Application of instantaneous profile method to determine the hydraulic conductivity of unsaturated natural stiff clay

In the laboratory, the instantaneous profile method has been routinely applied to determine the hydraulic conductivity of unsaturated compacted clayey soils. But the application of this method is often limited by the large-dimensions of water content probes such as TDR (Time Domaine Reflectometry) probes. Indeed, the existing commercial TDR probes are too large to match the small dimensions of laboratory samples. Furthermore, this method is difficult to be applied to natural stiff clays or other stiff materials (lime/cement treated soils for instance) owing to the surface irregularity created during the sample preparation. The voids between soil surface and testing cell would give rise to side wall leakage during water infiltration under constant-volume condition. This paper attempts to solve these problems by using resistant cohesive filament tape to wrap the soil sample for sidewall leakage prevention and keep the specimen in constant-volume condition. A set of home-made miniature TDR probes were developed for volumetric water content monitoring, together with a set of relative humidity sensors for suction monitoring. As expected, the hydraulic conductivity of stiff Teguline clay which was taken from the area of Albian Paris Basin was found decreasing with the increase of suction. Moreover, the value of hydraulic conductivity at zero suction determined by extrapolation is very close to that determined directly by constant head method. This confirms the promising use of miniature TDR probes in water content monitoring, as well as the effectiveness of resistant filament tape in sidewall leakage prevention under constant-volume condition. The successful incorporation of miniature TDR probes and resistant filament tape extends the common instantaneous profile method to small laboratory samples and stiff materials.