Stress dependency and unloading-induced swelling behaviour of a high plasticity overconsolidated clay of Paleogene origin

High plasticity clay of Paleogene origin in Denmark, possess unique deformational behaviour with significant stress dependent stiffness in oedometric loading/unloading. This is challenging settlement/heave predictions. The behaviour has been so far associated with high smectite content and complex geological loading history. Burial followed by erosion and then glaciotectonic activity, has resulted in fissures and slickensides, along with a high level of overconsolidation. Several currently planned and existing Danish construction projects encounter problems with such clays. Specifically, the immersed tunnel project connecting Denmark and Germany: “Fehmarn Fixed Link”, for which an extensive laboratory campaign of IL and CRS oedometer tests, has been carried out. These tests have been re-analysed, to obtain a better description of the swelling behaviour of Paleogene clays, due to the net unloading state expected for this immersed tunnel. The development of the swelling index during unloading, although non-linear, tends to reach a constant value at sufficient stress reduction. This constant value as well as the average swelling index increases with the stress at which unloading starts and increased smectite content. These trends are confirmed by an additional CRS test.