Characterization in one-dimensional compression of a Danish Paleogene Clay

The North Sea Basin is characterized by the presence of highly plastic, stiff Paleogene clays sedimented between 65.5 and 43 millions of years ago. During the Quaternary Period, this area underwent a series of cyclic glaciations that overconsolidated the underneath clay deposits and sheared the shallower layers. Different studies focusing on these superficial, folded strata are present in literature. On the other hand, less information are available regarding the mechanical behaviour of the deeper, intact layers. However, a good knowledge of these layers mechanical properties is necessary to guarantee an efficient design of the infrastructures built on these formations. This work focusses on the characterization of the Paleogene Røsnæs Clay Formation present in the Fehmarn Belt area, between southern Denmark and northern Germany, where a submerged tunnel will be constructed. The mechanical properties of the Intact Røsnæs Clay layer were determined and compared with those of the Folded Røsnæs Clay. Incremental Load Oedometer tests were performed by applying different load cycles in order to stress the sample below, in correspondence and above the estimated preconsolidation stress. The results were compared with those obtained from testing reconstituted samples in order to assess the influence of structure. This was done quantitatively by analysing the stiffness, the compressibility, and swelling indices. Environmental Scanning Electron Microscope micrographs were used to detect the presence of micro-folding. The mineralogical consistency among the different Oedometer tests was evaluated by means of X-Ray Diffraction. Despite the homogeneity in the mineralogy, differences in both the microstructure and the 1D behaviour of the Folded and Intact layers were observed. In particular, the Folded layers appear to have greater compressibility and swelling potential. These differences should be taken into consideration when building on partially pre-sheared Paleogene formations.