# Capillary Rise in Layered Soils
## 4 Results and Discussions  
In all of the columns, the capillary water increased gradually with time (Figure 5a). The capillary water entered the finer interlayer (k-8 soil in this case) if it was laid lower than the maximum capillary height of the underlying coarser soil. The highest maximum capillary height, 153 cm, which was higher than the maximum capillary height of the homogeneous k-7 soil, was obtained in column D. However, in the other three columns, the maximum capillary height was the same as that of the homogeneous k-7 soil column. Therefore, once the water flowed into the finer soil, whether the maximum capillary height of layered soil was higher than that of the homogeneous soil depended on the height where the finer layer was laid. If the sum of the finer layer thickness and the coarser layer thickness (distance from the water level) was less than the maximum capillary height of the coarser layer, the maximum capillary height of layered soil was equal to that of the homogeneous coarser soil; otherwise, the former was larger. The rate of capillary rise was different and not constant for each case. The capillary rate showed a power function relationship to the elapsed time; the corresponding regression equation is shown in Figure 5b. The tendency of the capillary rise rate in all of the layered cases was similar, which might be attributed to the fact that the thickness of the finer layer (20 cm) occupied a small portion of the total layered soil height, except in the case of column A. For column A, the finer soil layer was laid at a low position near the water level; it limited the water flow velocity because of its own lower hydraulic conductivity compared with that of the coarser soil in the same suction value. Figure 4: Schematic of the setup of the layered soil columns. Figure 6 shows the volumetric water content profiles of all of the six columns. The two solid curves are the water content profiles of the homogeneous k-7 and k-8 soil column, and the results of the other four layered cases are drawn as scatter plots. The water content value of the k-7 soil at the height greater than the maximum capillary height of the homogeneous k-7 soil was approximately equal to the residual water content. The water could be stored in the finer interlayer, and the amount of water was larger than that of the k-7 soil at the same height, but lower than that of the homogeneous k-8 soil at the same height. This result indicated a \"gap\" of the water content or suction between the interlayer k-8 soil and the homogeneous k-8 soil. According to the common view, the suction at the interface should be equal at two sides, therefore the water content at the two sides should be equal to the value of the homogeneous soil column at the height when the water flows reach at hydrostatic equilibrium condition. In other words, based on the observed water content, the suction of the layered soil might not be continuous because of the interface between the two types of soils due to the very different between the air entry values of the two kinds of soils. As shown in Figure 7, generally, at most of the observed points, the numerical results were consistent with the test results. However, the simulated results of the finer soil layer (k-8 layer) were lower than that of the test results (Figure 7b). This implied that even if the  
Figure 5: Comparison of the capillary rise height (**a**) and the rate (**b**) between the layered soil profile and the homogeneous soil profile. Figure 6: Water content distribution in the layered soil profile after the water capillary rise for 240 days. parameters in the simulation were same, it was difficult to ensure that the simulated results were consistent in all of the test cases. The effect of the interface played an important role in determining the characteristics of the water flow. Figure 7b shows that the measured volumetric water content increased with elapsed time. However, at observation point No.