Effects of Hydraulic Connections on Unsaturated Flow in Dual-Permeability Media

Field: Environmental Science (Physical Sciences)

A geological medium that exhibits two distinct types of flow is known as dual-permeability medium. Unconsolidated deposits composed of coarse (> 2 mm diameter) clasts (gravel, talus, rockslide debris), engineered systems (heap leach piles, capillary barriers, rock fill), and mine/construction waste fall into this category. The large inter-clast pores that are characteristic of this type of media will drain at near zero matric potentials constraining flow to the interiors of porous clasts and/or the clast surfaces. In either case, water must pass through hydraulic bridges (porous contacts and/or pendular water) that form physical connections between neighboring clasts. Therefore, properties of hydraulic connections place a primary control on flow structure. This dissertation presents three projects designed to study the influence of hydraulic connections on unsaturated flow in dual-permeability media. A numerical experiment performed to examine how the cross-sectional area and hydraulic conductivity of a bridge influence steady-state flow through a spherical clast is presented in the second chapter of the dissertation. The cross-sectional area of the bridges relative to that of the clast (Ar) was varied across six orders of magnitude between simulations. The ratio of hydraulic conductivity between bridges and clasts (Kb/Kc) was varied across 12 orders of magnitude to consider resistive, neutral, and conductive bridges. Results show a non-linear dependency of volumetric flow through the clast on both Ar and Kb/Kc. The intra-clast flow distribution shifts outwards as Ar increases. Conductive bridges promote this process and resistive bridges impede it. The third chapter presents a series of bench-scale experiments performed to evaluate the geometry of a pendular bridge under different flow rates through it and at different inclinations. Results show that bridge size increases in a nonlinear fashion with flow rate and decreases with inclination from vertical. The vertical profile of the bridge closely resembled a parabola in all experiments, in contrast to the profile of a static bridge that resembles the arc of a circle. Bridge geometry was independent of flow history. Flow is active through the entire volume of the bridge and exhibits non-laminar characteristics. The fourth chapter describes a series of bench-scale experiments designed to explore the influence of matrix-to-matrix hydraulic connections on two-dimensional transient wetting of a porous matrix. Cross-sectional area of the connection (Ar) relative to that of the matrix block and location of the connection relative to edges of the block were varied between trials. Results show that the rate of imbibition into the porous block nonlinearly increases with Ar. Moving the connection towards an edge of the block significantly decreases the imbibition rate. Saturation increase of the matrix block before the wetting front reaches all edges remains consistent independent of the connection. The last chapter summarizes results of the research and discusses about future research on this topic.