<p>One option for tailings disposal is filtering and compacting them following specified compaction degree and moisture content to form a dry stack structure. However, the moisture content varies in the filtering plant due to the tailings’ characteristics, operational features, and weather conditions. Also, the compaction characteristics are influenced by the gradation and mineralogy of tailings, which are highly dependent on the original ores and the beneficiation processes. This paper investigates the influence of grain size distribution and molding moisture content on the undrained behavior of iron ore tailings. Two materials with different fine contents were molded with distinct water contents and compacted at specific dry unit weights to reflect the diverse range of materials in a dry stack. The geotechnical behavior was investigated by simple shear tests to represent the in situ conditions regarding the dry stacks’ stability more realistically. When considering a unique dry unit weight in the field, materials with different grain size distributions will be compacted in different states. Moreover, the variation in moisture content can affect the strength and compressibility due to changes in fabric. When compacted with the same effort (optimum condition), mixtures with distinct gradings produce different grain arrangements that affect the behavior. Also, an increase in friction angle at the critical state with the fines content was found to be related to the particle characteristics (iron minerals and morphology). The results demonstrate the importance of appropriate field compaction control based on the material’s characteristics, being essential to consider the differences arising from the fabric for lower strains in dry stack design.</p>