- Title
- Water retention properties of engineered soils for mine rehabilitation
- Creator
- Vidler, Andrew Mark
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2022
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Mine site rehabilitation is the process of attempting to restore a mined area to a pre-mining state which among other requirements, usually calls for regrowth of vegetation, and is required whenever a mine reaches its end of life. The Hunter Valley region of NSW Australia, that as of writing, has around 40 operational coal mines that should all undergo a stage of rehabilitation. However, revegetating the land post mining, an inherent part of rehabilitation, can be challenging because of the degradation of topsoil quality following years of stockpiling during mining operations. In addition, the issue is exacerbated when the topsoil is of poor quality to start with, which is a common situation in the Hunter Valley. Another issue that arises during mining operations and that has a relevance for rehabilitation, is the management of coal tailings, a fine grained waste produced during the beneficiation of coal that can retain high volumes of water that generally does not contain toxic chemicals. The rationale behind the research conducted is that a topsoil can be improved, in terms of plant growth potential, by addition of coal tailings. The objective of the research is to develop and validate a design process that enables one to determine the preferred mixture proportions of a tailings and topsoil, given their properties, so that the mixture possesses targeted characteristics that are favourable to plant growth. Being able to efficiently improve a topsoil by tailings addition would not only ease rehabilitation and potentially allow for post mine areas to be used as economically productive agricultural land, but also reduce the volume of mine waste (tailings) that needs to be managed. Although many factors (chemical, physical, biological, geotechnical) of a soil influence plant growth, this thesis focuses on the significance of soil water retention and the evolution of soil strength with suction. Indeed, these two soil characteristics govern access to both water and oxygen for plant roots and the ability of roots to penetrate the soil. The design process involves predicting the water retention properties of a soil mixture of given proportions and adjusting those proportions to achieve specific retention characteristics. Through a literature review of plant growth research, those target characteristics were adopted as a saturation degree of 0.85 or lower (a value at which the air phase is usually continuous) at field capacity (i.e. suction of 30kPa) and maximising the volume of water between field capacity and wilting point (i.e. suction of 1500kPa). However, all of these characteristics may not be achievable simultaneously and depend on the water retention properties of the topsoil and tailings used. In that regard, the thesis explores several improvement scenarios based on the properties of the original components. The root penetration resistance of a soil mixture is not a design parameter as such but should be measured to assess whether it is likely to be detrimental to root growth or not. The development and evaluation of the proposed soil mixture design process involves several steps, which form the successive chapters of this thesis. First, the impact of the hydrophobicity of coal on water retention properties was assessed; then, a range of mine materials were collected and characterised in order to identify suitable tailings and topsoil to apply the design process; mixtures of the identified soils were then structured to representative in-situ conditions by wetting and drying cycles and their water retention curves were measured. The soil mixtures resistance to root penetration was also assessed, from field capacity to wilting point, after a penetrometer apparatus with similar dimensions and geometry to plant roots was designed and built. The predictive ability of different water retention curve models was then assessed, with the objective to identify the most appropriate predictive method to predict the water retention curves of the soil mixtures. Finally, the design process was evaluated using several possible design scenarios and materials. The mixtures of the identified materials were complemented by hypothetical mixtures of soils from the literature. It was found the water retention behaviour and available water and oxygen to a plant could be predicted relatively accurately for mixtures of the identified soils, when using a prediction method based on the water retention properties of pure tailings and pure topsoil. Design scenarios were demonstrated where either the plant available water or oxygen was improved through soil mixtures, but not both simultaneously; a design scenario where both the available water and oxygen are improved is believed to be possible, although it is hypothesised to occur from complex grain packing behaviour when mixing specific soils and such behaviour is not captured with the adopted prediction method. Another design scenario demonstrated a situation where any addition of material to a topsoil is only detrimental, and highlights that the design criteria for an improved soil mixture must be chosen carefully and should be based upon the water retention behaviour of the component soils (pure tailings and pure topsoil). This research suggests that the soil design process can indeed be used to determine preferred soil mixtures but it now needs to completed by plant trial tests to verify that plant growth is indeed enhanced in the designed mixture. This is out of the scope of this thesis and should be evaluated in further research. The soil mixture design process used herein requires laboratory measurement of the water retention curves for wet/dry cycled materials, which can be relatively time consuming. However, a number of promising approaches could be used to reduce the laboratory efforts, provided they are validated in future research. These approaches include methods to reduce the time necessary to appropriately structure soils, prediction of shear strength and penetration resistance of soil mixtures or only measuring saturation and volumetric water content at field capacity and wilting point, as opposed to the full water retention curve. Interesting and unexplained results were observed for materials that are either hydrophobic (crushed pure coal) or partially hydrophobic (due to presence of coal tailings). For example, crushed coal that was measured to be hydrophobic (i.e. contact angle of 90 degrees or larger) yet showed hydrophilic water retention behaviour, and a trend of reducing collapse as the proportion of partially hydrophobic material in a mixture increases. These findings warrant further research to understand the mechanisms at play.
- Subject
- water retention; soil mixture design; Hunter Valley; mine site rehabilitation
- Identifier
- http://hdl.handle.net/1959.13/1464300
- Identifier
- uon:46957
- Rights
- Copyright 2022 Andrew Mark Vidler
- Language
- eng
- Full Text
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