Geotechnical Centrifuge Modelling of Variably Saturated Flow at The Soil-Rock Interface

Abstract

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.