# The Experimental Characterization of Iron Ore Tailings from a Geotechnical Perspective
## 4 Discussion
The results obtained show that the shape of the compaction curve is similar to that observed in the vast majority of conventional geomaterials, with the possible exception of uniform sands. In fact, the existence of a peak in the compaction curve defining the optimum water content (w\({}_{\text{opt}}\)) and the maximum dry unit weight (\(\gamma_{\text{d}}\)\({}^{\text{max}}\)) is clearly observed. In addition, the value of w\({}_{\text{opt}}\) obtained (\(\approx\)9%) is consistent with the ranges for conventional geotechnical materials with similar plasticity characteristics. That said, it is important to emphasize that the value of \(\gamma_{\text{d}}\)\({}^{\text{max}}\) resulting from the modified Proctor test (\(\approx\)3 g/cm\({}^{3}\)) is much higher than that commonly observed in conventional geomaterials. This should not be understood as an experimental error nor as a sign of an extremely dense material, as, in fact, the value obtained would be physically unacceptable if the density of solid particles (G) would be in the usual range. But as previously described, iron ore tailings can have G values much higher than usual (4.7, in this case), which justifies the higher values of \(\gamma_{\text{d}}\)\({}^{\text{max}}\) in compacted iron ore tailings. Irrespective of the need to consider a particular use of iron ore tailings as a building material for geotechnical works or, as an alternative, to assess the stability of tailings storage facilities, the shear resistance of tailings, namely under undrained conditions, must be established. This is true both for static and cyclic loading conditions, with the latter being particularly important when tailings storage facilities are built in seismically active regions, where earthquake loading can have dramatic consequences. Overall, the undrained monotonic and cyclic behaviors observed in the triaxial tests carried out as part of this research suggest that tailings observe the basic features of behavior established for conventional granular materials. In fact, the principles of CrSL and PhTL govern the undrained response of iron ore tailings under monotonic and cyclic triaxial loading. In particular, in undrained monotonic compression carried out on samples prepared through the slurry-based method and potentially representing field conditions of some traditional tailings storage facilities, the effects of the effective confining stresses are clear both with respect to the increase in the contractive tendency during the early shearing stages (resulting in positive excess pore pressures in undrained tests) and also in the increase in the stiffness of the material at every loading stage. But even if in a fairly loose condition, both of the samples tested show a tendency to dilate as soon as the PhTL is reached. All these features of behavior are typically observed in sandy materials under similar loading conditions [18]. For larger axial strains, both samples follow stress paths corresponding to an effective angle of friction close to 42\({}^{\circ}\), a value that exceeds what is commonly observed in conventional geomaterials but is within the values observed in iron ore tailings in the region of Minas Gerais, Brazil, where values between 26 and 45 were reported [33]. Once again, to explain this value, the particular characteristics of tailings must be examined in detail. In fact, natural soil particles with dimensions comparable to those found in tailings are created naturally and tend to suffer long processes of weathering that result in near spherically shaped particles. In contrast, tailings result from the mechanical crushing and grinding of rocks that tend to form particles with a much more angular shape, as observed in optical and electron microscope photographs, which justifies the fairly large value of effective angle of friction found in the experiments and in other tailings storage deposits in the world [33].