Abstract
In order to better evaluate the applicability of the inverse analysis method for calculation and evaluation of hydraulic properties of unsaturated soil in more realistic conditions, a transient one – step outflow experiment for layered sands was applied in the desaturation process with the purpose to attain the profiles of suction, saturation and flow rate with time. In this study, the fine sand and medium sand were used with the same thickness of 40cm for each layer. The sand grains were mixed under water and scooped into the plexiglas column (H = 80cm, D = 28cm, wall thickness = 1cm) to prepare a fully saturated sample. For homogeneity within each sand layer, the density of two sands must be controlled during soil column construction. For numerical study, the inverse simulation and one straightforward calculation were carried out to determine the unsaturated hydraulic properties of sands. Unsaturated hydraulic parameters in the van Genuchten model were estimated using soil suction measurements at 10cm intervals and an outflow rate at the bottom of a layered sand column. To reduce the quantity of data for analysis and simulation but still keep enough typical information for the experiment, four data sets of soil suction and saturation at four locations (L2, L4, L5 and L8) were selected out of eight to compile the Soil Water Characteristic Curve. The comparison between predicted unsaturated hydraulic properties and the experimental unsaturated hydraulic properties shows good agreement in the case of the fine sand was overlaid with medium sand. The results concluded that besides the homogeneous sand, the inverse analysis based on the 1-D outflow experiment promises to be a useful method in determining the hydraulic properties for unsaturated heterogeneous sand.
Highlights
The unsaturated hydraulic properties of soils are very important in providing the essential knowledge for solving many engineering problems in the geoengineering such as seepage, slope stability, bearing capacity, consolidation and settlement, water contamination, etc
After opening the bottom valve to allow free drainage, the lower end of the fine sand column was exposed to atmospheric pressure, and soil is allowed to drain by gravity at that moment, the pore water pressure started to drop at every location in the sand column (Figure 5 a)
Since the inverse analysis was becoming a useful tool to determine the unsaturated hydraulic conductivity for homogeneous soil profile, many problems in Geoscience related to the unsaturated hydraulic parameters for heterogeneous soil profiles have been considered because of the high similar condition of these problems with real site condition
Summary
The unsaturated hydraulic properties of soils are very important in providing the essential knowledge for solving many engineering problems in the geoengineering such as seepage, slope stability, bearing capacity, consolidation and settlement, water contamination, etc. The most traditional determinations of unsaturated soil hydraulic properties require relatively restrictive initial conditions and boundary conditions, and can be time-consuming, laborious, expensive and uncertainty for most practical applications 1. To simplify and reduce the calculations of unsaturated hydraulic functions for soils, in general cases, the soils can be assumed as homogeneous soils. The values of hydraulic properties of unsaturated heterogeneous soils in many cases are more appropriate for solving many engineering problems. Finding out a good method to determine accurately the hydraulic properties of unsaturated layered soils with the real conditions are not easy but necessary
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