三轴加载条件下路基软土微观结构演化研究

Study on microstructure evolution of subgrade soft soil under triaxial loading

为了从宏、微观角度研究受力全过程中土的微观结构演化及损伤机制,对武汉地区路基软土开展了一系列K0(静止土压力系数)固结不排水三轴剪切试验、扫描电子显微镜试验和压汞试验。基于分维理论和图像处理技术,对微观结构参数进行了分析。研究结果表明:武汉地区路基软土的孔隙具有多重分形特征,并据此确定微孔隙、小孔隙、中孔隙和大孔隙的孔径分界点分别为0.03、0.50和7.00μm。其中孔径为0.50~7.00μm的孔隙整体上具有更强的自相似性,将其定义为特征区间,该区间的表面分形维数随着应变的增加而减小。软土损伤在微观上表现为团聚体解体,并伴随着孔径分布与孔隙形态的演变,孔隙面积比与孔隙形状系数随着应变的增加分别非线性增大和非线性减小。微观结构演化全过程可分为结构微调、损伤发展和结构破坏3个阶段。提出的微观结构因子能够有效揭示软土宏观力学性质的变化。

A series of K0(coefficient of earth pressure at rest)consolidated undrained triaxial shear tests,scanning electron microscope,and mercury intrusion tests were conducted on subgrade soft soil in Wuhan region.It aims to study the microstructural evolution and damage mechanism during the loading process from macro and micro perspectives.Microstructure parameters were analyzed based on fractal theory and image processing technology.The results show that soft soil pores have multifractal characteristics,according to which the pore size boundary points of the micro,small,medium and large pores are determined to be 0.03,0.50,and 7.00μm,respectively.Pores with diameters from 0.50 to 7.00μm exhibit stronger self-similarity overall.Therefore,0.50-7.00μm is defined as a characteristic interval,and the surface fractal dimension decreases with increasing strain in this interval.The microscopic damage of soft soil is characterized by aggregate disintegration,together with the evolution of pore size distribution and pore morphology.With increasing strain,the pore area ratio and shape factor manifest nonlinear increase and decrease,respectively.The microstructure evolution process can be divided into three phases:structural fine-tuning,damage development,and structural failure.The microstructure factor proposed in this study can effectively reveal changes in macro mechanical properties of soft soil.