堰塞湖坝体动力特性及加速度分布规律大型振动台模型试验研究

 2008年汶川八级地震形成了至少257个堰塞坝，主震后发生的大量余震可能会影响堰塞坝的动力安全状态。堰塞坝体的动力特性参数(包括自振频率和阻尼比等)和加速度分布规律是堰塞坝地震安全研究的基础内容。通过大型振动台模型试验，研究在余震作用下模型堰塞坝体的动力特性参数、加速度分布规律及二者的影响因素，并根据动力相似律，计算原型坝体的动力特性参数。共进行2组不同材料的振动台模型试验，分别模拟含黏粒较多且颗粒较小(坝体I)和基本不含黏粒且颗粒较大(坝体II)的2种坝体。在不同地震波形输入、不同加速度峰值和不同水位条件下进行振动台试验。研究成果表明：(1) 模型坝体具有较稳定的X向和Z向自振频率和阻尼比。(2) 先期振动使坝体自振频率降低，阻尼比有增大趋势；坝体I的自振频率小于坝体II。水位变化对2种坝体自振频率的影响规律不一致。(3) 加速度放大倍数随高程增大而增大，最大加速度发生在坝顶处；相同高程测点加速度放大倍数最大值出现在上游或下游靠近坝坡表面处，即“表面放大”效应明显，说明坝坡表面容易受地震作用破坏。(4) 所含频谱成分与坝体自振频率接近的地震波会引起最大的加速度反应。Z向振动使坝体测点X向加速度放大倍数增大。加速度放大倍数一般随输入加速度峰值的增大而减小。

LARGE-SCALE SHAKING TABLE MODEL TESTS ON DYNAMIC CHARACTERISTICS AND ACCELERATION DISTRIBUTION OF LANDSLIDE DAMS

At least 257 landslide dams were formed by Wenchuan earthquake in 2008. The aftershocks occurred after the major quake may influence the safety of those landslide dams. Dynamic characteristic parameters，including natural frequency and damping ratio，and distribution of acceleration amplification factor are the focus of the research of landslide dams. This paper presents the studies of large-scale shaking table tests on the dynamic characteristics and the acceleration distribution of landslide dams under aftershocks. The natural frequency，the damping ratio and the distribution of acceleration amplification factor of a model dam and their influencing factors are studied. The dynamic characteristic parameters of prototype landslide dams are calculated according to the similarity law. Two groups of shaking table tests are conducted to simulate the behaviors of two types of landslide dams：one with small particle size having the cohesive materials(dam I) and the other with large particle size not having the cohesive materials(dam II). The shaking table tests were conducted under different earthquake waves，different peak ground accelerations(PGA) and different water depths. The following conclusions are drawn：(1) The model dams have stable X-directional and Z-directional natural frequency and damping ratio. (2) Earlier shaking makes the natural frequency to decrease and the damping ratio to increase. The natural frequency of dam I is smaller than that of Dam II. The effects of water depth on the natural frequency of the two types of dams are different. (3) In vertical direction，acceleration amplification factor increases from the base to the top of the dam. The maximum accelerations occurred mainly on the top of the dams. In horizontal direction，the maximum accelerations occurred on the upstream and downstream surfaces of dam slopes，i.e. the surface amplification effect，which showed that the dam slope surfaces were prone to fail under earthquake action. (4) The earthquake waves with the predominant frequency close to the natural frequency of landslide dams induce the most prominent acceleration response. The earthquake in Z direction made the amplification factor of acceleration in X direction increase. The acceleration amplification factor decreases as PGA increases.