The water retention properties of a natural unsaturated loess from northern France

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 an in-house constructed 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 reasonably 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. It is hypothesised that this zone with no hysteresis corresponds to the natural variations of the water content under seasonal changes, providing a range of natural suction between 20 and 80 kPa, and it is suggested that this should be checked by in situ measurements. The loess microstructure was investigated by using scanning electron microscopy 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 intergrain 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 the smallest pores, a zone where the microstructure is sensitive to change in water content.