Collapse behaviour of a natural loess from Northern France

An investigation of the collapse behaviour of an intact loess samples extracted close to the high speed train line (TGV Nord linking Paris to Brussels and London) has been conducted in relation with some stability problems observed during heavy rain episodes. The effect of initial water content on the collapse obtained when soaking samples under the standard 200 kPa stress has been investigated, with significantly higher collapse potential in drier states. The effect of the applied stress has also been considered by running tests under 200 kPa (standard value) and the considerably smaller value 19 kPa (close to in-situ vertical stress of 16 kPa). Tests were performed by using two techniques: Constant Strain Rate (CSR) tests and Step Loading (SL) tests. The changes in suction upon loading were measured by means of an in-house constructed high capacity tensiometer. The results showed a higher collapse strain under the small in-situ vertical stress. Microstructure observation (SEM) suggested that the resistance of the microstructure should not be homogeneous given the irregular position of silt grains-clay aggregations between a metastable arrangement of clean silt grains. Clearly, collapse under wetting should occur by the densification of the areas where grains are clean with large pores around them. The zones in which the porosity is filled by clay aggregation should be more resistant and locally less sensitive to collapse.