DEM-PFV numerical investigation of the spatial heterogeneity induced by suffusion in sands

The permeation of Newtonian fluids through sandy soils is a crucial phenomenon in geology, environmental sciences, civil and petroleum engineering. Hydraulic conductivity is a key parameter for modelling permeation and is often assumed to be constant over time and in space, by disregarding suffusion phenomena that cause an evolution of local both grain size distribution and void ratio, due to soil mass transport/deposition processes in the porous medium. In this paper, the problem is approached at the pore-scale, where the previously mentioned processes occur, by using the DEM-PFV coupled method (Discrete Element Method and Pore-scale Finite Volume). Although this method has been originally conceived for spherical particles, a strategy to account for particle angularity, without explicitly modelling the full geometric complexity of real sandy grains, is proposed and validated against experimental permeability test results. The induced spatial heterogeneity and the evolution of hydraulic conductivity due to suffusion processes are then analysed by simulating erodimetric tests. An interpretative simplified approach, leveraging a state variable based on sand particle size distribution, is also suggested to predict the inception of suffusion. This state variable seems to be very useful for interpreting experimental results by (i) predicting the geometry of the final heterogeneous configuration (in terms of distance between two successive clogging zones), (ii) estimating the amount of eroded fines, and (iii) capturing the hydraulic conductivity evolution during permeation processes potentially causing suffusion.