32 research outputs found
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Numerical and Experimental Study of Issues Affecting Energy Piles in Unsaturated Silt
No full textThis study focuses on the coupled thermo-hydro-mechanical processes in unsaturated soils surrounding energy piles. Energy piles are deep foundations that are used to provide support to buildings and for exchanging heat between the ground and an overlying structure. While their performance in saturated and dry soil layers have been well investigated, their performance in unsaturated soils is an emerging topic that poses special challenges. Changes in temperature may induce drying of the soil, which will affect not only the heat transfer process but also the mechanical response at the soil-pile interface. In particular, this study evaluates the coupled processes in unsaturated soils surrounding energy piles using numerical simulations considering coupled heat transfer and water flow with nonequilibrium phase change and enhanced vapor diffusion processes. Further, this study evaluates the changes in yield stress of unsaturated soils during changes in suction which may be induced by thermally induced drying using a novel experimental technique. The numerical simulations and experiments in this study are focused on a low-plasticity soil referred to as Bonny silt, which was selected for this study because the solid particles are chemically inert, the soil retains water over a wide range of suction magnitudes, and many experiments and numerical studies have been performed on the soil that provide a point of comparison for the thermo-hydro-mechanical processes investigated.In the numerical simulations, a three-dimensional coupled heat transfer and water flow model was calibrated using tank-scale tests on Bonny silt then applied to investigate the long-term behavior of solitary energy piles and energy piles in groups having different boundary conditions and thermo-hydraulic initial conditions. The model was first applied to understand the impact of heating and cooling cycles for conditions representative of both heat exchange and heat storage for energy piles in geometric configurations. Changes in degree of saturation are observed during changes in temperature, which a magnitude of change dependent on the initial conditions of the soil and the magnitude of temperature applied. In particular, the water content and air entry suction control the changes in soil thermal behavior and water content. In some cases, a permanent decrease in degree of saturation is observed due to cyclic heating and cooling when the cooling temperature is higher than the initial soil temperature. The overall degree of saturation was found to affect the amount of stored thermal energy and the performance of energy piles. The calibrated model was also applied to reinterpret the results from a geotechnical centrifuge study on the axial capacity of energy piles. Heating was found to lead to an increase in effective stress at the soil-pile interface, which was helpful in explaining the increase in axial capacity of the energy piles with increased temperature.
An important issue identified in the impact of unsaturated conditions on the behavior of energy piles is the role of matric suction on the yield stress encountered during mechanical loading of piles. An evaluation of compression results on unsaturated soils from the literature indicated that most studies were performed on soils in compacted conditions, which is not relevant to the behavior of unsaturated soils surrounding energy piles in the field. Energy piles are typically installed in naturally sedimented soil deposits that may have a very different soil structure from compacted soils. Accordingly, an experimental study was performed on sedimented Bonny silt that was then desaturated under different suction magnitudes then compressed to high stresses to infer the relationship between suction and yield stress. Sedimented Bonny silt was found to have a greater yield stress in general than compacted Bonny silt. Sedimented soil was also found to have a higher air entry suction and lower changes in volume of void than the compacted soil due to the dispersed soil structure. The results confirm that the soil structure associated with the formation of soil specimens has important role in the soil compression behavior, which can have significant impacts on the mechanical simulation of energy piles in unsaturated soils
Impacts of Unsaturated Conditions on The Ultimate Axial Capacity of Energy Piles
No full textThis study uses concepts from unsaturated soil mechanics to explain changes in axial capacity observed in geotechnical centrifuge experiments on semi-floating energy piles in unsaturated silt heated monotonically to different temperatures. Thermally-induced drying of the unsaturated silt surrounding energy piles was observed during heating using temperature-corrected dielectric sensor readings. An effective stress-based equation for estimating the ultimate capacity was calibrated using the load-settlement curves for a pile at room-temperature, which was then used to estimate the ultimate capacities of energy piles under elevated temperatures using measured changes in degree of saturation near the energy pile. The predicted capacity matched well with the capacity from the experimental load-settlement curves, confirming the relevance of the effective stress principle in unsaturated soils in nonisothermal conditions and the importance of considering coupled heat transfer and water flow in unsaturated soils surrounding energy piles
Simulation of the thermo-hydraulic response of energy piles in unsaturated soils
