松散砂土静态液化失稳机理的模型试验及离散元模拟

Static liquefaction (diffuse instability) usually occurs in saturated loose sand and casues flow failures of slopes. Bearing in mind the particulate nature of loose sand, this study performs discrete element method (DEM) to simulate the static liquefaction phenomenon observed in laboratory test. The evolution of microstructure during the shearing is investitated and related to the macroscopic soil behavior. It attempts to explore the fundamental mechanisms of the static liquefaction and provide the basic theory for static liquefaction prediction. The Hill's stability condition in contineous medium is extened to the discrete medium, which provides the theoretical crieterion for static liquefaction predcition. Model test combined with digital image analysis technique is also carried out to study the developments of stress and strain in the slope, and the progressive failure of slope based on particle movement. DEM is also used to simulate the model test. The Hill stability condition in discrete medium is used to analyze the slope stability in model test and the results are compared with those in model test to discuss the applicability of Hill stability condition in diffuse instability prediction. This study plays an important role in understanding the fundametnal mechanisms of static liquefation and the prediciton of static liquefaction, and it is meaningful for preventing disasters such as landslide and failure of dams induced by static liquefaction.

饱和松散土体往往会发生静态液化（扩散性失稳）现象，造成边坡等发生流滑灾害。本研究针对松散砂土等离散介质的特性，采用离散元方法模拟静态液化的室内单元试验，研究土体微观结构在剪切过程中的演化，建立微观结构和土体宏观力学行为的内在联系，从根本上阐释静态液化失稳的机理，为准确判别砂土静态液化的可能性提供先决条件；基于微观力学，将连续介质理论中的Hill稳定条件推导扩展到离散介质中，为判别松散砂土失稳提供理论依据；通过模型试验，采用数字照相技术研究边坡失稳过程中应力、应变的发展，研究基于颗粒运动的边坡失稳渐进过程，并用离散元对模型试验进行模拟；基于推导的离散介质中的Hill稳定条件，对模型试验的边坡进行稳定分析，并与模型试验的结果进行对比，探讨Hill稳定条件判别扩散性失稳的合理性。本研究对揭示土体静态液化的机理和准确判别土体静态液化失稳提供基础性理论依据，对避免滑坡等灾害的发生有重要意义。

饱和松散土体往往会发生静态液化（扩散性失稳）现象，造成边坡等发生流滑灾害。本研究针对松散砂土等离散介质的特性，采用离散元方法模拟和理论推导等研究砂土的静态液化等宏观力学特性，以及其微观机理。主要研究内容和成果包括：（1）利用离散元方法模拟室内单元试验，并研究土体微观结构在剪切过程中的演化，建立微观结构和土体宏观力学行为的内在联系，从根本上阐释静态液化失稳的机理。离散元模拟研究结果表明：砂土的宏观力学有很强的状态相关（包括初始围压和孔隙比）特性，饱和松散砂土才会出现静态液化现象。土体的微观结构决定着土体的宏观力学特性，特别是颗粒接触配位数和接触法向的分布。静态液化发生时，颗粒的接触配位数为4.0左右，然后突然降为0，表示砂土结构的崩解。同时砂土的微观结构也存在临界状态线，跟土体的宏观临界状态线相对应。（2）研究了边坡渐进破坏过程中剪切带的形成和发展，研究结果表明：剪切带从加载初期许多的细微潜在剪切带发展而来。剪切带中土体由于强剪胀性而导致大孔隙结构，伴随着很大的颗粒累积转动位移。（3）研究了影响土体初始应力状态（静止土压力系数K0）的主要因素，研究表明土体的K0跟土体的颗粒接触配位数存在着本质联系。（4）推导了基于推导的离散介质中的Hill稳定条件，研究了土体初始应力状态对土体失稳的影响。研究结果表明：系统二阶功的减小量约为系统动能的增量。在不等向应力（或者有偏应力）条件下，二阶功跟应变路径角度有明显的相关性。只有应变方向处于失稳域（一般分布角度为130°-190°）中，试样才会发生失稳。砂土试样失稳的微观表现形式是系统动能的突然性爆发式增加，表明了Hill稳定条件在离散介质中的适用性。