- Title
- Reliable shear strength estimation for very-high spoil dumps
- Creator
- Bradfield, Leonie
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2018
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The shearing behaviour of current and planned coal mine spoil dumps up to 400m in height is studied using large-specimen-high-stress direct shear tests performed on a range of materials that are commonly encountered in the Hunter and Bowen Basin coalfields. The motivation for this research is to address industry concerns that some current spoil dump heights (>350m) are surpassing the scale (≤120m) for which reliable design information exists, and because standard geotechnical laboratory equipment is not able to test representative spoil specimens at field-scale stresses. A Large Direct Shear Machine (LDSM) with specimen size 720mm x 720mm x 600mm, and a normal stress capacity of 4.6MPa was custom designed and constructed to generate the data presented in this thesis. Bulk samples comprising 120 x 200L drums of five spoil types from the Bowen Basin and Hunter Coalfields were collected and tested. Detailed geotechnical characterisation of these materials was carried out. LDSM data validation involved comparisons with direct shear strength tests carried out, using 100mm and 300mm Direct Shear Machines (DSMs), on a previously well-studied, uniformly-graded sand and one of the spoils. The LDSM testing program consisted of thirteen sand tests and fifty-two spoil tests. The DSM and LDSM results were critically analysed to provide insights into a range of prior uncertainties regarding the shear strength of coal mine spoil: (i) shear strength and behaviour for a wide range of spoils compressed under normal stresses between 500kPa- 4600kPa; (ii) the significance of moisture content for shear strength; (iii) the potential for mechanical (compression-induced) saturation to occur at high stress; (iv) the significance of scale effects for spoil dump design; (v) applicability of the previously widely-used BMA Coal (BMAC) shear strength framework to higher stress situations and widely varying spoil types. A fundamental distinction between “rock-like” and “soil-like” spoils is identified on the basis of simple, visual-tactile observations, and evaluation of the uncertainties described above is made with respect to these two types of spoils. The shearing behaviour for “rock-like” spoils prepared to unsaturated (air-dried) and “saturated” (inundated under load until no further deformation) moisture conditions can be described using trilinear or linear envelopes, depending on clast strength. Trilinear envelopes are appropriate for stronger “rock-like” spoils (clast strength ≥5MPa). Within each normal stress zone of the trilinear envelope there is a prevailing shearing mechanism that is an artefact of the structure of the spoil fabric and its response to the level of compression applied. Clast strength is found to determine the transition stresses that separate these trilinear zones. For weaker “rock-like” spoils (clast strength 1-5MPa), the trilinear lines converge to a sigmoidal curve to indicate more gradual changes in the shearing mechanism, and as such, a linear failure envelope is equally appropriate. A curvilinear failure envelope (power-law trendline) is found to be suitable for characterising the shearing behaviour for “soil-like” spoil. The shear strength of “rock-like” and “soil-like” spoils is found to be discernible by clast strength, particularly at medium-high normal stress (>1500kPa). For unsaturated conditions, the corresponding BMAC failure envelope can be notionally related to strength of the most abundant clasts within a specimen. Shear strength and horizontal strain at peak strength are found to be sensitive to moisture content for all spoil types. Lower shear strengths and smaller horizontal strains at peak strength are associated with the saturated moisture condition. The effect of moisture content on strength for a commonly-encountered “rock-like” BMAC-Category 2 spoil, was accounted for by a “moisture content correction factor”. This correction factor accounts for the rate of strength change with respect to water content, and it is normal stress dependent. For the Category 2 material studied, shear strength increases by (25σ’n) kPa for every percentage decrease in water content below saturation, where σ’n is the numerical value of the normal stress in MPa, under which the shear strength was measured in the saturated condition. Mechanical (compression-induced) saturation (Tarantino, 2009) is not achieved for any of the “rock-like” spoils that were air-dried and then compressed up to stresses representative of “very-high” dumps (500kPa- 4600kPa). For the “soil-like” spoil, a high degree of saturation (>90%) is achieved when compressed at very-high normal stress (4200kPa), and a further increase in overall (average) degree of saturation occurs when shear failure is then mobilised (approached 100%). This finding implies that, since shear deformations are concentrated in the shear zone, shearing of highly-loaded “soil-like” spoils could lead to localised mechanical saturation of the shear zone, whilst the rest of the spoil pile remains unsaturated. For “rock-like” spoils, shear strength (as defined by the secant friction angle) is found to be scale-dependent in terms of both specimen size and the magnitude of normal stress. These scale effects are significant for dump design of current-built and planned heights because they imply that the adoption of shear strength parameters measured from standard-sized DSMs (100mm and 300mm) will overestimate the shear strength for dumps with height-equivalent stresses ranging between 450kPa-4600kPa. The largest values of bulk unit weight are associated with the saturated moisture condition for all spoil types. The LDSM data suggests that linear depth-based equations are appropriate for estimating the bulk unit weight profile for spoils exposed to increasing depths of burial. This finding is important because it controverts the current approach by industry, whereby a constant value for bulk unit weight is adopted, irrespective of the dump height. For some materials tested, the shear strength measured by the LDSM was different from the BMAC framework strengths, both within the BMAC-applicable stress range, and when extrapolated to higher stress levels. This is fundamentally important because it identifies several non-compliant spoils for which correct application of the BMAC categorisation process will not provide reliable shear strength parameters. The research outcomes suggest that the BMAC framework can be used for reliable estimation of shear strength for spoil dumps of current and planned heights under the following conditions: (i) CAT2 or CAT3 materials which contain clasts with UCS ≥ 5MPa; (ii) the fine fraction of the spoil has a liquid limit ≤ 35%; (iii) Clasts have low slake and swell potentials, and low or moderate dispersion potential; and (iv) CAT1 strengths are only adopted within the BMAC-intended stress range (i.e. for dumps up to 120m in height).
- Subject
- coal mine; mine spoil; high dumps; shear strength; direct shear; large specimen; high stress; large direct shear machine; big john
- Identifier
- http://hdl.handle.net/1959.13/1385398
- Identifier
- uon:32220
- Rights
- Copyright 2018 Leonie Bradfield
- Language
- eng
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| View Details Download | ATTACHMENT02 | Chapters 1-6 | 10 MB | Adobe Acrobat PDF | View Details Download | ||
| View Details Download | ATTACHMENT03 | Chapters 7-14 | 21 MB | Adobe Acrobat PDF | View Details Download |