Abstract

The MOSTAP sampling apparatus is a popular tool in geotechnical investigation, since MOSTAP sampling can be done efficiently and cost effectively with the same rig used for CPT testing. Although few publications exist discussing the nuances and application of the MOSTAP sampler in practice, it is generally found that the MOSTAP sampler provides lower quality undisturbed samples when compared to piston tube or block samples. This paper discusses the experiences and learnings from geotechnical investigations where the MOSTAP sampler was employed in platinum and discard sand tailings in Southern Africa. A comparison between different sampler diameters at the same site showed significant improvements in sample recovery with larger diameter samplers. Ancillary equipment such as a core catcher and nylon stockings have significant effects on sample recovery and quality. The soil type and degree of saturation of samples also have a significant effect on sample recovery and sample quality. Transportation and storage of samples were identified as major contributors in moisture loss and sample disturbance. Even with the potential challenges in obtaining a representative sample at depth, MOSTAP samples are shown to be greatly beneficial in evaluating a soil profile and density determination with high confidence, especially when paired with index tests. The opportunity is identified where diligent sample measurement and tracking can provide reliable and invaluable information about a relevant soil stratum. Advanced laboratory testing (e.g., triaxial and oedometer tests) is not recommended on undisturbed MOSTAP samples, these should rather be remolded for critical state line testing, if relevant.

Abstract

This paper presents the implementation of a Cone Penetration Test (CPT)-based methodology to identify fluid-like tailings in the Tailings Storage Facility (TSF) pond to support a TSF deconstruction. The TSF discussed in this paper was partially raised upstream and classified as very high risk due to its location, next to a creek and a short distance to the downstream community. The owner's preferred mitigation strategy was to proceed with the TSF deconstruction. To inform the deconstruction planning, a comprehensive geotechnical site investigation was conducted, which included CPTu and boreholes with continuous thin wall tube sampling from a floating platform. The investigation identified tailings in the pond as high plasticity clays (CH) with a geotechnical fines content exceeding 95 %. The CPT results indicated low tip resistance, nearly linear pore pressure measurements, and minimal sleeve friction, suggesting the presence of fluid-like tailings in a significant portion of the TSF. The implementation of this methodology allowed to systematically confirm the presence of fluid-like tailings across the pond and define their extent. This information supported the owner’s decision to employ mechanical excavation taking advantage of the tailings flowability. The observations made during the tailings excavation confirmed a good agreement between the CPT assessment and the actual material behaviour. These findings underscore the efficacy of the CPT-based method in guiding safe TSF deconstruction projects

Abstract

Historically, the analyses of tailings storage facilities (TSFs) have primarily focused on understanding the characteristics of tailings, while often overlooking a comprehensive evaluation of the foundation, as seen at Mount Polley in 2014. The Global Industry Standard on Tailings Management (GISTM) Requirement 5.4 stresses the importance of comprehensively addressing all potential failure modes. Numerous platinum TSFs in Southern Africa are underlain by residual clay, specifically a residual mafic rock from the Bushveld Complex. Similar soils, known as "tropical black clay soils", are found in other regions of the world. Surprisingly, there's limited public information on testing and modelling the behaviour of this clay foundation. This paper presents a case study that employs a novel approach to assess the impact of loading from tailings deposition on the underlying clay foundation of an upstream TSF. The analysis investigates how shear behaviour and consolidation characteristics of the clay foundation change with varying TSF heights and construction rates. The approach includes quantifying excess pore water pressures and their influence on the clay foundation's effective stress. The analyses primarily employ traditional limit equilibrium methods to assess TSF stability and foundation behaviour, with the potential to expand to numerical modelling. The study concludes that excess pore water pressures will significantly affect the Factor of Safety (FoS) of a TSF, particularly with adverse consolidation characteristics and increasing TSF height. This is primarily due to the low permeability and changing consolidation coefficient (cv) of the clay layer as the surcharge load increases. Furthermore, the research reveals that, depending on the rate of rise, tailings deposition may induce excess pore water pressures, potentially reducing the FoS. The ability to quantify excess pore water pressures using this novel approach enables a more accurate estimation of the FoS for facilities underlain by low-permeability materials, either residual or transported.

Abstract

A geotechnical assessment of mine tailings state conditions using the static cone penetration test performed at different tailings storage facilities (TSF’s) will be presented in this paper. Both tailings evaluated herein are deposited as a slurry (hydraulic deposition) and have similar grain-size distribution curves. A set of Cone Penetration Tests (CPTu) with pore pressure measurements were performed at each site to evaluate the state of the tailings. To assess the contractive-dilative behavior classical methodologies were adopted such as i) the contractive/dilative boundary suggested by Robertson (2016) ii) the approach suggested by Plewes et al. (1992) and cited by Jefferies and Been (2016) and iii) the yield stress ratio method proposed by Mayne and Sharp (2019). Partial drainage effects will be identified with classical methodologies. The results were compared to evaluate the difference and limitations of each methodology. Comments on the similarity between the two tailings evaluated herein will also be presented to explain the differences in behavior due to aspects such as mineralogy, gradation, stress history and deposition.

Abstract

Cone penetration testing with pore pressure measurement (CPTu) represents a state of practice tool to assess the in situ state parameter, strength, and liquefaction susceptibility of sandy soils and mine tailings. Many techniques for the interpretation of CPTu data are based on the results of calibration chamber test programs on sand and, more recently, mine tailings. While these efforts have led to the current methods to interpret CPTu data, two factors relevant to CPTu interpretation require consideration: (i) the available calibration chamber data is dominated by tests with consolidated mean effective stresses < 200 kPa; and (ii) tailings storage facilities are being constructed to heights such that in situ effective stresses are far higher than those of the available calibration chamber test database. While much of CPTu interpretation is carried out in a dimensionless framework, there is evidence that existing relationships between stressnormalised tip resistance and state parameter are dependent on effective stress. This stress-dependence has been attributed to a variation in shear rigidity with effective stress, which is not accounted for in many interpretation techniques. However, at high stresses, other factors such as the curvature of the critical state line in an e-log(p’) plane may contribute. To assess CPTu of sands at high stresses, a novel small-scale calibration chamber employing a miniature cone capable of testing soils consolidated to a mean effective stress up to 2,000 kPa is outlined. Test results are presented for tests carried out over a range of mean effective stresses up to 1,000 kPa.

Abstract

The Cone Penetration Test (CPT) is the primary site investigation tool in silt-rich tailings. The permeability and compressibility range of tailings often puts the standard CPT penetration rate in the partial drainage range where the tip resistance, pore water pressure, and sleeve friction become functions of not only the state, but also drainage conditions. This makes interpretation of the state parameter, which represents liquefaction susceptibility and residual strength of tailings particularly challenging. It is not clear whether existing CPT interpretation frameworks are capable of accounting for effects of partial drainage. Developing field correlations for interpreting partially drained CPT data in tailings is hampered by spatial variability and sampling disturbance. CPT tests were performed in a calibration chamber and in the field on a gold tailings material. Dissipation tests were performed, and the pore water pressures on the cone at the u2 position were monitored. The degree of partial drainage was estimated based on the coefficient of consolidation inferred from these dissipation tests. Similarities and differences of the calibration chamber and field CPTs were discussed.