Critical State Analysis for Iron Ore Tailings with a Fine-Grained Interlayer: Effects of Layering Thickness and Dip Angle

The formation of layering during the sedimentation process of tailings makes it of great significance to investigate tailings and to analyze their susceptibility to flow liquefaction. In this study, homogeneous iron ore tailings (IOTs) specimens were reconstituted with pure coarser grains and pure finer grains sampled from a typical tailings storage facility. Additionally, an improved sample preparation method was developed to create heterogeneous IOTs samples containing a fine-grained interlayer with different thicknesses and dip angles using the above two materials. A series of standard drained and undrained triaxial compression tests were conducted to investigate the effects of the presence of a layered structure and its geometry on the stress–strain responses, and the properties of the IOTs under the critical state soil mechanics framework, which has been widely adopted in the analysis of liquefaction in mine tailings. The results showed that for the two homogeneous specimens, unique critical state lines (CSLs) can be identified, but they have different degrees of curvature in the e-ln p′ plane, causing a decrease in the susceptibility to liquefaction with increasing fines content. With increasing fine-grained interlayer thickness (FGLT) within 0–40 mm, the critical state friction angle (φcs) decreased steadily, while the CSLs in the e-ln p′ plane translated upward. This may be because the morphology of the microstructure within the fine-grained interlayer restricted the compression of the intergranular pores. With increasing fine-grained interlayer dip angle (FGLA) within the range 0–30°, φcs decreased until a discontinuity occurred at a dip angle of 15°, while the CSLs in the e-ln p′ plane rotated clockwise through a pivot point. Different FGLAs could change the contact area between the different layers and the axial distribution of the fine-grained interlayer and thus may further contribute to the rotation of the CSLs.