Article

Reliability of using laboratory-determined soil water characteristic data for mine waste cover design

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Abstract

Soil water characteristic curves (SWCCs) used for the design of covers over mine wastes, are typically determined in the laboratory by performing drying tests on samples in a Tempe cell. In some cases, re-wetting is also carried out. The advantage of using drying tests is that they are carried out from the saturated state at a given sample test density; so that there is some chance of obtaining repeatable results. However, re-wetting is carried out from the arbitrary end-point of a drying test, giving a different re-wetting curve from each of these points. SWCC data may also be measured directly in the laboratory and in the field. Depending on the uniformity, stress history, structure and any cementation of soils under laboratory or field conditions, their SWCC data may be very different from the SWCC data collected after sample preparation and saturation for laboratory Tempe cell drying testing. The paper presents in situ, laboratory and field SSWC data for mine waste covers that highlight the strong influence that structure, cementation, sampling disturbance, sample preparation, saturation, uniformity, construction control and stress history can have on the data. They cast some doubt on the design of mine waste covers based solely on the use of laboratory-determined SWCCs.

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... Over the 4-year monitoring period, as wetting-up fingers penetrated Cadia's trial waste rock dump (as shown in the laboratory; see Figure 10), the rainfall amount required to trigger some base seepage reduced from about 30 mm initially to less than 2 mm, and the lag time (delay) before the emergence of base seepage reduced from about 15 days to less than 2 days (see Figure 18; Williams and Rohde, 2009). Table 2 presents the average initial and 4-year moisture states of Cadia's trial waste rock dump. ...
... When base seepage has been recorded, lysimeters beneath the flat top of Cadia's trial waste rock dump have flowed for about twice as long (40 to 120 days, implying an average hydraulic conductivity during drain-down of 4.3 x 10 -6 to 1.4 x 10 -6 m/s) as those beneath the angle of repose slopes (20 to 60 days, implying an average hydraulic conductivity during drain-down of 8.7 x 10 -6 to 2.9 x 10 -6 m/s), due to the greater height of waste rock involved. Over the 4-year monitoring period, as wetting-up fingers penetrated Cadia's trial waste rock dump (as shown in the laboratory; seeFigure 10), the rainfall amount required to trigger some base seepage reduced from about 30 mm initially to less than 2 mm, and the lag time (delay) before the emergence of base seepage reduced from about 15 days to less than 2 days (seeFigure 18; Williams and Rohde, 2009).Table 2 presents the average initial and 4-year moisture states of Cadia's trial waste rock dump. After 4 years, the degree of saturation is starting to approach 60%, at which continuum breakthrough would be expected for well-graded waste rock. ...
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Unsaturated soil mechanics continues to play poor relation to saturated soil mechanics, although an unsaturated soil at a given density is stronger, less compressible and less permeable (i.e. performs better) than the same soil in a saturated state. There are many examples of unsaturated conditions in the mining field, including the wetting-up and drain-down of initially dry surface waste rock dumps; the irrigation and drain-down of heap leach materials; the drain-down, desiccation and rewetting of mine tailings; the dewatering of mineral products such as coal; the shear strength and compressibility of stored mine wastes; and the performance of geo-covers placed on mine wastes on rehabilitation. This paper highlights the key unsaturated soil mechanics parameters, overviews the nature of mining and processing wastes, and some products, and discusses the issues involved. Some applications of unsaturated soil mechanics addressing the shear strength, compressibility and permeability of mine wastes, and mineral products, are presented, together with data to highlight them.
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