Investigation of Influence on Porosity and Mixed Porous Material to Water Retention Curve and Infiltration

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.