三轴剪切过程中软黏土的微观结构及分形特征

Microstructure and Fractal Characteristics of Soft Clay during Triaxial Shear Process

土的物质组成和内部结构是决定土物理力学性质的主要因素;当土性质变化时,土中孔隙大小和形状变化最直观.为研究三轴剪切过程中软黏土微观结构变化特征,利用扫描电子显微镜获取了软黏土在不同应变速率和不同应变阶段的三轴固结不排水剪切试验后土试样的SEM图像;计算得到了软土的等效孔径和孔隙(颗粒)分维值.研究表明,该软黏土具有团粒-絮凝结构.试验前后软黏土中的孔隙均以小孔径、中孔径为主,大孔径孔隙分布较少;大孔隙主要存在于原状土和剪切破坏后土样中.剪切过程中存在一个影响孔隙分布随应变速率变化的应变阈值.软黏土具明显分形特征,孔隙分维值为1.891~1.750,颗粒分维值为1.825~1.908;在剪切过程中,颗粒分布分维值增大,孔隙分布分维值减小,孔隙大小和形状变化最显著.

The mineral composition and internal structure are the key factors in the determination of the physical and mechanical characteristics of the soft soil, and the variation of pore size and shape is obvious with the changes of soil properties. In order to investigate the microstructure and fractal characteristics of soft clay during the triaxial consolidated undrained shearing process, a lot of scanning electron microscopy（SEM） images of soft clay at different strain rates and strain stages were acquired. The equivalent pore size and fractal dimension of the soft soil were calculated. The results show that the particles and pores of soft clay have obvious fractal dimension characteristics while soft clay has the agglomerate-flocculation microstructure. The pores of soft clay before and after triaxial test are dominated by small pore size and medium pore size, and the large pore size distribution is less,which mainly exists in the undisturbed soil or the sheared soil. Soft clay has strain threshold affecting pore size distribution with strain rate in the shear process. With obvious fractal characteristics, the fractal dimension values（FDV） of pores varying from 1.891 to 1.750, the fractal dimension values of particles from 1.825 to 1.908, the FDV of particles increase and the FDV of pores decrease during the triaxial consolidated undrained shearing process, the size and shape of pores being relatively obvious.