地铁致凸形地貌地表振动局部放大现象实测及机理分析

法源寺文保区毗邻地铁4号线,区域受地铁振动影响较大。该研究通过对地铁运行影响下法源寺文保区的现场振动测试,发现浏阳会馆位置存在明显的地表振动局部放大现象,通过相关分析得出结论:(1)浏阳会馆位置为典型的凸形地貌,其地表振动"局部放大区"出现在振中距1.5~3.0倍左线隧道埋设范围内,这与现有研究成果(约1.0~1.5倍隧道埋深)并不相同;(2)基于弹性波场理论,分析了地下埋置振源下凸形地貌地表振动放大区形成机理——传递到凸形地貌内的振动波一方面会在坡脚产生绕射,形成以坡脚为新的振源点的波场,并部分转化为面波沿坡面传播,这样远离坡脚的界面透射波、坡脚新绕射波、坡面面波及表层土内的多次反射波便会在凸形地貌地表进行叠加;另一方面凸形地貌为若干介质层的组合,其波阻抗一般会随地层深度的减小而减小,传递到凸形地貌的透射波在向上传递的过程中,波速会不断放大,最终导致凸形地貌地表振动的局部放大;(3)采用二维有限元模型对地铁致凸形地貌地表振动局部放大现象进行了数值验证,结果发现凸形地貌地表竖向加速度峰值在其波阻抗由深及表逐渐衰减时,凸形地貌地表振动局部放大效应最为明显,数值计算结果与实测值基本吻合;数值计算结果还发现凸形地貌地表竖向加速度峰值随着地貌高度的增大而增加,达到临界值后,又会随着地貌高度的增大而降低,表明凸形地貌的振动放大效应并非会随着高度增加的而无限增大,其只在一定高度范围内存在。

In-situ measurement and mechanism analysis for local amplification phenomena of ground vibration in the protruding topography induced by subway

The Fayuan Temple historic reservation is adjacent to Metro Line 4, the residents of which place are affected by subway vibration for a long time. Based on the in-situ vibration measurements, the local amplification phenomena was found in the region where the Liuyang Guild Hall is located. A further study was conducted, and results show that:(1)Liuyang Guild Hall is the protruding topography, and it is a characteristic region called the local amplification region(LAR), where the ground vibration does not attenuate monotonically with the increasing of the distance from the epicenter, but there exists vibration amplification region somewhere else. LAR of the Liuyang Guild Hall appears with a distance of about 1.5-3.0 burial depth from the epicenter, which is different from existing literature(about 1.0-1.5 burial depth).(2)Base on the theory of elastic wave propagation, mechanism for local amplification phenomena of ground vibration in the protruding topography induced by the subway was analyzed. On the one hand, the vibration wave transmitted to the protruding topography will diffract at the foot of the slope, so a wave field is formed with the foot of the slope as a new vibration source point, and partially converts to surface wave to propagate at surface. The transmitted wave, the new diffraction wave at the foot of the slope, the surface wave, and the reflected wave of the topsoil will superpose on the surface of the protruding topography. On the other hand, the protruding topography is a combination of several media layers, and its wave impedance will generally decrease with the decrease of the depth of the ground layer. In the process of the transmitted wave transmitting up to the ground, the wave velocity will be continuously enlarged, resulting in the local amplification phenomena of ground vibration in the protruding topography.(3)The two-dimensional finite element model was used to verify the local amplification phenomena of ground vibration in the protruding topography induced by the subway. Numerical results show that the local amplification local amplification phenomena is the most obvious when the wave impedance decrease with the decrease of the depth of the ground layer. The numerical results are in good agreement with the in-situ measurement, which can validate the correctness of mechanism analysis for local amplification phenomena. It is also found that the peak value of vertical acceleration can magnify with the increase of the height, and then decreases with the increase of the height when it reaches the critical value. It indicates that the local amplification phenomena of ground vibration in the protruding topography does not magnify infinitely with the increase of height, but only exists in a certain height range.