The relationship between the microstructure and the water retention properties of a natural 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 events. Its water retention curve was obtained by using two approaches of the filter paper method and an in-house constructed high capacity tensiometer (HCT). A good agreement between all the results was observed. 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. The loess microstructure was investigated 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 others characterized by the presence of clay aggregations into a well graded intergrains 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 large pores between clean grains whereas clay adsorption becomes dominant in smaller pores, a zone where the microstructure is sensitive to changes in water content.