Characterization of unsaturated flow in dual-porosity granular media.

摘要

A geological medium made up of uncemented coarse porous rock fragments may be described as a "dual-porosity granular medium" due to the presence of two types of pores; small pores within individual fragments and large pores between the fragments. Crushed stone found in heap leach piles, mine waste, backfills, rock drains, and engineered capillary barrier systems fit this description. Unsaturated flow in a dual-porosity granular medium will occur both through the fragments (matrix flow), and on the fragment surfaces (film flow). The relative influence of gravity and capillary forces on these two flow regimes will be largely different. Therefore, unsaturated flow in this type of media is expected to differ from the conventional concepts developed for application to single porosity systems.;A test column (30 cm diameter, 100 cm tall) was filled with crushed sandstone (∼1.5--3 cm diameter) as a dual-porosity granular media. Water entered the column from the top through a point source at a steady rate (8.0 ml/minute) and exited through nine equal-area sections at the bottom of the column. Inflow, outflow from each of the nine sections, weight of the column, temperature (ambient and inside the column), humidity (ambient and inside the column) and barometric pressure were measured at 2 minute intervals in four trials (2--40 days long) under variable conditions.;It was found that unsaturated flow in dual-porosity granular media is spatially non-uniform, and likely occurs in the form of narrowly focused discrete pathways. The resulting flow structure limits the matrix saturation of the rock fragments to well below 100%. The distribution of flow was observed to change spontaneously, without any apparent external perturbation. Furthermore, the flow distribution was observed to change in response to external perturbations (inflow interruption, relocation of the inlet, and reducing evaporative loss); however, the occurrence and magnitude of redistribution were not predictable.