真砂を使った浸透実験の一考察

Abstract

土壌の流出特性を定める指標として使われる, 円筒型浸透計による最終浸透能と鉛直降下浸透における透水係数の関係を実験的に比較検討した。供試土として, 粒径分布の比較的狭い標準砂と滋賀県田上山の裸地斜面のマサを使った。鉛直降下浸透の実験結果によれば, 浸透量の時間的変化はGreen-Amptの式によって与えられる関係をほぼ満足している。この式はその物理的意味が明確でないという欠点はあるが, この実験条件の範囲では, 浸透係数Kに対し, 不飽和非定常の初期浸潤段階における係数K_I, ほぼ定常的な段階に達したときの係数K_P (最終浸透能に対応する) 及び飽和状態における係数K_o (Darcyの透水係数) はそれぞれ異なった値をとる一定値として取扱って考察を進めた。この結果, 標準砂の場合は, K_IとK_Pはほぼ近い値であり, さらにK_PはK_oとも一致した。つぎにマサの場合は, K_I ≈ K_Pではあったが, K_IはK_oのほぼ4倍程度であった。また, 毛管前進力には初期含水率の影響は, 明確にはあらわれなかった。一方, 円筒型浸透計を想定した, 水平方向にも土壌水が移動する場合の最終浸透能をK_Iと較べると, 標準砂の場合4倍程度であり, マサの場合2倍程度であった。結局, マサにおける最終浸透能はK_oの10倍以内であった。

The relations between the infiltration capacity measured by Musgrave-type infiltrometer and the hydraulic conductivity of vertical infiltration into uniform soil column were clarified experimentally by means of two kinds of apparatus. Firstly, the infiltration of water into vertical soil columns were examined. The columns had 14.4cm in diameter, and 120cm in height, and the materials were packed uniformly in transparent cylinder under dry or slightly damp conditions. The top of soil column was maintained on overflown stage with constant water depth throughout the experiment. In the infiltration stage (from the begginning to the time when the wetting front reaches down to the bottom of soil column), the rate of water entry and the depth of wetting zone were measured. From these data, the rate of water entry (q) was plotted as a linear function of the depth of infiltration zone (y). Threrfore, the infiltration low of Green and Ampt is passably applicable for these experimental conditions. Then the rate of water entry (q) is given as following equation. [Figure omitted] where, ho: initial potential of wayer at the entrance of soil (cm) hk: capillary or moisture potential at the front of wetting zone (cm) hw: loss of pressure potential in wetting zone (cm) KI : hydraulic conductivity of infiltration stage (cm/sec) The experimental result shows that the values of ho, hk, hw and KI are maintained as constant values throughout the infiltration stage. In the percolation stage (after the time when the wetting zone reached to the bottom of soil column, and the flow of water percolating into soil becomes to almost steady), the rate of water flow was measured. Then the hydraulic conductivity of percolation stage (Kp) was decided. Secondarily, the model test corresponded with the infiltration from Musgrave-type infilrometer was executed, in the box of 120cm in height, 120cm in and 60cm in length. The front surface of the box is made by transparent plate, and the model is constituted as the symmetric dispersion of wetting zone is cutted halfly at the vertical section, and then, the variation of wetting zone is visible at the front surface of box. The value of infiltration capacity (f) was given as the rate of water entry, when the rate has become constant. The materials used were Toyoura standard sand (A) and the weathered granitic soil at Tanakamiyama, Otsu City (B). The results are as follows. 1. In the first experiments using material 〔A〕, 〔KI〕 neally equals 〔Kp〕 and 〔Ko〕. The values are 1.5--1.8×10-2 cm/sec. 2. In the first experiments using material 〔B〕, 〔KI〕 neally equals 〔Kp〕, but is about four times of 〔Ko〕. 3. In second experiments, the values of 〔f〕 using material 〔A〕 are 7.1--8.0×10-2 cm/sec. The values are about four times of 〔KI〕 and the value of 〔f〕 is 1.2×10-2 cm/sec, and is a twice of 〔KI〕 at the same material. The values of 〔f〕 are from a twice to three times of 〔KI〕. 4. The infiltration capacity measured by Musgrave-type infiltrometer is passably applicable for the approximate value of hydraulic conductivity of vertical infiltration in practical problems. 5. The difference of initial water content doesn't bring apparent effect to the capillary potential.