放大谱
Figure 9. Site response amplification spectra at surface of typical free fields
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图 10 场地基本周期与放大系数随基底输入峰值的关系
Figure 10. Relationship among basic period, amplification factor and the peak value of base input
表 1 模型土体的基本物理性质指标
Table 1 Basic physical properties of model soil
密度ρ/(g·m-3) 含水率w/% 塑限/% 液限/% 孔隙比e 1.61 36.0 26.0 48.0 1.170 
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表 2 典型自由场离心模型试验基底输入时程参数
Table 2 Actual input ground motion parameters of free-field centrifugal model tests
地震波类型 工况 幅值 差异率/% 持时/s 无水/g 有水/g 白噪声扫频 BZS 5.683 5.691 0.1 110 El Centro波 EL-1 46.16 41.13 10.9 60 EL-2 81.40 77.57 4.7 EL-3 146.80 143.00 2.6 EL-4 250.20 266.60 6.6 EL-5 395.30 387.40 2.0
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表 3 地表与基底峰值加速度量测结果及放大倍数
Table 3 Peak accelerations and magnification of surface and basement
模型工况 无水模型 有水模型 基底PGA/g 地表PGA/g 放大倍数 基底PGA/g 地表PGA/g 放大倍数 EL-1 46.46 125.5 2.70 41.13 130.0 3.16 EL-2 81.40 172.7 2.12 77.57 117.2 1.51 EL-3 146.8 267.8 1.82 143.0 96.94 0.68 EL-4 250.2 249.3 0.99 266.6 141.3 0.53 EL-5 395.3 251.8 0.64 387.4 204.0 0.53
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表 4 模型体系动力特性
Table 4 Dynamic characteristics of model system
加载工况 无水模型 有水模型 fd/Hz Tb/s fd/Hz Tb/s 白噪声扫频BZS 0.682 1.47 0.819 1.22
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表 5 地震动振型放大系数及其相应的自振频率
Table 5 Amplification factors and their corresponding natural frequencies of ground motion modes
加载工况 无水模型 有水模型 fd/Hz AF fd/Hz AF EL-1 0.330* 6.671 0.352 5.583 EL-2 0.488 3.932 0.315 3.365 EL-3 0.403 3.492 0.277 2.155 EL-4 0.317 2.596 0.242 0.848 EL-5 0.312 2.318 0.480* 0.466 注： *表示试验非正常点。
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表 6 不同工况条件下两组离心模型的基本周期Tb及差异率δ
Table 6 Basic periods and difference rates of two groups of centrifugal models under different working conditions
模型工况 无水模型Tb/s 有水模型Tb/s 差异率δ/% EL-1 3.03 2.84 -6.7 EL-2 2.05 3.17 35.5 EL-3 2.48 3.56 30.3 EL-4 3.15 4.13 23.7 EL-5 3.21 2.08 -35.2
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