The water retention properties of a natural loess from Northern France were investigated on intact block samples that were excavated along a high speed train line (TGV) that experienced stability problems during heavy rain episodes. Suction measurements were made by using the filter paper method and a home made high capacity tensiometer (HCT) so as to determine the water retention curve of the loess. The results showed that an alternative approach using a previously wetted filter paper could fruitfully be used with suction values fairly compatible with the HCT measurements. The water retention curve obtained exhibited a peculiar form with no hysteresis observed around the natural water content (14.4%) and two hysteresis loops on both the wet and dry sides of the curve. This zone with no hysteresis is supposed to correspond to the natural variations of the water content under seasonal changes, providing a range of natural suction between 20 and 80 kPa that should obviously checked by in-situ measurements. The loess microstructure was investigated by using scanning electron microscope and mercury intrusion porosimetry. It appeared that the clay fraction (16%) was not uniformly distributed with some areas composed of clean grains and a well graded corresponding inter-grains pore population. Conversely, the clay aggregations define a smaller sized porosity. The calculation of a water retention curve derived from the pore size distribution curve showed that water retention is governed by capillarity in the largest pores between clean grains whereas clay adsorption becomes dominant in smallest pores, a zone where the microstructure is sensitive to change in water content.

International audienceThe compression and collapse behaviour of a natural unsaturated loess from Northern France was investigated by running a series of constant rate of strain oedometric compression tests (together with some step loading tests) while monitoring suction by means of a high capacity tensiometer. The reasonably constant suction measured during constant water compression tests was linked to the collapse of the largest dry pores. Time dependent behaviour was also observed, with different responses obtained at different strain rates. In a standard fashion, collapse tests appeared to bring the representative point from the constant water content curve to the zero suction compression curve, indicating the existence of a maximum collapse stress. Data were qualitatively interpreted by using the Loading Collapse (LC) yield curve of the Barcelona basic model. Changes in the LC yield curve inclinations could account for the existence of a maximum collapse value

International audienceTo provide a complete local monitoring of the state of an unsaturated soil sample during triaxial testing, a local water content measurement device was adapted to a triaxial device comprising the measurement of local displacements (Hall effect transducers) and suction (high-capacity transducer). Water content was locally monitored by means of a resistivity probe. The water content/resistivity calibration curves of an intact, natural, unsaturated loess from northern France extracted by block sampling at two depths (1 and 3.3 m) were carefully determined, showing good accuracy and repeatability. The validity of two models giving the resistivity of unsaturated soils with respect to their water content was examined. The first triaxial tests carried out with this device in the range of in situ stresses gave satisfactory results, but with some effects of the applied cell stress on the water content measurements. Some preliminary behaviour characteristics of the natural unsaturated loess, a material rarely tested up to now in the literature, were evidenced. Also, the yield stresses appeared significantly higher than the in situ stresses, confirming the combined effect of partial saturation and bonding

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Non-destructive methods of measuring water content in soils have been extensively developed in the last decades, especially in soil science. Among these methods, the measurements based on the electrical resistivity are simple and reliable thanks to the clear relationship between the water content and the electrical resistivity of soils. In this work, a new electrical resistivity probe was developed to monitor the change in local water content in the triaxial apparatus. The probe is composed of two-pair of electrodes, and an electrical current is induced through the soil at the vicinity of the contact between the probe and the specimen. Some experimental data on the changes in resistivity with the degree of saturation were obtained in specimens of a natural unsaturated loess from Northern France. Two theoretical models of resistivity were also used to analyze the obtained data. Results are finally discussed with respect to the loess’s water retention properties.