穿越不同密实度饱和砂土地层的盾构隧道地震响应三维数值分析

Numerical study on seismic behavior of shield tunnel crossing saturated sandy strata with different densities

地震液化对隧道结构有重大威胁,且位于不同抗液化能力地层交界处的盾构隧道段更易发生严重的地震破坏。采用三维数值方法研究穿越不同密实度状态饱和砂土地层的盾构隧道的地震响应规律。饱和砂土用一种描述不同密实度砂土液化行为的边界面模型进行模拟,首先通过隧道液化上浮的振动台试验结果验证该本构模型的合理性。其次,应用多自由度连接弹簧表征管片环间相互作用,采用文献中的拼装管片的逐级加载试验结果验证该方法的可行性。最后,建立穿越两种不同密实度饱和砂土地层的盾构隧道三维数值模型,研究相对密实度、输入加速度峰值和交界面倾角对砂土地层-盾构隧道系统动力响应的影响。结果表明,可液化地层中隧道结构位移模式是水平地震激励下产生的水平位移与由于液化上浮效应产生的竖向位移的耦合作用,加之隧道在不同土层中变形存在差异,从而导致隧道呈现扭转的变形形态。在靠近交界面处,隧道整体上浮量急剧变化且该处结构上浮量随着交界面倾角增大而增大,同时管片结构弯矩出现突变,接头螺栓的环间剪切和拉伸位移也显著增加。分析结果进一步印证地震作用下盾构隧道在不同性质饱和砂土地层交界面处更易破坏,在设计阶段应予以重点关注。

Earthquake-induced liquefaction poses a significant threat to tunnel structures. Particularly, shield tunnel crossing sandy stratums with different liquefaction susceptibilities could suffer more severe seismic damages near the soil interface. In this paper, a three-dimensional numerical study was carried out to investigate the seismic response of a shield tunnel passing through saturated sandy strata with two different relative densities. Firstly, a practice-oriented two-surface plasticity sand model was employed to model the sandy soil and was validated by shaking table experiments on a tunnel structure embedded in liquefiable soil. Secondly, a deformable force-displacement link model for circumferential joints between each successive segmental ring was employed to model the interactions between segmental rings. The approach was validated using the results of two loading experiments on model segmental linings from the literature. Finally, the 3D numerical model was established considering various relative densities of soil,peak input accelerations, and the dip angle of the interface. The results indicate that the tunnel's horizontal displacements due to seismic excitations are coupled with the liquefaction-induced vertical uplift displacements, and the tunnel's deformation is not simultaneous in the two soil strata, resulting in twisting distortion of the tunnel structure. The uplifts of tunnel change rapidly and are increased by the rising of the dip angle near the soil interface. Also, the bending moments suddenly change, and the shearing/tensile displacements of joints increase remarkably, which confirms that the seismic design of shield tunnel segments near the soil interface is a critical issue.