Micro-scale flow and induced contact erosion in granular media

Abstract

Internal erosion is the loosening, detachment and transport of fine particles through the pore structure of a coarse material due to a seepage flow. Once erosion is initiated and fine particles are not blocked within the coarse material, particles can be washed out leading to larger scale deformations. This phenomenon can be frequently seen in water retaining structures like dams, levees and embankments. This kind of hydraulically induced erosion processes caused enormous damage to infrastructure and buildings as well as loss of human lives. Prevention of these catastrophic events includes a better understanding of the initiation of this process. Although an extensive number of studies have been carried out on this topic, the hydro-mechanical mechanisms of the onset of erosion at micro scale are still poorly understood. In the presented thesis, contact erosion induced by hydraulic flow perpendicular to the interface was studied. Base material can be dislodged and transported into the filter material due to upward water flow. The study focuses on the hydraulic changes happening at the interface between fine and coarse layer and the conditions leading to first detachment of fine particles. In the sequence of processes leading to erosion and as consequence the failure of the structure, it is the onset of erosion indicating the initiation of the process, which is of interest in this research. Initiation of contact erosion process takes place locally at the pore scale and therefore, measurements of the pore scale parameters are necessary. The major difficulty of a micro-scale experiment is the optical opaque nature of real samples that do not allow observations inside of samples. Due to this limitation, most of the studies have been carried out based on the observations at the wall of the test cell, measurements of the outflow turbidity, settlement of the sample at the wall of the sample cell. To overcome this problem, transparent soil has been introduced for our lab-scaled experiments using Refractive Index Matched (RIM) media technique. RIM medium was obtained by using the same refractive index for both fluid phase and solid phase. Hydro-gel beads were used for the soil matrix, and pure water was used for the fluid phase which creates a transparent medium. Visualization of the local flow field inside the transparent soil was done by Particle Image Velocimetry (PIV) technique. Rather than using an expensive laser systems, Light Emitting Diode (LED) based light sheet was used for the illumination. This developed experimental apparatus was first used to identify the porous flow characteristics inside a mono-dispersed packing. Results were successfully compared with the available literature data on porous flow studies. Formation of preferential flow channels, flow characteristics for different pore geometries, pore scale measurements for the deviation of laminar flow were identified. Finally, experimental results for the layered porous medium was used to validate a numerical model which was developed using an existing code to simulate contact erosion models as same as in the experimental conditions. Discrete Element Method (DEM) was used to model the solid phase while Lattice Boltzmann Method (LBM) was used for the fluid phase. First, a simple fluidized bed problem was developed and identified the mechanism for particle dislodgement and transportation due to an applied upward flow. Then contact erosion was simulated for several particle size combinations where erosion is geometrically possible. Local flow behaviour of different pore shapes at the contact zone was studied and their influence for the contact erosion was quantified. Main outcomes of the study were presented through three journal papers and one conference paper which form the core of this thesis. Key findings can be summarized as: 1. Development of an experimental method for measuring pore scale flow characteristics inside the porous medium 2. Identification of preferential flow channels and flow characteristics for different pore geometries 3. Modification of an existing numerical code for contact erosion simulations and validation of it through experimental results 4. Quantification of hydro-geometrical influence for onset of contact erosion