Partially saturated flow from sand into a discrete smooth open vertical fracture at the soil–rock interface: experimental studies

The two proposed conceptual models explaining partially saturated flow from soil into fractured rock in the intermediate fractured vadose zone have not been confirmed due to the difficulty involved in observing the soil–rock interface in situ. To address that challenge, this paper presents a series of newly developed physical experiments using a geotechnical centrifuge model of sand overlying a single dry clean smooth vertical fracture. The model shows the development of a perched water system and a saturated wetting front that progresses transversely along the interface, while breaching through the interface occurs via multiple point sources. The dominant flow mechanisms within the fracture comprise droplets, discontinuous rivulets with droplet formation, and continuous rivulets. A maximum drainage area of 30% of the width of the fracture contributes to the flow in the model, and this drainage area decreases with depth due to the merging of oscillating rivulets in the upper regions of the fracture. The presence of evidence supporting both conceptual models shows that a combined conceptual model is required to accurately explain partially saturated flow at the soil–rock interface.