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[AGE30-P01] Water Retention in Model Soil Aggregates
Keywords:aggregated structure, water content, microscopic pore, macroscopic pore
Water retention in porous media has been studied in various fields. Porous media near the Earth surface sometimes have hierarchical structures. In terms of soil, tiny particles such as clay grains often form aggregated structures using organic substances as a glue to stick together. Several researches to reveal the relation between such structural factors and the water retention have been conducted from the viewpoint of water content and pressure head [1]. However, considering the rainwater permeating to soil, the amount of retained water is determined by the balance between the drainage rate and the suction rate. Also, since drying process of the retained water depends on initial water content, whole phases (from wetting to drying) should be taken into account in order to evaluate the actual water retention in soil. Here, we focus on the relation between this kind of water retention and hierarchical pore structure in this study.
Although it is qualitatively considered that the aggregated soil can efficiently retain more water compared with the simple soil (without aggregates), quantitative analysis of the retained water depending on pore size distribution in aggregated soil has not been understood well thus far. In order to focus on the effect of pore structure and neglect the complex effects such as advection and sedimentation due to clay and organic substances, some previous model experiments using spherical glass beads have been performed [2]. In other words, however, hierarchical structures in soil were not considered in these experiments because of the monodisperse structure of the glass beads.
Therefore, in this research, we conducted experiments of water retention using model soil consisting of aggregates. Concretely, we prepared the hierarchically structured soil by sintering (650°C, 1 hour) a cluster of tiny monomer glass beads and crushing it to form aggregated model soil. Aggregated particles of 100 g were poured in a vessel whose bottom consists of a sieve. Then, fixed amount of water (100 g) was sprinkled on its surface. The temporal variation of the weight of water and soil was measured throughout the experiment under the constant temperature condition (35°C). The dependence of the amount of retained water on two parameters: size of aggregates D (162, 545, 1420, 3380 μm) and monomer particle (original glass beads) size d (5, 18, 100, 400 μm) were analyzed. Identical measurements using glass beads without aggregation were also conducted.
From the measurement, the water drainage (dripping from the sieve) was observed only right after the initial water addition and the remained water was gradually dried by evaporation. Thus, we measured the initially retained water W0 at the moment when the water drainage stopped.
As a result, we found some characteristic features of water retention in aggregated soil. First, W0 in aggregated soil was larger than that of non-aggregated soil. Secondly, W0 decreases as d increases and becomes maximum around D =0.5mm. Namely, our result indicates that smaller d and D = 0.5mm is the best condition to maximize the initial water retention. It is considered that the aggregated soil can efficiently retain water not only in microscopic pores within each aggregated particle but also between the aggregates around the size of D = 0.5mm. Details on the relation between pore water content and whole amount of the retained water will be discussed in the presentation.
References
[1] Ken’ichirou KOSUGI (2007): Reviewing classical studies in soil physics. J. Jpn. Soc. Soil Phys. No.106, 47-60
[2] Kondo, J., A. Yanagihara and N. Saigusa (1993): An experimental study on evaporation parameters of soi1. Tenki, 40(12), 873-879
Although it is qualitatively considered that the aggregated soil can efficiently retain more water compared with the simple soil (without aggregates), quantitative analysis of the retained water depending on pore size distribution in aggregated soil has not been understood well thus far. In order to focus on the effect of pore structure and neglect the complex effects such as advection and sedimentation due to clay and organic substances, some previous model experiments using spherical glass beads have been performed [2]. In other words, however, hierarchical structures in soil were not considered in these experiments because of the monodisperse structure of the glass beads.
Therefore, in this research, we conducted experiments of water retention using model soil consisting of aggregates. Concretely, we prepared the hierarchically structured soil by sintering (650°C, 1 hour) a cluster of tiny monomer glass beads and crushing it to form aggregated model soil. Aggregated particles of 100 g were poured in a vessel whose bottom consists of a sieve. Then, fixed amount of water (100 g) was sprinkled on its surface. The temporal variation of the weight of water and soil was measured throughout the experiment under the constant temperature condition (35°C). The dependence of the amount of retained water on two parameters: size of aggregates D (162, 545, 1420, 3380 μm) and monomer particle (original glass beads) size d (5, 18, 100, 400 μm) were analyzed. Identical measurements using glass beads without aggregation were also conducted.
From the measurement, the water drainage (dripping from the sieve) was observed only right after the initial water addition and the remained water was gradually dried by evaporation. Thus, we measured the initially retained water W0 at the moment when the water drainage stopped.
As a result, we found some characteristic features of water retention in aggregated soil. First, W0 in aggregated soil was larger than that of non-aggregated soil. Secondly, W0 decreases as d increases and becomes maximum around D =0.5mm. Namely, our result indicates that smaller d and D = 0.5mm is the best condition to maximize the initial water retention. It is considered that the aggregated soil can efficiently retain water not only in microscopic pores within each aggregated particle but also between the aggregates around the size of D = 0.5mm. Details on the relation between pore water content and whole amount of the retained water will be discussed in the presentation.
References
[1] Ken’ichirou KOSUGI (2007): Reviewing classical studies in soil physics. J. Jpn. Soc. Soil Phys. No.106, 47-60
[2] Kondo, J., A. Yanagihara and N. Saigusa (1993): An experimental study on evaporation parameters of soi1. Tenki, 40(12), 873-879