No full textThis paper focuses on the simulation of the coupled heat transfer and water flow in unsaturated soil layers surrounding a solitary energy pile undergoing heating and cooling cycles typical of a field-scale energy pile. The results indicate that heating leads to drying of the soil surrounding the energy pile, which has been shown in previous studies to result in an increase in axial capacity. During cooling, the degree of saturation was observed to recover to the value present before the start of heating initially, however, it will not recover in the following years. Which will lead to a cumulative effect after several cycles of heating and cooling. Heating and cooling cycles lead to an overall reduction in the thermal conductivity of the subsurface, reducing the heat transfer from the energy pile but also leading to greater storage of heat in the subsurface surrounding the pile
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The role of sovereign wealth funds in the global capital markets
No full textThis thesis examines the topic of sovereign wealth fund (SWF) sustainability and its
relationship with a country’s strategic resources. When the relevant literature on SWFs is
reviewed, it is found that these topics are seldom examined. There are studies that focus on the
financial performance of SWFs, but the sustainability of SWFs did not attract enough attention
in the academic and business circles. This thesis fills this conceptual gap by developing
measures of SWF sustainability and country-level strategic resources and then connecting them
in a novel conceptual model of SWF sustainability. Towards this aim, three sustainability
measures are identified. These measures are investments in alternative asset classes, the
employment of external fund managers, and spending on social and environmental projects and
causes. When these measures are reviewed through different theories, it is found that the
resource-based theory can justify the employment of external fund managers, while the natural
resource-based theory justifies the spending on social and environmental projects and causes,
and the portfolio diversification theory can be used to justify the investments in alternative
asset classes. The resource-based theory is also used in establishing the new conceptual model
of SWF sustainability. This theory argues that the resources and capabilities that companies
possess matter for their competitive advantage and sustainability. However, this theory does
not take country-level factors into account. In this context, the thesis extends the resourcebased theory to the SWFs domain and incorporate the country-level strategic resources into the
new model. So, this model of SWF sustainability postulates that countries’ strategic resources
would positively affect their SWFs’ sustainability.
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To test the relevant research hypotheses of the SWF sustainability model, the thesis
uses firm-level data from 56 SWFs, covering the period of 2007-2017. There are four
independent variables (i.e., the human development index, the innovation index, the reputation
index, and FDI inflows) representing the country-level strategic resources and three dependent
variables (i.e., the dummy for alternative asset classes, the dummy for employing external fund
managers, and the dummy for social and environmental expenditures). The relationships
between these dependent and independent variables are examined quantitatively using
descriptive statistics, pairwise correlations, scatter plots, t-tests, and finally logistic and ordered
logistic regressions. The results from different quantitative methods usually support each other.
Overall, these quantitative results show that there is strong empirical support for the first two
dimensions of SWF sustainability, which are investing in alternative asset classes and
employing external fund managers. However, empirical evidence for the third SWFs dimension
is relatively weak
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Hydromechanical behavior of unsaturated soils: Interpretation of compression curves in terms of effective stress
No full textThis state-of-the-art paper on the hydromechanical behavior of unsaturated soils focuses on the interpretation of the compression curves of unsaturated soils in terms of effective stress, with the goal of understanding the relative impacts of suction on the effective stress, net yield stress, effective yield stress and slope of the virgin compression line (VCL) during a monotonic increase in net stress. A database of compression curves was compiled for both high and low plasticity fine-grained soils under a wide range of suctions, isotropic or oedometric stress states, drainage conditions (constant suction or constant water content) and preparation techniques (impact compaction, static compaction, consolidation from slurry). Most of the compression curves plotted in terms of effective stress revealed a consistent hardening response with increasing suction and a slight suction dependency on the slope VCL. Interpretation of the compression curves in terms of effective stress led to load-collapse curves with a similar shape for a wide range of soils. Most soils evaluated had a greater rate of increase in effective yield stress with suction than the rate of increase in suction stress with suction, implying that these compacted soils may be susceptible to collapse upon wetting. Inconsistent trends were observed in some studies, which were attributed partially to natural variability but also experimental issues and limitations on the range of conditions investigated. Accordingly, recommendations are provided for future studies on the compression curves of unsaturated soils to ensure that results can be clearly interpreted in terms of effective stress
