Department of Civil Engineering Graduate Theses
http://hdl.handle.net/1974/791
2019-10-15T14:51:12ZHeat Migration and Solute Transport in a Discrete Fracture
http://hdl.handle.net/1974/26704
Heat Migration and Solute Transport in a Discrete Fracture
Bou Jaoude, Issam
The study of heat and solute transport in fractured rock can provide complementary information in aid of understanding the interaction between surface and groundwater, the long-term isolation of energy by products, the application of renewable energy storage systems, and the treatment of contaminated sites. Employing numerical modeling, this research was undertaken to assess the most influential factors controlling heat migration in discretely fractured rock under natural groundwater flow conditions, to address the effect of fracture aperture variability on the spatial distribution of a migrating thermal front, and to compare heat and solute transport mechanisms. Using factorial analyses, it is shown that the most influential factor controlling heat propagation in a single fracture setting is the velocity of the fluid in the fracture. The combination of effects of the thermal conductivity of the matrix with the velocity of the fluid, and of the thermal conductivity of the matrix with the aperture of the fracture dominantly control the attenuation of the thermal front migration. By integrating variable aperture fields with contact points, it is demonstrated that the effect of aperture variability on the spatial distribution of the thermal front is defined mainly by the thermal conductivity of the rock matrix. The effect of groundwater flow channeling on the spatial distribution of the thermal front is small, contrary to solute transport in a discrete fracture setting, where channeling is sometimes a major contributor to widespread solute migration rates and directions. The thermal plume in the fracture does not reach equilibrium over the 3-year period of the simulation in contrast to the solute plume that reaches steady state in less than ten days, mainly due to thermal conduction in the matrix which remains in disequilibrium. Two-dimensional conduction in the plane of the fracture and three-dimensional conduction in the matrix are important factors to consider when assessing the thermal plume in contrast to solute transport, whereas one-dimensional diffusion in the matrix and two-dimensional dispersion in the fracture are good assumptions.
Probabilistic Analysis of Unreinforced Slopes and Reinforced Footings Using the Random Finite Element Method
http://hdl.handle.net/1974/26700
Probabilistic Analysis of Unreinforced Slopes and Reinforced Footings Using the Random Finite Element Method
Shafiee, Navid
The focus of this thesis was on probabilistic analysis of unreinforced cohesive soil slopes and geosynthetic reinforced and unreinforced footings over cohesive foundations.
Two open-source random finite element method (RFEM) programs written in FORTRAN were modified to accommodate larger problem domains analysed in this study. For footing analyses, subroutines were added to model geosynthetic reinforcement layers as linear elastic-plastic bar elements together with two-layer random fields.
The modified RFEM code for the foundation problem was used to examine the case of the footing sitting directly on a cohesive soil layer, and the same footing seated on unreinforced and geosynthetic reinforced granular layers. Simulations were carried out assuming different isotropic and anisotropic spatial correlation lengths for the undrained shear strength of the foundation soil. The influence of reinforced and unreinforced granular layers on worst-case spatial correlation lengths was identified. The footing investigation showed that introduction of geosynthetic layers in the granular layer placed between the footing and underlying cohesive soil foundation significantly reduced the probability of footing failure. The correlation lengths for isotropic and anisotropic spatial variability of the foundation undrained shear strength that were at or close to the width of the footing corresponded to worst-case scenarios with respect to probability of failure regardless of the number of reinforcement layers and reinforcement stiffness.
The modified slope stability code was used to investigate the sensitivity of RFEM analysis outcomes to numerical parameters used to describe the random fields. The results of RFEM analyses were compared to results using the random limit equilibrium method (RLEM) for the same cohesive slope problem. The conditions that are necessary for the two methods to achieve practical agreement with respect to probability of failure and worst-case correlation lengths were identified. The slope investigation showed that critical correlation lengths deduced from probabilistic analyses using RFEM are sensitive to the precision of variables in numerical computations and to the ratio of random field cell size to finite element mesh size. The probabilities of failure computed using RFEM can be replicated with RLEM using Janbu method of slices and with less computational time.
