Visualization experiments study on coupling process of flow-erosion in rough fractures and evolution of permeability

The karst areas are widely distributed in Southwest China where there are abundant water resources. They will be the main battlefield of development of water resources in China. It is very important to explore the basic law of the coupling process of flow and erosion and its influences on the permeability of the rock mass so as to ensure the rationality of the site selection, the safety of the construction and the long-term operation of these hydraulic engineering facilities in the karst areas. For this purpose, a flow visualization experiment device with a soluble transparent rough fracture is designed. The erosion experiments in rough fractures are carried out to investigate the influences of the flow rate on the erosion morphologies, erosion rate, transition of erosion patterns and evolution of permeability of the fractures. The results show that with the increase of the flow rate, the erosion patterns shift from compact to wormhole, and finally to uniform patterns. Based on the penetration theory, an efficient Da number Da_{eff, L} is defined. Then, the criteria for the transition of erosion patterns from compact to wormhole and from wormhole to uniform are defined as Da_{eff, L}≈10 and Da_{eff, L}≈1 respectively. The criteria are validated by the experimental results. The coupling effects of flow and erosion increase the permeability of fracture, and their trends do not accord with the cubic law. With the same aperture enlargement, the permeability of wormhole erosion increases the most rapidly and is much greater than the cubic law. The required breakthrough volume of "wormhole" patterns is the smallest. This research can provide a theoretical and experimental basis for the prediction and evolution of permeability control of karst processes in natural and engineering applications.