UNSATURATED FLOW THROUGH A FRACTURE-MATRIX-NETWORK: LABORATORY EXPERIMENTS AT THE METER SCALE

Unsaturated flow through fractured rock remains a topic of considerable uncertainty. We designed a meter-scale laboratory experiment to explore how local processes (single-fracture and single-fracture-matrix) interact at the network scale. A thin, uncemented wall of porous bricks was constructed within a two-dimensional load frame. In a first experiment, water, chemically equilibrated with the bricks, was supplied to a fracture in the middle of the top of the initially dry system, and subsequent system behavior was followed photographically over a 71 day period. Processes acting within the fracture network placed a strong control on the temporal development of flow pathways, demonstrating the schizophrenic roles of fractures as both flow conductors and capillary barriers. Pathways formed primarily within the fractures, with minimal lateral matrix interaction across surrounding non-flowing fractures. Multiple pathways were also found to form over time. Finally, evaporation and the associated chemical precipitation led to a constraining of the flow field, where some pathways were strengthened and others starved of fluid.

A second more sophisticated experiment minimized the influence of external perturbations, principally evaporation, and included sensor arrays to measure pressure, temperature, and individual fracture outflow. During wetting, the system once again showed the critical influence of fractures. Over time, the wetted structure widened with depth, as might be simulated using standard single or dual permeability models; however, fracture outflow showed erratic temporal behavior throughout the course of the 15 month experiment. These results suggest flow processes within the fracture-matrix network, along with feedback from very small external forcing, to generate a time-local 'unpredictable' behavior that is not simply random. Instead, analysis of the outflow time series shows similarities with that found in systems where self organized criticality induces a response that has been defined mathematically as 'complex'. Taken in their entirety, the results from these experiments suggest unsaturated flow through fractured rock can exhibit behavior at all temporal scales, and that such behavior is intrinsically unstable to perturbations.