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Piezocone technology for the geoenvironmental characterization of mine tailings Davies, Michael Paul

Abstract

The mining industry produces large volumes of milled mineral tailings. These tailings are separated/removed from ore by mechanical, chemical and/or biological means during processing sequences aimed at isolating the economic commodity. In many cases, particularly in metaliferous and oil sands mining, the economic commodity is a very small amount of the total mass of ore processed. Modern mining is increasingly processing greater tonnages as the economies of small-scale mining decrease. It is often the case that the tonnage of ore processed is essentially equal to the tonnage of tailings produced and, consequently, mine tailings storage facilities are often required to be very large. Mine tailings storage facilities often represent the single largest environmental liability with the mining process. To design and operate against the large number of potential physical and chemical failure modes, designers and operators of tailings storage facilities must adequately characterize the nature of the tailings within these facilities and then manage the given facility accordingly. The large size of many of these facilities makes adequate characterization a challenging task. As if to add credibility to the nature of this challenge, of the roughly two major tailings storage facility failures per year over the past four decades, many of the failures have been directly attributed to a lack of understanding of the in-situ character of the given tailings. Piezocone testing, carried out in-situ under prevailing physico-chemical stresses, was assessed to see i f it could offer any advantageous technology for the geoevironmental characterization of mine tailings. In particular, an assessment of new developments in resistivity piezocone technology were applied to mine tailings for the first time to see if there was any ability to characterize these materials for a very broad range of key physical and chemical traits. Besides a general assessment of the suitability of piezocone technology for characterizing mine tailings, the research included more in-depth assessments of several key aspects of the geoenvironmental character of mine tailings. In the process of these assessments, several modest contributions appear to have been developed. Amongst these original contributions are: • introduction of the resistivity piezocone to mine tailings including new generation resistivity modules with isolated measurement technology for linearity of calibration and greater range and reliability of response; • introduction of the concept of a material index for assessing the soil behavior type of mine tailings; • introduction of a fines content assessment procedure from material index; • development of a method to estimate tailings compressibility from material index; • suggested approach for estimating static and cyclic liquefaction susceptibility directly from piezocone tests using concepts of material state; • rationalization of constrained versus unconstrained undrained strength of liquefied tailings and the introduction of piezocone-direct techniques for estimating the appropriate undrained strength in either case; • introduction of a case history of a mine tailings static liquefaction event; • a suggested approach to estimate hydraulic conductivity anisotropy of tailings deposits; • introduction of resistivity piezocone technology to the assessment of sulphide tailings deposits including assessment of several ionic strength-bulk conductivity trends; • a suggested normalization technique for bulk resistivity measurements; and • an initial effort at combining/comparing electromagnetic signatures from surface commercial techniques with subsurface resistivity piezocone measurements. From an overall geotechnical contribution, the research also adds to the relatively sparse literature on cohesionless silt-sized soils. Mine tailings typically have unique fabric, gradation, grain angularity and stress-history when compared to most cohesionless soil deposits. Understanding these differences is important when "importing" traditional soil mechanics to mine tailings. The research work supporting the thesis included over 200 piezocone soundings from 8 minesites and 4 well-characterized tool calibration sites in a wide variety of physico-chemical conditions. Over 100 pore water samples with geochemical analyses complimented this piezocone database. In addition to introducing existing and proposed evaluative procedures, the research database was used for verification of the proposed procedures. Part of this verification process included clearly identifying differences between mine tailings and natural mineral soils and these differences can affect traditional piezocone interpretive techniques; e.g. the use of shear wave velocity to predict in-situ state. The main conclusion developed from this research is that current piezocone technology, when applied within an appropriate assessment framework, can characterize to a screening level many aspects of the geoevironmental nature of a wide variety of mine tailings. Several new procedures for estimating key geoevironmental parameters of mine tailings are provided. Additional research is required to determine which of these procedures may be globally tenable and to what degree the procedures require modification.

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