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Geotechnical support for open pit coal mining
Geotechnical considerations for support of coal mining are described in this review, which is an updated version of an earlier review published in 1995. A thorough working knowledge of the geological environment and of modern coal mining operations is required to provide specialist geotechnical advice. The mining industry operates within a range of constraints and drivers that are quite different in some respects to those encountered in the wider geotechnical community that supports civil projects. Fundamental geotechnical requirements remain the same: a sound working knowledge of applicable geotechnical parameters and groundwater conditions and reliable analytical tools. Opportunities for data gathering are limited and much reliance is placed on experience, judgement and consideration of mine slope forming processes and the operating requirements of equipment. Principles for stability assessment of both rock and spoil slopes are outlined. The implications of modern risk management procedures are discussed along with future developments that are anticipated.
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Reuse of Iron Exploitation Waste as a New Binder for Tailings Stabilisation in Dry Stacks: Circular Economy Approach
The dried allocation of the tailings, rather than the disposal in a slurry form, appears as an alternative to attend new legislation and improve the safety of mines operation. Also, the use of cementing agents in dry stacking facilities can enhance aspects of operations such as guaranteeing dilatant behaviour at the base and increasing tailings’ strength. The present research assesses the technical and environmental viability of a new alkali-activated cement (AAC) in iron ore tailings stabilization. The mechanical response of compacted tailings-AAC specimens was evaluated through strength and shear modulus tests while Life Cycle Assessment (LCA) was performed to verify the sustainability of this new binder when compared to conventional AAC. This new binder is derived from the residues of iron exploitation and is intended for use in new disposal schemes, such as dry stacks. The AAC is mainly composed of metakaolin (MK), produced from the residual soil removed during the mining activity, and sodium silicate (SS), produced with sandy tailings. Using tailings and waste in AAC production aligns with sustainable practices, minimizing resource consumption and promoting waste recovery. Also, LCA demonstrates a lower impact for tailings AAC when compared to conventional AAC. In addition to environmental and mechanical aspects, using this AAC supports the application of circular economy in mining since it enables the reuse of waste produced in mine operation as a substitute for conventional cement (that involves another industry and raw materials).
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Managing Geotechnical Uncertainty With Simulation Models: An Introduction
In a standard deterministic analysis discrete scenarios are considered, and a moderately conservative “characteristic” value is used as a design basis. However, fixed or exact values in a real-world geotechnical site seldom occurs. Deterministic approaches may not explicitly consider the ground uncertainty. Simulations using various probabilities provides for this uncertainty as each parameter input is treated as a random variable within certain measured ranges or ability to evaluate. Monte Carlo (MC) sampling is a traditional technique for generating random numbers to sample from a probability distribution. When low probability events occur, a small number of MC iterations might not sample sufficient quantities of these outcomes for inclusion in the simulation model. Latin Hypercube (LH) sampling uses stratification of the input probability distributions, to overcome the limitations of Monte Carlo sampling. The simulation results show low probability outcomes are included in the sampling for the simulation model. At a high number of simulation iterations both provide similar outputs, but at low simulation iterations the LH is more reliable. However, both the MC and LH sampling suffer from impractical values at low or high probability events when the normal probability density function (PDF) is adopted. The normal PDF is commonly used in statistical modelling. Non-normal PDFs often represent the best fit PDF when a goodness of fit test is carried out. The errors associated with using the common normal PDF are shown with the above-mentioned simulation models. This best fit PDF applies whether simulation models as described above is used or even with simple “what if” sensitivity models in traditional analysis.
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Evaluating effective stress parameters and undrained shear strengths of soft-firm clays from CPTu and DMT
Results from piezocone penetration (CPTu) and flat dilatometer tests (DMT) can be used to evaluate the stratigraphy, soil types, and a suite of engineering parameters that are needed for geotechnical analysis and design, especially for finite element methods (FEM). Of particular interest herein is the utilization of in situ test data for assessing the effective stress strength envelope (c’ and f’) in soft to firm clays, as well as undrained shear strengths (su), since many FEM codes have built-in constitutive soil models that are based on critical-state soil mechanics and require effective stress parameters as input. An existing undrained limit plasticity solution for evaluating f’ in clays from CPTu is reviewed and then extended to the DMT via a link established through spherical cavity expansion theory. Laboratory and field results on soft Bothkennar clay at the British national test site and additional CPTu data in clays are used to illustrate the methodologies.
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Unsaturated free-standing mainline railway embankments – Part 2: An example of handling the awkward truth
The presence of negative pore pressures within cuttings and embankments, and the benefit of the consequent reduction of the likelihood of instability (also known as increased stability), have long been recognised by members of the profession. Negative pore pressures are usually a consequence of environmental influences upon clay soils in particular, and are frequently termed “soil suction”.
The recognition of suctions in the assessment of potential instability, by way of stability analyses, is less common, albeit that the tools are available to conduct such analyses, once the boundary conditions are understood.
Measurement of suction values in the field assists the selection of suction values appropriate for such analyses.
In the companion paper, the authors develop a philosophy and present a defensible model for analysis of free-standing embankments (Hull & Leventhal, 2019). Herein, a case history is presented that demonstrates one such analysis, being for Main Line Railway infrastructure. The results indicate the benefit accrued through recognition of suction in the estimation of potential instability of free-standing embankments.
The paper is intended to alert the profession to an improved assessment technique that incorporates these effects.
