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To understand the hydraulic properties of soils, accurate prediction of the unsaturated hydraulic conductivity is needed, which is commonly derived from the soil water release characteristic. In fine-textured soils, the modified Mualem-van Genuchten (MMVG) function has been used extensively to predict the change in relative conductivity with changes in matric potential, but this assumes that the soil is rigid with a constant porosity (CP), which is unrealistic for shrinking soils. We used a triaxial cell apparatus to accurately monitor soil volume changes during water release in a clay soil. From this we derived relative hydraulic conductivity functions based on either MMVG (CP), or based on geometrically similar shrinkage (GSS) of all pore sizes by a constant factor as calculated from the measured shrinking porosity (SP). The MMVG (CP) function predicted a decline in relative conductivity of up to three orders of magnitude from 0 to -400 kPa matric potential. This was similar to published data on the response of conductivity in the same soil to consolidation of larger pores. Based on SP, however, the soil remained effectively tension saturated (>0.97) down to a matric potential of-85 kPa due to normal shrinkage. The corresponding GSS (SP) function predicted only a modest decrease in relative conductivity from I to 0.85 across this range. In this case the larger pores, although reduced in size, are assumed to be still water filled and conducting. The GSS function was probably the most realistic portrayal of hydraulic functioning in a tensionsaturated shrinking clay soil. [ABSTRACT FROM AUTHOR]

The management of clay-rich soils in agriculture is mainly dependent on their hydromechanical properties that govern their structural evolutions. These structural evolutions are commonly described along the shrinkage curves obtained in a laboratory. Nevertheless, the field survey needs tools of in situ investigations able to characterize the structural profile evolutions of soils through the seasons. This paper presents a method based on the cone penetration and resistivity measurement by penetrometer-salinometer coupling that was gauged in the West Marsh of the French Atlantic Coast. First, the parallel measurements of water and cone resistance (Qd) profiles in clay dominant soils, characterized by very large ranges of water content (W), allows the calculation of a Perdock's-type equation that links the Qd profiles and the shrinkage curve of the clay dominant material. Second, thanks to the homogeneity of mineralogy and cationic exchange capacity of the soil, the structure-resistivity relationship was gauged according to the Archie's law taking into account porosity (Φ), saturation index of the soil (Sat), and the resistivity of the wetting fluid (ρw). The vertical evolutions of structure, resistivity, and cone resistance can be represented in a crossed diagram showing the shrinkage curve-resistivity and Qd profile relationships. Finally, the structural profiles of soils can be modeled from the recorded resistivity and Qd profiles taking into account the ρw. [ABSTRACT FROM AUTHOR]