显微CT扫描南京粉砂空间孔隙结构的精细化表征

Fine characterization of spatial pore structure of Nanjing silty sand using micro-CT

孔隙结构是反映土体物理、力学性质变化的本征指标,传统医用CT分辨率尺度不足以辨识表征岩土体细观结构的单个颗粒或孔隙。使用了14μm高空间分辨率的工业显微CT,对砂雨法生成的南京粉砂试样进行了扫描。对CT图像预处理后所得二值数字图像序列进行了三维重构,实现了任意表征单元体(REV)提取,并可对REV采用三维二值矩阵加以表征与定量分析。通过分析断层图像序列,计算出土样整体的体积孔隙率,与实验孔隙率误差仅3.93%。沿试样高度计算表观孔隙率在20.97%~46.77%范围内波动。从试样底部一REV中提取出水平方向、与水平面成60°角以及两正交垂直方向共4个典型切面,对其孔隙定向性进行统计分析,结果表明,水平切面具有最小主定向角,两正交垂直切面中,一个具有最大主定向角,另一具有最小各向异性率,斜切面孔隙呈近似等向分布,无明显定向性。采用孔隙网络模型进行空间孔径定量分析,结果发现,所提取出3个REV的最大孔径分布在629~696μm,最小孔径分布在54~77μm,平均孔径分布在166~185μm,孔径分布在100~200μm范围内最为集中。方法可用于热、水、力等各类因素作用下土体空间孔隙结构的无损定量分析。

Pore structure is the index reflecting the nature of the physical and mechanical properties of the soil. The resolution scale of the traditional medical CT is not high enough to identify the single grain or pore characterizing the meso-structure of geo-materials. An industrial micro-CT, with a high spatial resolution of 14 μm, is used to scan a Nanjing silty sand specimen fabricated by pluviation. The binary digital image sequence is acquired after preprocessing the CT initial images, and three-dimensional reconstruction is implemented. Arbitrary representative elementary volume（REV） is extracted, and three-dimensional binary matrix is used for the characterization of the REV and for the quantitative calculation. By analyzing the tomographic images, the volume porosity of the whole specimen is calculated, with an error of only 3.93% compared with the experimental porosity. The apparent porosity along the specimen fluctuates between 20.97 and 46.77%. Four typical sections, one horizontal, one tilting with an angle of 60 degrees to the horizon and two orthogonal vertical sections, are extracted from the REV at the bottom of the specimen. Statistical analysis of the preferred orientation is performed, and the results show that the minimum orientation angle occurs at the horizontal section, and one of the two orthogonal vertical sections exhibits the maximum orientation angle and the other exhibits the maximum anisotropic ratio, and the oblique section exhibits approximate isotropy, namely no obvious preferred orientation. The pore network model is used for spatial pore diameter analysis of three extracted REVs, and the results indicate that the maximum pore diameter distributes from 629 to 696 μm, and the minimum pore diameter does from 54 to 77 μm, and the average pore diameter does from 166 to 185 μm, and the pore diameter concentrates from 100 to 200 μm. The proposed method is capable of nondestructive quantitative analysis of pore structure variation of the spatial soil under any effect induced by heat, water or mechanics.