地铁车站结构上穿可液化土层地震响应分析

Seismic Response Analysis of Liquefiable Soil Layer on Subway Station Structure

地基液化是导致地下结构在地震中发生严重震害的重要威胁之一。以北京某箱型框架式地铁车站上穿一定厚度的可液化土层为工程背景,基于有限差分软件FLAC_(3D)建立了含有可液化土层的场地土-地下结构相互作用数值模型,分析了场地液化分布特征、周围场地位移沉降及矢量特征、地下结构动力反应及上浮特征等。结果表明,地下结构的存在加大了可液化场地的地层变形,但会显著降低结构上方一定范围内地表土体的地震响应;可液化土层中的孔压发展表现为“起始缓慢增长、最后急速增加至峰值并保持一定时间,最后缓慢消散”的规律,孔压达到峰值的时刻与输入地震动的峰值时刻接近;结构的竖向位移变化表现出“起始少量下沉,然后振荡上升,随后急剧上浮,最后缓慢下降”的发展阶段;液化场地中,地铁车站结构附近的土体部分上浮、部分沉陷是造成地表土体开裂的内因;下部可液化土层对上穿的地下结构的地震响应具有一定的隔震效果。

Ground liquefaction is one of the important threats that cause serious damage to underground structures in earthquakes.Taking the actual engineering of a subway station upper crossing a certain thickness of liquefiable soil layer,a numerical model of the site soil with the liquefiable soil layer-underground structure interaction was established based on the finite difference software FLAC3D to study the liquefaction distribution characteristics,displacement settlement and vector characteristics,the stress response and floating characteristics of underground structure etc.The measured data substantiate that under the action of input ground motion,the existence of underground structures increases the stratum deformation of the liquefiable site,but significantly reduces the seismic response of the surface soil in a certain range above the structure.The pore pressure in the liquefiable soil layer develops as“initial slow growth,and then rapidly increases to the peak value for a certain period of time,and finally slowly dissipates”.The time when the pore pressure reaches the peak value is close to the peak time of the input ground motion.The vertical displacement of the underground structure shows the development stage of“starting with a small amount of sinking,then oscillating up,rising sharply afterwards,and finally falling slowly”.The partial floating and partial subsidence of the soil near the shallow underground structure are the internal causes of the cracking of the surface soil in the liquefaction site.The lower saturated silty sand layer has a certain seismic isolation effect on the seismic response of the above-crossing underground structure.