Water flow between soil aggregates

Abstract

Aggregated soils are structured systems susceptible to non-uniform flow. The hydraulic properties depend on the aggregate fabric and the way the aggregates are assembled. We examined the hydraulic behavior of an aggregate packing. We focused on conditions when water mostly flows through the aggregates, leaving the inter-aggregate pore space air-filled. The aggregates were packed in 3 mm thick slabs forming a quasi two-dimensional bedding. The larger aggregates were wetted with water and embedded in smaller aggregates equilibrated at a lower water content. The water exchange between wet and drier aggregates was monitored by neutron radiography. The three-dimensional arrangement of the aggregates was reconstructed by neutron tomography. The water flow turned out to be controlled by the contacts between aggregates, bottle-necks that slow down the flow. The bottle-neck effect is due to the narrow flow cross section of the contacts. The water exchange was simulated by considering the contact area between aggregates as the key parameter. In order to match the observed water flow, the contact area must be reduced by one to two orders of magnitude relative to that obtained from image analysis. The narrowness of the contacts is due to air-filled voids within the contacts. A. Carminati et al.

Unsaturated water flow across soil aggregate contacts

“…At h ¼ À1:4 m the large pores between the aggregates and the aggregate fissures appear to be drained. From a previous study [19] we know that at h ¼ À1:4 m the aggregates are still relatively wet. This means that water is retained in the fine pores of the aggregate matrix and is not discernible at the used resolution.…”

“…The conductivity of the aggregates has been measured in previous experiments by matching simulated and observed water flow in aggregate packings [19] and [13]. The cross-sectional conductivity of liquid films has been determined by Tuller and Or [20].…”

“…where a o agg is the area of the solid contacts and h agg ðhÞ is the water retention curve of the aggregate matrix as measured in a preceding study [19].…”

Modeling Macroporous Soils with a Two-Phase Dual-Permeability Model

“…For example, in a three-dimensional volume of a structured soil, the matrix can be separated into soil blocks/aggregates and voids (macropores). When the soil saturation is low, the water flow mainly occurs between the aggregates and depends on the aggregate bridging (see Carminati et al 2007). In less structured soils, macropores (e.g., earthworm burrows and decayed plant roots) may penetrate the matrix.…”

Recent developments in neutron imaging with applications for porous media research

“…This makes neutrons an isotope-sensitive probe. Neutron imaging (both radiography and tomography) can quantify very small amounts of fluid, even when the pore space itself cannot be resolved, as demonstrated in several porous media studies (Schaap et al, 2008;Carminati et al, 2007;Robinson et al, 2008;Rudolph et al, 2012;Zarebanadkouki et al, 2014;Zhang et al, 2010;Lal et al, 2014). It is particularly relevant for investigations of water displacement in bulk samples where the detailed structure of the pore space is known, is unchanging, or is of lesser importance in understanding the fluid processes.…”