基于孔隙重构机理的饱和砂土液化大变形研究

Large liquefaction-induced deformation for saturated sand is one of the most challenging problem in the field of geotechnical earthquake engineering in China. Some phenomena of the large liquefaction-induced deformation for gentle slope are not explained well based on the current awareness of liquefaction mechanism. For this case, the void redistribution mechanism is proposed to carry out the research of large liquefaction-induced deformation. Utilizing the technology of void testing and non-contact relative density testing, derict tube tests, dynamic torsion shear tests and small scale shake table tests for saturated sand are carried out to study following aspects. First, the relationship between the void redistibution and the gradient of excess pore water pressure is analyzed to reveal the mechanism of the void redistribution occur in the saturated sand. Secondly the triggering factors of leading to the water film, videlicet an extreme case of the void redistribution, is investigated, and the decisive effect of the generation of water film and its size on the large liquefaction-induced deformation of saturated sand for gentle slop is proved to deduce the mechanism of large liquefaction-induced deformation caused by the void redistribution of saturated sand. And then a parameter of void redistibution is proposed to establish a constitutive relation reflected the void redistribution of saturated sand by studing the internal relations between the parameter of void redistribution and the sand properties, the thickness of the sand layer, the dip angle of upper soil layer and the stress-strain relationship. Finally the numerical analysis method is studied based on the constitutive realtion.These work will innovative enrich the knowledger of the seismic-induced liquefaction mechanism and the theory of deformation analysis, and have great theoretical significance in revealing the regularity of large liquefaction-induced deformation.

饱和砂土地震液化大变形问题是我国岩土地震工程领域最具挑战性的课题之一。当前对液化机理的认识尚无法很好地解释缓坡场地液化大变形的某些震害现象，针对这一问题，提出从孔隙重构机理的观点开展饱和砂土液化大变形研究。结合孔隙率测试和非接触式相对密度测试技术，开展饱和砂土震动液化直筒试验、动扭剪试验和小型振动台试验；首先研究饱和砂土的孔隙重构与超静孔压梯度的关系，揭示饱和砂土发生孔隙重构的机理；其次研究触发孔隙重构极端情况"水夹层"产生的条件，证明水夹层的产生及其规模对缓坡场地液化大变形的决定性作用，揭示孔隙重构导致饱和砂土液化大变形的机理；然后提出孔隙重构参量，研究孔隙重构参量与砂土土性、土层厚度、地层倾角、应力应变关系的内在联系，建立反映孔隙重构的饱和砂土本构模型；最后运用该本构模型开展数值分析方法研究。研究成果将丰富并创新地震液化机理和变形分析理论，对揭示液化大变形规律具有重要的理论意义。

地震液化是地震灾害之一。现有成果主要集中于是否会触发震动液化的研究，对发生震动液化后的危害研究比较薄弱。其中，缓坡场地地震液化震害中出现侧向大变形的机理问题仍存在争议。针对这一现状，本项目引入孔隙重构概念，开展地震液化产生大变形的机理及其危害研究。主要研究内容及重要结果包括：（一）、研发基于图像处理的砂颗粒识别技术和基于电阻率的孔隙率测试技术，研制电阻率测试系统、砂土多功能测试容器和一维透明土箱等设备。（二）、开展砂土电阻率特性研究，通过接触面积、孔隙水电阻率、砂土饱和度和孔隙率等影响因素测试试验，综合分析各因素的影响程度，建立基于电阻率的砂土孔隙率测试方法，给出饱和砂土孔隙率与其电阻率之间的定量关系。（三）、开展震动液化试验，再现了孔隙重构导致的水夹层和颗粒运移等非常见震动液化现象，分析电阻率和超静孔压等试验结果，发现：（1）震动液化前后，饱和砂土孔隙率总体呈变小趋势，且其变小的速率受到上覆土层渗透性的影响，渗透性越小，其变小的速率越慢；（2）在震动后相对较短时间内，饱和砂土电阻率出现先减小再增大的变化，体现出饱和砂土震动密实之前经历了强烈的孔隙重构过程；（3）电阻率先减小再增大的转折时刻与超静孔压快速消散的起始时刻吻合。（四）、振动液化试验中，通过图像处理技术意外捕获了砂土层顶面的瞬时沉降量（1/6秒时间内发生约1mm的沉降量），经论证发现：瞬时沉降只可能是如下两个原因引起，一是液化状态下砂土的渗透系数变大，二是液化状态下砂土中孔隙水的渗流为非稳定流，不满足达西定律运用条件。（五）、揭示缓坡场地地震液化大变形的孔隙重构机理。在特殊土层条件下，受震动密实趋势及渗透力的耦合作用下可液化土体中产生的孔隙重构使可液化土层上部产生了水夹层，致使缓坡场地侧向大变形的发展。通过本项目研究，深化了饱和砂土震动液化过程的认识，为孔隙重构机理导致液化大变形分析奠定良好基础。