Air permeability and trapped‐air content in two soils

Abstract

To improve understanding of hysteretic air permeability relations, a need exists for data on the water content dependence of air permeability, matric pressure, and air trapping (especially for wetting‐drying cycles). To obtain these data, a special instrument was designed. The instrument is a combination of a gas permeameter (for air permeability determination), a suction plate apparatus (for retentivity curve determination), and an air pycnometer (for trapped‐air‐volume determination). This design allowed values of air permeability, matric pressure, and air trapping to be codetermined, i.e., determined at the same values of water content using the same sample and the same inflow‐outflow boundaries. Such data were obtained for two nonswelling soils. The validity of the air permeability determinations was repeatedly confirmed by rigorous tests of Darcy's law. During initial drying from complete water saturation, supplementary measurements were made to assess the magnitude of gas slip. The extended Darcy equation accurately described the measured flux gradient relations for each condition of absolute gas pressure tested. Air permeability functions exhibited zero‐permeability regions at high water contents as well as an abruptly appearing hysteresis at low water contents. Measurements in the zero‐permeability regions revealed that the total amount of air in general exceeded the amount of trapped air. This indicates that the medium' s air space is partitioned into three measurable domains: through‐flowing air, locally accessible air (i.e., air accessible from only one flow boundary), and trapped air. During repeated wetting and drying, the disappearance and reappearance of air permeability coincided closely with the reappearance and disappearance, respectively, of trapped air. The observed relation between critical features of the air permeability functions and those of the air‐trapping functions suggest that water‐based blockages play a significant role in the disruption of gas‐phase connectivity and in preventing air flow, and must be considered in any effectual model of air permeability relations.

A comparison of relative nonwetting‐phase permeability models

“…The first data set was obtained for Oakley sand [Stonestrom and Rubin, 1989]. The second data set for a quartz sand mixture was taken from Dury et al [1998].…”

“…Following the terminology of Stonestrom and Rubin [1989], we denote the saturations at which permeability disappears and reappears as extinction and emergence points, respectively. These points delimit, in other words, a range of saturation (for a given hysteresis branch of the phase saturation characteristics) in which both phases �re continuous from adjacent ranges in which the medium is impermeable for one phase [White et al, 1972; Luckner et al, 1989;Stonestrom and Rubin, 1989].…”

A falling-pressure method for measuring air permeability of asphalt in laboratory

“…76 -78;Stonestrom and Rubin, 1989) [L] along the one-dimensional air flow in the sample (z ¼ 0 at the interface of the sample and the sample-confined air chamber, z ¼ Z at the upper surface of the sample, see Fig. 1), t is time [T].…”

“…The common method for determining the air permeability of unsaturated rock, soil or other materials is to measure the steady-state air flux through the soil sample (e.g. Stonestrom and Rubin, 1989;Springer et al, 1998). Although the data analysis of this method is simple, the experiment apparatus is complex and expensive in order to maintain and measure the steady-state air flux.…”

A Fractal Model for Predicting Water and Air Permeabilities of Unsaturated Fractured Rocks

“…In contrast, when S w approximates to 1, the air phase becomes disconnected and no air flow can be detected (Kaviany 1995). These facts limit the "effective" water saturation to a narrower range, which can be defined as 0 < S w,e ≤ S w < 1 for water flow and 0 < S w ≤ S a,e < 1 for air flow, where S w,e and S a,e are the emergence points for water and air flows, respectively (Stonestrom and Rubin 1989;Dury et al 1999). The emergence point of water flow S w,e must be addressed as the value of water saturation above which water phase becomes connected and water flow takes place.…”