Microanisotropy and preferred orientation of grains and aggregates (POGA) of the Malan loess in Yan’an, China: a profile study

It is well known that the microfabric formed by the shape, size, and preferred orientation of grains and aggregates (POGA) is the primary cause of the macroscopic anisotropy of soil. Therefore, this study investigated the vertical and horizontal anisotropic characteristics of the POGA at different burial depths in the Malan loess of Yan’an. Quantitative information of particle aggregates from scanning electron microscopy (SEM) photographs of vertical and horizontal solidified slice specimens was derived via the ArcGIS vectorization tool. The variation in the roundness (R), anisotropy index (Ia), and probability entropy (Hm) of clay grain aggregates was analyzed, and the results show that the azimuthal of the POGA in the horizontal direction is between 130° and 150° (N40°W–N60°W), while the POGA in the vertical direction is parallel to the ground plane. Moreover, the deeper the burial depth is, the more significant the preferred orientation (PO). Both vertical and horizontal grain aggregates are based on rounded grains, and the Ia of soil increases with the burial depth. The Hm shows little change in the horizontal direction with the burial depth but decreases in the vertical direction, and the POGA of the vertical direction is better than that of the horizontal direction. Finally, the Ia and Hm of undisturbed loess are compared with that of remolded clay in the published literature, and the relationship between microanisotropy and POGA is discussed.