桩端岩溶顶板地震动力特性的振动台试验研究

Shaking table test of seismic dynamic characteristics of karst roof under pile load

摘要:

为研究溶洞顶板在桩端荷载及地震共同作用下的动力特性和破坏特征,依托渝黔铁路复线周家湾大桥桩基工程,基于分离相似设计方法确定岩质材料、模型桩和地震波的主控参数,通过19组配比试验得到振动台试验用的岩体相似材料配比指标,最后考虑不同顶板厚度及溶洞直径的影响开展岩溶桩基小型振动台模型试验.结果表明:溶洞的存在改变模型的局部基频及动力特性,对地震波的传递起到阻碍及能量消散作用；波的频谱变化与上部荷载大小无直接关系,桩端荷载和地震共同作用下产生的桩端裂隙造成地震波整体幅值的下降,引起模型基频的变化；模型内部裂纹随地震波加速度的增强而增多,造成阻尼比逐渐增大,使得振动主频降低；同类型地震波之间仅幅值有所差别,不同类型地震波作用下的频谱分布差异较大；各测点的应变均随地震峰值的增加而增大,溶洞直径较小时顶板临空面处易发生剪切破坏,而溶洞直径较大时表现为整体的冲-剪破坏.

Abstract:

To study the dynamic properties and failure characteristics of karst cave roof under combined action of pile load and earthquake, supported by the pile foundation project of the Zhoujiawan Bridge of the Chongqing-Guizhou Railway, the main control parameters of rock materials, model piles, and seismic waves were determined based on the separation similarity method. Through 19 groups of mixture ratio tests, the index of the similar material ratios of rock masses used in shaking table tests were obtained, and the small-scale shaking table model tests of karst pile foundation were conducted considering the influence of different thicknesses of the roof and diameters of the cave. The results showed that the existence of caves changed the local fundamental frequency and dynamic characteristics of the model, and it hindered the transmission of seismic waves and dissipated energy. The frequency spectrum of the wave had no direct relation with the size of the upper load. The cracks in the pile-end caused by the combined action of the pile-end load and the earthquake gave rise to the overall amplitude decrease of the seismic waves, which caused the change of the fundamental frequency of the model. The internal cracks in the model increased with the enhancement of the acceleration of the seismic waves, resulting in a progressive increase in the damping ratio, which caused the main frequency of the vibration to decrease. Variation of the same kind of seismic wave only altered the wave amplitude, while different kinds of seismic waves led to the distribution changes of the fundamental frequency. The strain of the monitoring point increased with the increment of the peak of the seismic waves. Shearing failure of the karst roof occurred when the diameter of the cave was small, but punching-shearing failure happened when the diameter was larger.