Geotechnical Centrifuge Modelling of Variably Saturated Flow at The Soil-Rock Interface
The underlying mechanisms governing unsaturated flow from soil into fractured rock in the intermediate fractured vadose zone is still poorly understood despite occurring in numerous areas of application. To gain a better understanding of these mechanisms, a series of physical experiments using a...
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Language: | en |
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University of Pretoria
2017
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Online Access: | http://hdl.handle.net/2263/63229 Brouwers, LB 2017, Geotechnical Centrifuge Modelling of Variably Saturated Flow at The Soil-Rock Interface, MSc Dissertation, University of Pretoria, Pretoria, viewed yymmdd <http://hdl.handle.net/2263/63229> |
Summary: | The underlying mechanisms governing unsaturated flow from soil into fractured rock in
the intermediate fractured vadose zone is still poorly understood despite occurring in
numerous areas of application. To gain a better understanding of these mechanisms, a
series of physical experiments using a geotechnical centrifuge are performed. The
centrifuge model comprises of two Perspex® sheets bent to form a clean smooth parallel
1mm aperture single discrete fracture that is inclined 90º, 75º and 60º from horizontal,
where dry sand is placed on top and water is supplied as continuous and intermittent
influx styles. The presence of the interface shows the development of perched water
system and saturated wetting front along the interface, which supports the capillarydominant
conceptual models for the fractured vadose zone. Breaching through the
interface occurred from preferential feeding pathways in the soil as multiple point
sources in the fracture and flow regimes composing of droplets, tendrils with droplet
formation and numerous types of rivulets indicating that fluxes within the fracture
range between 1 x 10-8 < Q < 1 x 10-4 m3/s per m. Changing the influx styles did not
alter the flow mechanisms occurring within the fracture. However, intermittent influx
did provide larger saturated wetting fronts along the interface while continuous influx
promotes flow instability within the fracture. Altering the fracture inclination influenced
the dominant flow mechanisms within the fracture but full saturation is never achieved
but rather only a potential 5 - 30% cross-sectional area contributes to flow, which
further decreases with depth due to merging of rivulets in upper regions of the fracture.
Although the results from the geotechnical centrifuge model may not be scaled to
prototype conditions due to similitude not being achieved between the Capillary and
Bond numbers, observations of breaching of the soil-rock interface, flow mechanisms
and flow instabilities within the fracture are similar in both the 1g and 20g experiments
indicating gravity driven flow instability maintains similitude and the geotechnical
centrifuge model offers a representative indication of natural conditions. The
contrasting support for both conceptual models regardless of fracture inclination
indicates that variably saturated flow at the soil-rock interface is a combination of the
two current conceptual models, with capillary flow dominating in soil material and the
dominant flow regime present within the fracture dependant on the interaction of
interfacial capillarity, gravitational and viscous forces. Ultimately the improved
conceptualisation and understanding gained from these experiments will benefit
hydrogeological as well as geotechnical areas of application such as hillslope hydrology,
contaminant transport, groundwater recharge, slope stability, differential settlement,
waste disposal, rock mass stress distributions, grouting and seepage into excavations. === Dissertation (MSc)--University of Pretoria, 2017. === Water Research Commission === Geology === MSc === Unrestricted |
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