Quantification of macropores of Malan loess and the hydraulic significance on slope stability by X-ray computed tomography

The accurate identification and quantitative characterization of loess structural properties at the pore scale are important in the study of macroscopic permeability. To characterise the macropore structure of Malan loess systematically, a non-destructive detecting method, that is, X-ray computed tomography (CT), was adopted for scanning five undisturbed specimens. A series of processing steps, including image filtering by a novel compositing image filter, image segmentation with the effective combination of threshold and top hat method, and three-dimensional (3D) reconstruction and visualisation by marching cube algorithm and volume-rendering technique were finished in AVIZO (R) software to acquire the 3D pore network model and to extract the two-dimensional (2D) and 3D structural parameters, such as porosity, equal diameter, aspect ratio, shape factor (SF), node density, number of terminal and branching nodes, coordinate number, dip angle, dip direction angle and tortuosity. Results show that (1) Malan loess is a kind of porous geological material with strong verticality and spatial anisotropy reflected by the 2D parameters including porosity, equal diameters and aspect ratio as well as the 3D dip angle and 3D-visualised loess macropores; (2) Malan loess has higher permeability in the vertical direction than that in the horizontal direction so that prone to induce excessive infiltration and preferential flow, thereby threatening the loess slope stability; (3) SF is an effective parameter for pore classification in both 2D and 3D scales; (4) the macropores with a large diameter have a larger volume fraction, better connectivity (effective porosity) and greater contribution to water permeability; (5) the larger the coordinate number, the greater the hydraulic conductivity, nevertheless other than the aggregates for the water repellency caused by organic matter. In conclusion, the combination of CT and AVIZO (R) is excellent for quantifying the key macropore structural parameters (e.g., shape factor and tortuosity) and their hydraulic significance on slope stability.