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Dry stacking is a disposition method that is quickly becoming a trend for the mining industry in Brazil. With several cases in which compacted mine tailings are positioned on the top of hydraulically disposed tailings which were placed into pits. The present research assessed the mechanical response of hydraulically disposed iron ore tailings collected from a pit that was formerly a mining site. To better understand the mechanical behaviour of mine tailings under such conditions, the investigation of their monotonic response is of great importance. The studied material was classified as a silty sand with traces of clay size. A very loose condition with distinct confining stresses was tested monotonically on a simple shear equipment. Simple shear tests were performed in an apparatus that can directly measure pore pressure, guaranteeing an effective stresses analysis. A plane strain condition was applied on fully saturated samples under undrained loading conditions. Results showed the agreement of the strength envelopes of monotonic stress paths, the effective friction angle of the material was 33.2° and no cohesive intercept.

Mine tailings have been disposed of in a slurry form in tailings dams for many years. However, recent disasters involving conventional disposal in dams reinforced the need for alternative structures to store these materials safely. One alternative is dry stacking of tailings. In these structures, tailings are filtered to low moisture content and then compacted in layers. Due to material compaction, dry stacks tend to be stable and are usually built with elevated heights to use the available area better. So, it becomes essential to understand the mechanical behavior of tailings subjected to high pressures, especially concerning the possibility of grain breakage. In this context, the present research focuses on studying the geotechnical behavior of iron ore tailings from different stages of ore beneficiation plants in Quadrilátero Ferrífero, Brazil, when subjected to high pressures both in compression and in shear paths. The results demonstrate that the inclusion of fines can change the geotechnical performance of the dry stack tailings considering the same compaction energy (greater strength, stiffness, and lower permeability) both in drained and undrained conditions. No breakage could be identified for the stress level studied (6 MPa).

Regulations prohibiting the construction of new tailings dams using the upstream method and requiring the closure of the existing ones have been recently introduced in Brazil. This scenario demands the sought for alternatives to manage both the previously disposed tailings as well as the upcoming generated mining residues. Fortunately, advances in dewatering technologies have enabled the tailings’ dried disposal in stacks having hundreds of meters rather than the traditional slurry deposition. Eventually, a cement material can be incorporated into the tailings before the stacking aiming to enhance the overall material’s performance, permitting the construction of higher and more inclined piles. In this regard,the present study evaluates the effect of key variables on the stiffness and strength of compacted iron ore tailings-cement blends. Specifically, the effect of the following variables was statistically evaluated through a full factorial design set: dry unit weight (17, 18, and 19 kN/m3), amount of cement (1, 3, and 5%), curing period (7, 28, and 90 days) and cement type (pozzolana Portland cement and high early strength Portland cement). The results have shown that all the main factors, and most of the second-order interactions, were significant in altering both the strength and stiffness of the studied mixtures, with a highlight to the amount of cement, curing time, and dry density. The type of cement, in turn, has exerted a marginal influence. As well, both responses could be successfully correlated to the adjusted porosity/cement index via power-type functions.

The incorporation of cement to improve soil properties and attract the re-use of locally available materials has become a part of current geotechnical engineering projects. Its applicability varies from construction of pavement base layers, slope protection for earth dams, to support layer for shallow foundations. Unconfined behaviour might be used to evaluate the basic mechanical properties and efficiency of the cemented soils. The proposed relationship between porosity () and volumetric cement content (Civ), presented as porosity cement index (/Civ), was shown to play an important role in the initial shear stiffness (G0), tensile (qt) and compressive (qu) strength of cemented materials. This research aims to assess these parameters through experimental investigation and modelling predictions. Bender elements testing were carried out to assess the maximum initial shear stiffness (G0) prior to specimens testing, and assessing G0 evolution during cement hydration, focusing on the anisotropic/isotropic behaviour established during specimen moulding. Results show a good correlation between experimental and numerical data. It was also observed that cementation isotropises G0 during curing.

In soil improvement, the insertion of cement to the soil matrix provides an increase in strength and rigidity of the material, whereas fibre addition provides an increase in ductility, reducing post-peak loss in strength. This research aims to experimentally analyse the mechanical behaviour of unreinforced and fibre-reinforced cemented sands under flexural tensile monotonic loading conditions. In order to do so, an experimental program was planned using silty sand (Arenito Botucatu), early strength Portland cement – CPIII, and polypropylene fibres 24 mm long and 0.023 mm thick. Three different dry unit weights (γd) (18, 19 and 19.7 kN/m3), three cement contents (3, 5 and 7%), and two fibre contents (0 and 0.5%) were chosen. Analysing the results, an increase in cement content generates an increase in flexural strength (qf) for all concentrations and the increase in γd generated smaller qf gains, especially for lower cement contents. The addition of fibres generated a qf increase for lower γd but at higher γd there was a reduction in the influence of fibre addition on strength gain. The reduction can be attributed to an entanglement of the fibres in higher compaction strengths, disrupting the formation of cementitious bonds. However, based on a qualitative assessment, there was a noticeable reduction in post-peak strength loss due to fibre insertion. The parameter porosity/volumetric cement content (η/Civ0.28) was considered suitable to predict the qf mechanical behaviour of the studied composite.

The effect of the addition of 0.5% of randomly distributed polypropylene fibres in a round grained sand, composed mainly of silica, is presented on laboratory scale. The samples were compacted with two void ratios (0.75 and 0.63), corresponding to values of relative density of 50% and 90% respectively. 24 monotonic triaxial tests were performed, with initial effective mean pressures of 20, 100 and 200 kPa. Half of samples were tested in drained conditions, and the other half in undrained conditions. For the samples subjected to drained conditions, the effect of the fibres was identified at the post-failure stage, by showing sustained increase of strength. The positive effect of fibres could also be observed through the increase of material friction angle. In the samples subjected to undrained tests, the addition of fibres reduced the contractive behaviour, mainly for those more compacted.