Study on the seismic damage mechanism of buildings on the river terrace in Wenchuan earthquake
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摘要:
基于2008年汶川MS8.0地震现场的震害调查资料，揭示了河流阶地地形对建筑物震害分布的影响规律。阶地前缘冲洪积层较厚区域的建筑物遭受了严重的破坏，而阶地后缘坡积层较薄区域则相对受损较轻，总体上呈现出前缘震害显著大于后缘的特征。为研究该震害特征的机制，利用FLAC3D有限差分软件构建三维阶地模型，模拟分析其在脉冲荷载作用下的地震动响应。计算结果表明：各级阶地前缘的地震动水平均要显著高于后缘，同时随上覆土层增厚，加速度峰值、相对持时值、反应谱平台值以及放大系数均呈现上升趋势。而随阶地级数的减小，峰值、持时和反应谱特征周期却呈下降趋势，且级数的增加还会显著增强地震动低频成分的放大效应。因此阶地级数与上覆土层厚度是影响地震动响应的关键因素，高阶地和厚覆盖土层区域对地震动的放大效应更强，从而导致建筑物破坏严重。
Abstract:The seismic ground motion topographic effect, as an important research content in the field of seismic engineering, the study of the mechanism of complex terrain on the ground motion characteristics can provide basis for engineering seismic defense.A large number of post-earthquake site investigations have shown that the complexity of local terrain has a significant impact on the distribution of seismic damage, especially the irregular terrain can change the intensity and spectral characteristics of ground motion. As a type of local irregular topography widely found in nature, the unique geometric shape of river terraces can cause complex scattering and diffraction of seismic waves, resulting in differences of ground motion in its local areas, and then affecting the seismic damage degree of surrounding buildings.
Based on the on-site seismic damage investigation data of the river terraces in Wenchuan MS8.0 earthquake in 2008, it was found that buildings in the area with thicker alluvial at the front edge of the terrace suffered serious damage, while those in the area with thinner slope deposit at the rear edge of the terrace suffered relatively less damage. In general, the seismic damage at the front edge was significantly greater than that at the rear edge. At the same time, in order to study the mechanism of this seismic damage feature, the river terraces were selected as the research object, and the FLAC3D finite difference software was used to establish three-dimensional river terrace analysis models with different thicknesses of overburden soil layers, simulating and calculating the ground motion response under impulse loading, further revealing the influence law and internal mechanism of river terrace topography on ground motion characteristics and the distribution of seismic damage to buildings.
For the same level terrace, the peak values of the horizontal and vertical acceleration and the 90% energy duration all show an upward trend with the increase of the overlying soil thickness, reaching the maximum value at the front edge of the terrace and the transition point with the steep slope. Meanwhile, the ground motion level at the front edge of each terrace is significantly higher than that at the rear edge. As the terrace grade decreases, the peak values of the horizontal and vertical acceleration and the 90% energy duration at the corresponding area also gradually decrease. Similarly, the trend of the Fourier spectrum amplitude and ratio at different monitoring points on the same level terrace is basically consistent, but the amplitude and ratio increase gradually as the monitoring point approaches the edge of the terrace facing the steep slope and the front edge of the area with increased soil thickness.As the terrace grade increases （the terrain height rises）, the natural frequency of the structure increases accordingly, thereby significantly enhancing the amplification effect of low-frequency ground motion. The characteristic period value, platform value, and platform amplification coefficient in the standard response spectrum of seismic acceleration for different monitoring points are all affected by the terrace grade and overlying soil thickness. The Tg value decreases gradually as the terrace grade decreases; the platform value increases as the overlying soil thickness at the front edge of the terrace increases; however, the platform amplification coefficient β value decreases as the terrace grade increases, and the amplification coefficient at the front edge of the terrace is significantly larger than that at the rear edge.
River terraces have a significant impact on the propagation of ground motion and the degree of seismic damage to buildings. The change of the thickness of the overlying soil layer leads to different distributions of building damage by affecting the amplification of ground motion, while the number of terrace grades exacerbates or mitigates the seismic damage by affecting the spectral characteristics and overall level of ground motion.Therefore, the number of terrace grades and overlying soil thickness are the key factors affecting ground motion response, and the amplification effect of ground motion is stronger in high terrace and thick covering soil layer, which leads to serious damage of buildings.
