孔隙率及混合孔隙材料影響保水曲線 與入滲之研究

Abstract

研究以實驗方法預先夯實土壤（以兩種石英砂為土壤樣本），形成土壤之孔隙率變化，藉以探討對土壤保水曲線影響，再將實驗所得土壤基本性質，以數值模擬方式，模擬水分入滲及排水時，土壤體積含水比與深度之關係。另外再藉由不同配比混合粗粒料孔隙材料（以生態環保磚材料為例）及細粒料孔隙材料（以其中一種石英砂為例），探討保水曲線受到粗、細混合配比孔隙材料之影響，最後利用數值分析了解各種配比下之入滲曲線變化情況。 試驗結果顯示，對單一種級配之石英砂而言，飽和含水比較高（即孔隙率較高）之土壤，其殘餘含水比較低，而空氣侵入壓力亦較低。形狀因子α，會隨著孔隙率增加而增加，形狀因子n則會隨著孔隙率增加而減少。若將各形狀因子對孔隙率分別進行線性迴歸，相關半徑皆達0.93以上，此結果將可預測該砂種在實驗範圍中之孔隙率改變時，其保水曲線變化之情形。而於其入滲數值模擬中發現，土壤初始含水比固定、當上方邊界為定通量、下方邊界為定水頭，而孔隙率因夯實而越低時，上邊界附近對應之含水比越高，溼鋒深度則為降低。而排水過程中，最大含水比之深度亦會隨著孔隙率之減少而減少。此外，不論在溼潤或排水過程中，溼鋒發生的深度永遠是高孔隙率之土壤較深。 另外於配比試驗中顯示，且不同配比下之粗粒料與細粒料所產生保水曲線之差異，會隨著細粒料佔整體孔隙材料之比例越高而越明顯。另在入滲模擬時發現，孔隙率較高之孔隙材料（或是飽和水力傳導係數較高），其水份入滲速度在未飽和狀態下不一定較快，而需視當時含水比大小所計算而得之未飽和水力傳導係數而定。

This research is to investigate the effect of porosity and the mixture of different porous material on the water retention curve and infiltration curve. A sand box test and a numerical model are adopted to study both effect. In the sand box test, the porosity is changed by consolidating the porous material. Besides, a coarse material (using Ecological Environmental Protection Brick material) and a fine material (using quartz sand) are mix in different ratio to form the mixture. The numerical model is a finite difference model which can characterize the infiltration properties of porous media. In the sand box test of porosity, for one kind of quartz sand, the experiment result shows that the higher saturated water content of the soil, the lower residual water content and lower air entry pressure it will have. The shape factor α will increase with porosity, but the shape factor n will decrease with it. If we use a linear regression on the shape factors and porosity, the R-squared will all be higher than 0.93. So we may predict the shape of water retention curve by this linear relationship within the experimental range we’ve conducted in the sand box test. In the infiltration simulation, initial water content is set constant, the top boundary flux is set constant and the bottom boundary head is set constant. Because the porosity becomes smaller due to compaction, higher water content appears near the top boundary, and the wetting front depth decreases with the porosity. When top boundary flux is set to zero, the depth of the highest water content decreases with porosity. Besides, no matter the top boundary is zero or non-zero, the deepest wetting front always occur in the soil of the highest porosity. In the sand box test of mixing different porous material, result shows that the higher ratio of the fine material in the mixed porous material, the hysteresis would be more obvious. The results also show that higher porosity (or higher saturated hydraulic conductivity) does not necessarily result in higher infiltration velocity under unsaturated condition. The unsaturated infiltration velocity depends on the unsaturated hydraulic conductivity calculated from the water content.