Effect of Wall Roughness and Concentration of Clay on Erosion in the Hole Erosion Test

Problem statement: Internal soil erosion is a real threat for hydraulic infrastructures. In its final stage it develops in piping involving the formation and progression of a continuous void inside the soil between the upstream and downstream sides. The hole erosion test was introduced to characterize kinematics of piping in terms of the time left to rupture. Actual modeling approaches of this test are essentially one dimensional. The wall shear stress generated by the flow is assumed to be uniform, so that erosion rate is also uniform along the hole length. Experimental observations show however an irregular profile of the eroded hole. Approach: In this study an axisymmetric extension representation of the hole erosion test was performed. The biphasic flow at the origin of surface erosion occurring in the porous soil sample was modeled by means of the renormalization group based k-ε turbulence equations. Fluent software package was used to perform the numerical modeling. Results: This had enabled to estimate the wall shear stress which was found to be non uniform along the hole length. Erosion rate was then estimated by using a classical law. Its variations as affected by the applied gradient pressure, fluid density as well as the actual fluid/soil interface roughness were analyzed. In particular, wall roughness and clay concentration were found to increase noticeably the erosion rate. Conclusion/Recommendations: Predicting erosion rate at the start of piping formation can be done by the proposed model. Flow features are however very complex in the real HET configuration. In particular, clay concentration does not vary equally along the hole length. The erosion law coefficients are variable. Transport phenomenon of some soils particles that detach is present in the problem. Further investigations including these aspects should be performed in order to render more profoundly the complex physics involved in this experiment.