Visual experimental study on the coupling process of seepage-erosion in rough fracture and the evolution of permeability

The karst areas are widely distributed in southwest China where have abundant water resources. There will be the main battlefield of water resources development in China. It is very important to exploring the basic law of the coupling process of seepage and erosion and its influence on the permeability of the rock mass, to ensure the rationality of the site selection, the safety of the construction and the long-term operation of these hydraulic engineering facilities in karst areas. For this purpose, we designed a self-designed flow visualization experiment device with a soluble transparent rough fracture. The erosion experiments in rough fractures are carried out to investigate the influence of the flow rate on the erosion morphologies, the erosion rate, the transitions of erosion patterns and the evolutions of permeability of the fractures. The results show that with the increase of flow rate, the erosion patterns shift from compact to wormhole, and finally to uniform patterns. Based on the penetration theory, an efficient Da number Daeff,L is defined. Then, the criteria for the transition of erosion patterns from compact to wormhole and from wormhole to uniform are defined as Daeff,L≈10 and Daeff,L≈1 respectively. The criteria are validated by our experimental results. The coupling effect of seepage and erosion increased the fracture permeability, 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. And 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 nature and engineering applications.