Stability of a proposed steepened beach

Abstract

Existing topography at an operational lateritic nickel facility is such that if the tailings beach slope could be increased to 1-2% for approximately one year, significant costs related to wall building could be deferred. As additional thickening capacity would require significant capital expenditure, polymer treatment technologies have instead been investigated. As part of the trial process for polymer treatment, the stability of the potential steepened beach was assessed. While beach slopes of the range targeted here would, for many materials, be unlikely to produce an unstable landform, lateritic nickel tailings develop very low dry densities, consolidate slowly, and contain hypersaline pore fluid. This combination results in a lower resistance to slope instability, all else being equal. Further, polymer treatment technologies, while providing benefits through rapid dewatering and beach slope development, have been shown to result in an increased brittleness for some materials. Laboratory characterisation of the polymer treated material was first undertaken, by means of (i) monotonic, cyclic and post-cyclic simple shear testing, to assess the potential for strength loss of the material, (ii) laboratory-scale shear vane testing, to estimate residual shear strength, and, (iii) settling and consolidation tests, to provide input to consolidation modelling. A consolidation model of the proposed steepened beach was then developed. Outputs from this model were used as the basis for infinite slope stability analyses on a variety of scenarios. It was found that over the one year planned deposition period, beach slopes of 1.5% or less are likely to result in a satisfactory factor of safety (FS) for the steepened beach. 2 Previous relevant studies 2.1 Steepened beach stability A number of previous studies have been undertaken to assess the expected stability of proposed unconstrained tailings beaches, most of which were developed from thickened tailings deposition. Each of

The influence of slurry density on in situ density

“…Therefore they provided a useful source to perform additional tests to assess the effects of initial slurry density. The results of some of the testing on one of the gold tailings (1) and the iron ore tailings were previously reported by Reid and Fourie (2014), while further details regarding the lateritic nickel tailings are outlined by Reid and Boshoff (2015). Consolidation tests were undertaken using either a 63 mm diameter oedometer, or a 71 mm diameter slurry consolidometer (Sheeran & Krizek 1971), dependent on availability.…”

Geotechnical effects of polymer treatment on tailings – state of knowledge review

“… Post-cyclic test comparisons using a DSS device outlined by Reid and Boshoff (2015), Reid et al (2015a), and Reid and Fourie (2017a). Most of this testing did not suggest an increase in post-cyclic strength loss in PT samples.…”

“…Further, cyclic liquefaction behaviour is affected by a number of material properties such as density, fabric, and deposit age. Three published examples of cyclic testing are available to enable comparison between PT and UT material -testing by Reid and Boshoff (2015) on laterite nickel samples, Reid et al (2015a) on iron ore tailings, and by Reid and Fourie (2017a) as part of their study of PT effects on a low-plasticity synthetic tailings. The results of these three test programs are summarised together in Figure 2.…”

Large strain shearing behaviour of untreated and polymer treated clayey silt slurry specimens

“…Owing to the increased application of forms of PT, studies to characterise the effects PT may have on post-deposition geotechnical behaviour of the treated slurry have been undertaken. Broadly, these studies have indicated the following effects of PT: lower consolidated densities at a given vertical effective stress and higher permeability at a given density (Jeeravipoolvarn et al 2009;Yao 2012;Manzotti et al 2014;Beveridge et al 2015;Gholami & Simms 2015;Reid & Boshoff 2015;Riley et al 2015;Znidarcic et al 2015;Reid & Fourie 2016, 2018aAbdulnabi et al 2022), higher undrained shear strengths at a given density (Jeeravipoolvarn et al 2009;Yao 2012;Beier et al 2013;Manzotti et al 2014;Cooling & Beveridge 2015;Gholami & Simms 2015;Reid & Fourie 2016Abdulnabi et al 2022) and higher CSL elevation (Reid & Fourie 2016), and various contrasting outcomes when examining the effects of PT on brittleness in undrained monotonic or post-cyclic shearing (Beier et al 2013;Gholami & Simms 2015;Reid & Fourie 2017) and drastically different penetrometer responses (Reid and Fourie 2018a), with a summary of recent studies provided by Reid and Fourie (2018b).…”