Numerical Modelling of Methane Emissions From Peatlands
http://hdl.handle.net/1974/26629
Numerical Modelling of Methane Emissions From Peatlands
Wisheart, Caroline
Peatlands could play a significant role in global warming for two reasons: 1) peatlands release methane (CH4), which is a very potent greenhouse gas estimated to be responsible for a quarter of the temperature rise the world has experienced to date, and 2) peatlands are a natural source of atmospheric greenhouse gases, which means that unlike anthropogenic sources, they cannot be controlled through regulations or sustainable policies. Because emissions from peatlands are inevitable, it is paramount that they are quantified, and accounted for in the climate models used to determine emission reduction targets. Quantifying CH4 releases from peatlands is challenging because peatlands are highly variable ecosystems, and as a result they generate highly variable emissions. CH4 gets released from peatlands by diffusion in the aqueous phase, or by ebullition in the gas phase. Since diffusion is a constant process, it is more easily measured in the field, and therefore most previous emission estimates have been based solely on diffusion. However, it is now believed that ebullitive emissions make up the majority of methane emissions from peatlands.
Ebullitive releases are poorly understood and are therefore challenging to predict. In order to better understand peatland CH4 emissions, a numerical model of CH4 flux from the water table in peatlands was developed. The model is based on physical processes and simulates diffusion, ebullition and gas-water mass transfer, and is populated by parameters measured in peat. The model was used to conduct a sensitivity analysis on the effect that different conceptual models of the peat profile have on emissions. It was found that changing the physical parameters of peat, such as its degree of decomposition or heterogeneity, did not affect the total flux from the water table, but did affect the magnitude and distribution of ebullition events. This is an important finding because large release events may bypass consumption above the water table and result in considerably more CH4 reaching the atmosphere than previously anticipated. It was also found that if only CH4 was modelled, and the carbon dioxide that is also present in peat was ignored, the outputs were comparable to reducing the CH4 generation rate by an order of magnitude.
Simulating solute transport in discretely fractured rocks: Computational limitations and a hybrid method
http://hdl.handle.net/1974/26613
Simulating solute transport in discretely fractured rocks: Computational limitations and a hybrid method
Campbell, Wesley
To investigate the computational limitations arising when simulating large-scale groundwater flow
and solute transport using a 3D control volume finite element model (the FE model) in discrete-fracture
mode, numerical simulations were matched to a semi-analytical solution for radial divergent
solute transport in a single fracture. Discretization parameters and computation times were
compared for two sets of simulations. Depending on the model parameters, transport simulations
in the FE model can take longer to compute with parallel versus serial processing. The transport solution
was the largest limiting factor with respect to computational cost for our models. Transport
computations, for example, took 86-98% of the total computation time, and the relative flow and
transport times changed little with increasing scale. Next, an efficient hybrid numerical-analytical
method (the NA method) for simulating solute transport in 2D fracture networks is proposed. The
NA method can be used to compute solute breakthrough curves in a 2D network of fractures of
varying aperture and fluid velocity by applying the Tang et al. (1981) solution to an equivalent fracture
pathway (EFP) which is generated by a numerical model. The NA method was verified at
three different transport scales (i.e., 10m, 50m and 250m) using a range of realistic fracture apertures,
matrix porosities and hydraulic gradients, by comparing simulated breakthrough curves to
those generated by the FE model. Solute dilution can be accommodated by scaling a breakthrough
curve using discharge ratios in the fracture segments before and after dilution. The validity of
this method depends on the ratio of the Péclet number and the matrix porosity, which can serve
as an indication of the cases in which the NA method will be valid for a given fracture spacing
and transport distance. A 2k factorial analysis was also conducted which shows that increasing the
number of fracture intersections diminishes the ability of the NA method to match the FE model.
Simulations of realistic 2D fracture networks however show that in natural fractured rock systems,
the NA method could be a useful tool to simulate solute transport at an up to 85% reduced
computational cost.