石碑塬滑坡黄土液化特征及其影响因素研究

Loess liquefaction characteristics and its influential factors of Shibeiyuan landslide

石碑塬滑坡是1920年海原地震触发的大型黄土流滑,认识其破坏特征与发生机制对于黄土边坡长距离液化失稳机制的研究非常重要。对石碑塬黄土滑坡的调查和研究表明,饱和黄土或高含水率黄土具有很高的液化势和流态破坏势,在强震作用下,饱和黄土易发生液化或流滑。对石碑塬滑坡的7组原状黄土样品进行振动三轴剪切试验,并结合其微观特征分析,探讨了循环振动荷载作用下的饱和黄土孔隙水压力-应变增长模型,分析了振动液化过程中液化应力比与黄土粒度组成、土体微观结构参数及饱和度之间的关系。结果表明:黏粒含量越低,振动作用下饱和黄土孔隙水压力响应越快,液化应力比越低;黄土孔隙比越大,孔隙结构分形维数越大,液化应力比越低,振动液化后黄土孔隙分形维数降低,结构较液化之前更为致密;饱和度对黄土粒间胶结物质的赋存状态及黄土结构强度影响很大,同一土体饱和度越高,溶滤于孔隙水中的离子浓度越高,土体粒间接触点(或胶结点)越容易发生断裂,使得黄土结构强度降低,液化应力比降低。

The Shibeiyuan landslide is a typically large loess flow slide which was triggered by Haiyuan earthquake in 1920. Understanding its behavior and mechanism is very important to study liquefied slope failures with long travel distance in the loess area. The in-situ investigation of the Shibeiyuan landslide shows that the saturated loess or the loess with high water content has high liquefaction and destructive potential which could cause liquefaction or flow slide under strong earthquake. The dynamic triaxial shear tests are performed on seven intact soil specimens taken from three places at the landslide. In addition, the scanning electron microscope （SEM） microscopic characteristics of the soil specimens are used to clarify the shear behaviors of these soil specimens. According to the results, the relation between water pore pressure and strain, as well as the relations between liquefied stress ratio and particle size, microstructure parameter, and degree of saturation are analyzed. The analytical results reveal that the liquefied stress ratio decreases and pore pressure response is stronger with the increasing of clay content. Meanwhile, the specimens with greater porosity have greater fractal dimension and less liquefied stress ratio. Moreover, the liquefied specimens are denser and its fractal dimension is greater compared to that before the liquefaction. Additionally, the degree of saturation has great influence on the state of interparticle cement and the microstructure in loess. The saturating process shows that the ionic concentration of the pore water is greater and the rupture of the interparticle contacts is easier when the degree of saturation becomes greater. As a result, the structural strength of the specimens decreases, leading to the decrease of the liquefaction stress ratio.