# The Experimental Characterization of Iron Ore Tailings from a Geotechnical Perspective
## 3 Results
### Undrained Monotonic Triaxial Compression Tests  
The stress-strain shear resistance response in the undrained monotonic triaxial compression (UMTC) of reconstituted samples representing field behavior, reconstituted through the SDM, was assessed through the behavior of isotropically consolidated samples to effective confining stresses of 50 and 200 kPa (Figure 7). The stress paths described by both samples (Figure 7a) under undrained triaxial compression indicate that, during the early loading stages, the samples show a tendency to contract until reaching a common line passing through the origin. The so-called phase transformation line (PhTL) observed in Figure 7a marks the instant where the samples change from a contractive to a dilatant tendency, with the stress paths curving to the right-hand side of the plane p'-q and progressive shearing causing an increase in the effective confining stresses (p'). For a certain level of axial strain, the stress paths tend to follow another common line passing through the origin, which may possibly coincide with the so-called critical state line (CrSL). During shearing, the stress-strain responses of the two samples also show similar features (Figure 7b). Until they come near to the PhTL, there is a fairly linear stress-strain response of the samples. But as the PhTL is crossed, the samples show decreasing stiffness and strains increase at a larger rate with the increase in shear stress. Finally, as the samples approach the CrSL, the samples show progressively increasing stiffness with strain. As expected, during all the described stages, the sample consolidated at higher effective confining stresses exhibits a stiffer response due to the well-known effect of p' on the stiffness of soils [18]. In fact, this well-known effect of p' on the stiffness of soils during shearing is also clear in the case of this unconventional geomaterial. Figure 7c reinforces some of the features of behavior previously described, namely with respect to the tendency for the effective stress ratio (\(\sigma^{\prime}1/\sigma^{\prime}3\)) to reach a similar and fairly constant value for the two samples tested, which is close to 5. This value corresponds to an effective friction angle of 42\({}^{\circ}\), which is a fairly high value in comparison with a conventional soil with the characteristics of the tailings tested. Still, this value is within the range of values observed in the field, namely in Campo Grande iron ore tailings dam (Mariana/MG, Brazil), possibly due to the very important angularity of the grains formed by rock crushing and grinding, as shown by optical and electron microscope photographs [33]. It is also worth mentioning that the sample consolidated to a higher effective confining stress shows a higher initial tendency to contract, which results in a large reduction in p' during the earlier loading stages. This is a feature that is also typical of the behavior of granular soils under undrained monotonic shearing, which also tend to show a larger contractive tendency (responsible for higher excess pore pressure generation) when consolidated under larger effective confining stresses. Figure 7d emphasizes the tendency of the samples to change their behavior from contractive to dilative during monotonic shearing, which is another feature of behavior that is also commonly shown by granular soils [18]. In the case of the undrained monotonic triaxial compression tests carried out, this tendency can be observed through the variation in the excess pore pressure with the axial strain imposed to the sample. In fact, as the PhTL  
Figure 7: Stress–strain shear resistance response of slurry-based reconstituted samples isotropically consolidated to effective confining stresses of 50 and 200 kPa under undrained monotonic triaxial compression: (**a**) effective stress path; (**b**) stress–strain response; (**c**) variation in the effective stress ratio with p’; (**d**) variation in the excess pore pressure with axial strain.