Characterization in one-dimensional compression of a Danish Paleogene Clay

Abstract

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.

Mode-based characterisation of swell deformations in a high-plasticity Paleogene clay

“…Figure 9 shows that C sp increases with σ' v-max for both SM2 and SM3 tests, as well as the intrinsic references by Di Remigio et al (2021). The SM2 tests form two groups, one for the outliers in Figure 3 and 4, which lies very close to the intrinsic trend and one for the regular tests, which positions below it.…”

“…Figure 4 is divided into subfigures based on these swelling modes, showing for each of them; the normalised unloading branch (4a 1 , 4b 1 and 4c 1 ) and the development of C s with UI (4a 2 , 4b 2 and 4c 2 ). For comparison to the intrinsic state, the two intrinsic swelling lines (starting from 2 MPa) performed in this study, as well as an intrinsic swelling line (starting from 7 MPa) reported by Di Remigio et al (2021), are also shown in Figure 4. Figure 4a 1 and 4a 2 show the SM1 tests, which display a continuous increase of C s (positive curvature) throughout the entire unloading branch.…”

“…Bjerrum (1967) compared intact and remoulded samples, which showed similar development of C s for the first part of the unloading but later diverged based on the presence of structure in the samples. Rocchi et al (2018) andDi Remigio et al (2021) show that C sp for intact samples remains lower than the corresponding intrinsic value after several unloading/reloading loops, suggesting that full destructuration from unloading/reloading loops is very unlikely. Cui et al (2013) suggest that the swelling response is dominated by mechanical interactions at higher 5 stresses and physicochemical interactions at lower stresses, resulting in the observed two-part behaviour.…”