Study on water retention capacity for drained soils with different textural layering

Abstract: The impact of textural layering on water retention capacity was evaluated through three different layer thicknesses soils (11.25, 22.5 and 45 cm), and two homogeneous soils (sand and sandy loess) as control in 90 cm long columns. The soil columns were completely saturated and then drained with a positive pressure of 2 cm at the bottoms of the soil columns. The changes of water storage with time were measured by weighing the columns and by measuring the soil water content profiles using TDR probes, respectively, at 1, 5, 24, 48, 72 and 96 hours of drainage. The hydraulic properties of the sand and sandy loess were measured in the laboratory and also optimized using numerical simulations for the two homogeneous soil columns. Comparing the measured hydraulic parameters with the optimized ones, we found that the optimized parameters could decrease the maximum relative error from 19% to 9%. These optimized properties were then used to simulate the drainage process of deep profiles in more typical field conditions. The results showed that: 1) The laboratory observations and simulations all confirm that the amount of water retention decreases as the thickness of the layers increased. When the combined pressure caused by the suction of the underlying coarser layer plus the hydrostatic pressure within the finer sandy loess layer exceeds the air entry value (AVE) of the finer sandy loess, the amount of the water retention capacity does not increase with decreasing thickness of layer in the textural soil. In this study, we found the minimum thickness of the fine sandy loess is about 5 cm; 2) The water content within the finer textural layers decreased only slightly from saturation, with almost all of the water loss occurring from the coarser textural layers. This phenomenon can be found in the water content profile. The water content profiles in all layered soil columns showed distinct breaks at the layer interfaces, and the water content of coarser layers decreased from saturated to only 0.04 cm3/cm3. Though the water content of finer sandy loess layers changed little, it also can be found that the water content of finer soil increased with the decreasing thickness. The results of this research can provide some recommendations for mine reclamation in arid and semiarid regions.