| Abstract | This thesis is concerned with the current contentious issue of environmental containment.
This work provides an assessment of contaminant migration through Geosynthetic Clay
Liners (GCLs). GCLs are thin liners usually consisting of three layers, defined in section
2.2.4. These are factory manufactured materials, used extensively in the waterproofing and
environmental containment markets. The predominant, if not the only "low flow" element
in GCLs is bentonite, described in section 2.4.1. Bentonite has been found to be most
advantageous as the waterproofing core of GCLs. During this research, emphasis was
placed on an investigation as to the nature of the low flow core of the GCLs in terms of
mineralogy and microstructure. GCL types can be divided into two generic categories;
granulated/powdered and extruded (pre-hydrated). One of each of the generic types were
selected for this study to provide a representative overview of the GCL product range.
The main type of laboratory assessment was that of fluid migration through the liners. Due
to the paucity of knowledge in relation to diffusion, emphasis was put on diffusion
assessment. At an early stage in this research, it was found that suitable equipment for the
assessment of diffusion was not available and, as a result the author developed two main
types of diffusion test cells which were key to this work. These cells were the constant
stress and constant volume diffusion cells. In the author's opinion, the constant stress cell,
in particular, offers many advantages over current diffusion equipment. During the
production of these cells, emphasis was placed on the following factors; sample size,
control of effective stress, mixing of fluid, reduction in sample and fluid contamination,
reservoir size, sample boundary, reduction of leaks and ease of sampling.
Following the development of the diffusion cells, extensive testing was carried out. Using
solutions of sodium, potassium and calcium chloride the assessment of contaminants was
selectively limited. A concentration gradient was created across the GCL sample by
placing a cation solution on one side and deionised water on the other. Following hydration
of the GCL samples with deionised water, 1000ppm of prepared solution was placed in the
source reservoir whilst fresh deionised water was placed in the receptor reservoir. Samples
were taken at regular intervals, from both source and receptor reservoirs, and their
concentrations assessed. Profiles of source and receptor concentrations were plotted versus time. A tabulated Microsoft Excel spread sheet was used in the determination of diffusion
coefficient. This was achieved by incorporating equations from the concentration plots
with Ficks' law.
Extensive hydraulic conductivity tests carried out by the author on the GCLs were
compared with the diffusion tests. Standard triaxial, Rowe & large column hydraulic
conductivity tests were produced.
A large part of the investigation was the assessment of the characteristics of the
waterproofing core of the GCLs. A number of techniques were adopted for this
assessment. These included the following; X-ray Diffraction (XRD), X-Ray Fluorescence
(XRF) & Scanning Electron Microscope Analysis (SEM). These assessments provided an
overview as to the characteristics of the bentonite core. The SEM, in particular, was used
to assess the microstructure of the two types of GCL under investigation and was used to
determine if there was any variance in the particle frequency, type, shape and orientation.
The XRF and XRD equipment were used to make an assessment of the mineralogy of the
bentonite core. These procedures were conducted on a number of available bentonite
granules and assessed relative to that of the core product of the GCL under investigation.
Tests were also conducted on samples used in the diffusion experiments. Sections of the
samples were removed from the source, middle and receptor sides of the samples and taken
for analysis.
The discussion and concluding remarks of this thesis have highlighted a number of key
findings. It was noted that there is a marked difference between the granulated and
extruded (pre-hydrated) GCLs assessed. It was found that the extruded (pre-hydrated) GCL
exhibits an orientated microstructure. The bentonite particles were predominantly
orientated flat across the width of the GCL. The granulated bentonite, when hydrated, did
not exhibit any preferential orientation of bentonite particles. |
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