THESIS
2022
1 online resource (xix, 176 pages) : illustrations (some color)
Abstract
The objective of this research is to investigate potential triggers and failure mechanisms of flow
and sliding failures of tailings dams and loose fill slopes. This research consists of three components.
The first component is the investigation of triggers and failure mechanisms of tailings dams. Two
model tailings dams were constructed from gold tailings using the moist tamping technique. The
model tailings dams were accelerated using a geotechnical centrifuge and subjected to a rising
groundwater table using a viscous pore fluid until failure was triggered. Two triggers and failure
mechanisms were identified. In the first centrifuge test, a retrogressive flow slide failure was
triggered by large seepage forces at the toe of the slope. The large seepage forces caused sloughing
at the t...[
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The objective of this research is to investigate potential triggers and failure mechanisms of flow
and sliding failures of tailings dams and loose fill slopes. This research consists of three components.
The first component is the investigation of triggers and failure mechanisms of tailings dams. Two
model tailings dams were constructed from gold tailings using the moist tamping technique. The
model tailings dams were accelerated using a geotechnical centrifuge and subjected to a rising
groundwater table using a viscous pore fluid until failure was triggered. Two triggers and failure
mechanisms were identified. In the first centrifuge test, a retrogressive flow slide failure was
triggered by large seepage forces at the toe of the slope. The large seepage forces caused sloughing
at the toe, which ultimately resulted in a rapid retrogressive failure. The rapid rate of shearing
caused the generation of significant excess pore pressures within the toe region of the slope, with
sufficiently large positive excess pore pressures to trigger localised liquefaction. The localised
liquefaction resulted in the tailings at the toe of the slope not being able to support the tailings
upstream of the toe, which triggered the retrogressive failure. In the second centrifuge test a drained
slip failure triggered at least partially undrained shearing of the soil within the slope, which
transitioned into a slide-to-flow failure. The rate of shearing was reduced compared to the first test,
which resulted in smaller positive excess pore pressures being generated within the slope. The
positive excess pore pressures generated were smaller compared to the first test due to the slower
rate of shearing. The slide-to-flow failure was predicted using the finite element method. During
the numerical back-analysis of the slide-to-flow failure, similar computed excess pore pressures
were generated within the model slope when compared with the measured excess pore pressures.
Thus, the slope only experienced instability. This confirms that a drained slip failure triggered by
large seepage forces caused the slide-to-flow failure observed in the centrifuge test.
Buttresses constructed at the toe of the model tailings dams were illustrated to improve the stability
of tailings dams. However, if a buttress is too small, a slide-to-flow failure can still be triggered in
the slope. In addition, if a buttress extends too far upstream of the tailings dam, it can also trigger a
slide-to-flow failure, as a section of the buttress acts as a destabilising weight. A design method for
buttresses for tailings dams was developed. An optimum height of the buttress can be determined
by identifying the centre of rotation of the critical slip surface and designing the buttress to be
constructed on the downstream side of the centre of rotation.
In the final component of this study a numerical back-analysis of a previously conducted centrifuge
test on loose fill slopes was conducted. It was previously hypothesised that a drained surface failure
was triggered by large seepage forces. The drained surface failure triggered the generation of
significant positive excess pore pressures within the slope, triggering a degree of localised static liquefaction of the soil within the slope. The numerical back-analysis confirmed the hypothesised
triggering mechanism. During the strength reduction phase of the numerical back-analysis a drained
slip failure caused the generation of significant positive excess pore pressures within the slope.
Localised liquefaction was triggered resulting in the failure of the entire slope. Soil nails were also
illustrated to be able to prevent the static liquefaction failure of the loose sand fill slope. Soil nailing
can prevent the peak shear strength of the soil from being mobilised. If the peak shear strength is
not mobilised, strain softening cannot be triggered in the soil. Thus, soil nailing limits the excessive
strains required to trigger static liquefaction.
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