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Use Of Bitumen Stabilised Limestone In Western Australian Road Pavements
Bitumen emulsion stabilisation of locally available Tamala Limestone was widely used by State and Local Governments from the mid-1960s through the late-1990s, however its use has declined in recent years. This paper aims to substantiate its benefits as a viable alternative material for use on heavily trafficked roads. The benefits provided through stabilisation of crushed limestone with bitumen emulsion include improved workability, reduced ravelling under construction traffic, lower moisture susceptibility and enhanced mechanical properties. Case studies are presented that show that satisfactory performance has been observed where Bitumen Stabilised Limestone (BSL) is used as a basecourse under heavily trafficked roads. The paper provides a construction methodology and discusses barriers and future opportunities. Two structural design approaches are presented for the use of BSL under sprayed seals and thin asphalt surfacings.
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Managing the risks associated with acid sulfate rock in NSW road projects
Acid sulfate rock (ASR) is unweathered rock that contains metal sulfide minerals (commonly iron sulfides). When ASR is exposed to both oxygen and water, oxidation of sulfides leads to the formation of sulfuric acid, sulfates and salts. The probability of ASR being present, can to some extent, be predicted from the geological origins of the rock or later hydrothermal depositions of sulfides. An ASR risk map has been prepared to assist in the pre-design phase of road construction projects.
ASR has the potential to be problematic (depending on concentrations) with respect to environmental, structural and durability risks. It is becoming increasingly common for ASR to be encountered by roadworks in New South Wales where designs include deeper cuttings into unweathered rock that has generally not been the case historically. Examples are given of New South Wales where ASR has been encountered, together with an American example where significant environmental penalties and remedial costs occurred.
Other than low risk geological formations, site investigation for roadworks must include identification of ASR and, where present, screening, detailed testing and interpretation of the distribution of sulfide contents. The details of each aspect of this assessment need to be fully understood.
Where ASR is present, the design, specification and construction must include control measures to reduce environmental risks associated with exposing ASR and potentially releasing leachate into the environment. Control measures include dilution, encapsulation and treatment with crushed limestone. Control measures must also be developed to protect structures such as bridges, culverts and retaining walls, stormwater drainage pipes and pavements.
The locations of where ASR is placed within the earthworks formation must be limited with respect to environmental, structural and durability constraints. For successful management of ASR in construction projects, careful planning and staging of the earthworks is critical.
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Evaluating liquefaction and lateral spreading in interbedded sand, silt and clay deposits using the cone penetrometer
Current procedures for evaluating potential earthquake-induced liquefaction and lateral spreading appear to have a tendency to over-predict liquefaction effects in interbedded sand, silt, and clay deposits. Possible reasons for overprediction of liquefaction effects are discussed, and investigations regarding some factors pertinent to use of the cone penetrometer are described. An axisymmetric direct cone penetration model is presented for use with the MIT-S1 constitutive model to explore cone penetration processes in a range of soil types; current efforts are focused on validating this new direct cone penetration model, beginning with simulations of cone penetration in soft clay. The relationship between cyclic strength and cone penetration resistance in non-plastic and low-plasticity fine-grained soils is examined by relating cyclic strengths from laboratory tests to cone penetration resistances from simulations. The performance of a site underlain by interbedded soils along the Çark canal during the 1999 M=7.5 Kocaeli earthquake is analysed using one-dimensional lateral displacement index procedures and two-dimensional nonlinear deformation analyses with spatially correlated stochastic models to illustrate how several factors can contribute to an over-prediction of liquefaction effects. Future research needs and directions for improving the ability to evaluate liquefaction effects in interbedded sand, silt, and clay deposits are discussed.
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Pile Design For Solar Farms And Reactive Clay Sites In Australia
There are many geotechnical aspects that require consideration during the design of solar farms. One key consideration, which is a major cost driver for these developments, is the design of the pile foundations supporting the solar arrays. The number of piles is often in the order of tens to hundreds of thousands and contributes to a significant portion of the capital cost of the project. The uptake of commercial scale solar is in its infancy in Australia with the first commercial size farm only commencing operation in late 2012. Consequently, we have limited local case studies for lightly loaded piles installed in reactive clay subjected to wetting and drying cycles on which to base a rational design methodology. Methods are available to analyse this type of problem. However, many are based on simplified assumptions, limited data sets, or can be difficult to apply in practice. Furthermore, some of the available methods rely on laboratory tests, such as swell pressure measurements, of which the results can be highly variable and difficult to rationalise, or insitu tests that are difficult to undertake in engineering practice. This paper provides an overview of the current state of practice in Australia for designing piles supporting solar arrays to resist expansive soil movements and presents some preliminary design charts developed using finite element analysis to assist designers with estimating the vertical pile movements and loads in piles associated with reactive ground movements.
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Mechanistic design of concrete block pavements
Concrete block pavements consist of a layer of rigid, specially shaped and jointed paving blocks that are overlaid on a bedding course layer. The joints of these pavers create an ‘interlocking’ action, which makes the pavement stiffer and stronger with progressive loading. Therefore the design concepts for flexible pavements cannot be directly adapted for concrete block pavements. Instead a mechanistic design method is required, which is too complex to be undertaken by hand calculation and requires the use of computer analysis. This paper describes the development of a new software program (DesignPave), which has been developed in conjunction with the Concrete Masonry Association of Australia. The program’s methodology and design procedure involve: a) estimation of the number of design traffic vehicles (NDT) and traffic loading spectra; b) modelling of progressive interlocking and stiffness development of the block layer; c) stress-strain analysis of a multi-layer pavement system; and d) application of rutting and fatigue criteria suitable for block pavements. This paper uses DesignPave to produce design curves that describe the relation between design thicknesses with NDT. The design curves for different layer systems are compared to identify appropriate subgrade CBR and NDT for a layer system. The different layer systems include a granular base course, a granular base course with a sub-base course and a granular base course with a stabilised sub-base course. The DesignPave program also produces extensive technical documentation for use in common engineering practice.