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图 1 南坝镇阶地前缘房屋倒塌严重
Figure 1. The houses at the front edge of the terrace in Nanba Town collapsed seriously
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图 2 南坝镇阶地后缘房屋基本完好
Figure 2. The houses at the rear edge of the terraces in Nanba Town were basically intact
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图 3 蓥华镇建筑物破坏点调查分布图
Figure 3. Distribution map of building damage points investigated in Yinghua Town
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图 4 蓥华镇阶地后缘民房基本完好
Figure 4. The houses at the rear edge of the terrace in Yinghua Town were basically intact
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图 5 阶地前缘蓥峰实业工厂建筑物破坏严重
Figure 5. The buildings of the Yingfeng Industrial Factory at the front edge of the terrace were severely damaged
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图 6 小鱼洞镇河流阶地地形剖面图
Figure 6. Topographic profile of the river terrace in Xiaoyudong Town
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图 7 小鱼洞镇建筑物破坏点调查分布图
Figure 7. Distribution map of building damage points investigated in Xiaoyudong Town
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图 8 小鱼洞镇阶地前缘房屋破坏严重
Figure 8. The houses at the front edge of the terrace in Xiaoyudong Town were seriously damaged
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图 9 罗阳村山前阶地后缘房屋基本完好
Figure 9. The houses at the rear edge of the mountain terrace in Luoyang Village were basically intact
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图 13 阶地模型加速度等值线云图
Figure 13. Cloud map of acceleration contour lines for the terrace model
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图 14 阶地模型监测点剖面加速度分布云图
Figure 14. Cloud map of acceleration distribution in the monitoring point profile for the terrace model
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图 15 阶地模型地表各监测点加速度峰值变化
Figure 15. Changes of peak acceleration at each monitoring points on the surface of the terrace model
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图 16 阶地模型地表各监测点90%能量持时变化
Figure 16. Changes of 90% energy time-holding at each monitoring points on the surface of the terrace model
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图 17 各级阶地的加速度傅里叶谱
Figure 17. Fourier spectrum of acceleration for terraces at all levels
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图 19 测点#36加速度规准反应谱曲线
Figure 19. Response spectrum curve of acceleration gauge at measuring point #36
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图 20 阶地监测点反应谱Tg值和Amax值变化趋势
Figure 20. Change trend of Tg and Amax values for response spectra at terrace monitoring points
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图 21 阶地监测点反应谱β放大系数变化趋势
Figure 21. Change trend of β amplification coefficient for response spectrum at terrace monitoring points
表 1 计算模型岩土体物理力学参数
Table 1 Physical and mechanical parameters of rock and soil mass for the calculation model
岩土类型 密度（kg/m3） 体积模量（GPa） 剪切模量（GPa） 粘聚力（MPa） 内摩擦角（°） 粉质黏土 1850 0.160 0.074 0.023 25 砂岩 2487 5.970 6.010 2.060 40 
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表 2 阶地模型地表监测点规准反应谱参数
Table 2 Standard response spectrum parameters of surface monitoring points in the terrace model
监测点号 T0（s） Tg（s） Sa（m/s2） β 监测点号 T0（s） Tg（s） Sa（m/s2） β 1 0.10 0.40 1.963 0.940 20 0.10 0.25 2.750 1.240 2 0.10 0.40 2.013 0.949 21 0.10 0.25 3.425 1.357 3 0.10 0.40 2.050 0.927 22 0.10 0.25 4.200 1.318 4 0.10 0.40 2.125 0.889 23 0.10 0.25 4.250 1.050 5 0.10 0.40 2.288 0.890 24 0.10 0.25 4.388 1.174 6 0.10 0.40 2.700 1.276 25 0.10 0.25 2.325 1.348 7 0.10 0.40 3.038 1.318 26 0.10 0.25 2.475 1.394 8 0.10 0.40 3.450 1.177 27 0.10 0.20 2.500 1.396 9 0.10 0.40 3.838 1.178 28 0.10 0.20 2.463 1.368 10 0.10 0.40 4.125 1.183 29 0.10 0.20 2.450 1.350 11 0.10 0.40 3.513 1.157 30 0.10 0.20 2.475 1.346 12 0.10 0.35 2.163 1.188 31 0.10 0.20 2.600 1.381 13 0.10 0.35 2.100 1.144 32 0.10 0.20 2.975 1.530 14 0.10 0.35 2.088 1.139 33 0.10 0.20 3.538 1.703 15 0.10 0.35 2.075 1.125 34 0.10 0.20 4.050 1.806 16 0.10 0.30 2.113 1.127 35 0.10 0.20 4.450 1.522 17 0.10 0.30 2.113 1.107 36 0.10 0.20 4.600 1.232 18 0.10 0.30 2.200 1.133 37 0.10 0.20 4.675 1.226 19 0.10 0.30 2.250 1.095
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