Background  Layer structure is the most common sedimentary structure in alluvial unconsolidated sediments, which usually exhibits special hydraulic properties to influence water flow and water retention ability. In this paper, a situ investigation on the water content distribution, grain-size and bulk density for a layered sediment profile was conducted at the floodplain of Weihe River in Xianyang, in the center of Shaanxi, then the effects of the grain-size and the capillary barriers on natural water content were discussed.

Methods  The experiment site outcrop was mapped into 11 macro layers having different texture and structures. With each layer, well developed laminations were observed. We investigated the characteristics of water content and its corresponding influence factors including soil bulk density, grain-size and matric suction in each layer, based on sampling analysis and the measurement of EC-5 Small Soil Moisture Sensor and MPS-6 Calibrated Water Potential Sensor. Firstly, we analyzed the relationship between grain-size and water content in each layer, based on this, the effect of capillary barriers on water content was discussed. Then, we analyzed the influence of capillary barriers in the process of precipitation infiltration and soil water evaporation.

Results  The results illustrated that in the layered alluvial sediments with alternating layers of coarse and fine sands, natural water content exhibited in a layered distribution on a form of interbedding high and low water content. The magnitude of the water content was negatively correlated with the grain-size, and was positively correlated with the bulk density. The water content of site outcrop was between 0.010 cm³/cm³-0.148 cm³/cm³, the median size ranged from 0.16 mm to 0.39 mm and the bulk density ranged from 1.32 g/cm³ to 1.53 g/cm³. The results also demonstrated that the layered sediments with alternating layers of coarse and fine sands formed the capillary barriers in the process of water flow, therefore by which the water content distribution was controlled. In both process of precipitation infiltration and soil water evaporation, the capillary barriers effects will lead to the water content in fine layer obviously higher than that in coarse layer.

Conculsions In conclusion, capillary barriers become a significant factor to control water flow in unsaturated soils. At the process of precipitation infiltration, capillary barriers have the effect on permeability reduction, which can be used in designing landfill anti-seepage system, optimized drainage layers and so on. Additionally, insights are potentially useful for designing mulching strategies and capillary barriers aimed at reducing evaporative losses, increasing water retention ability of soil, which is significantly important in arid and semi-arid regions.