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Determination of the preconsolidation stresses is a key tool in geotechnical engineering used to evaluate and estimate the behavior of soils. However, it has proven difficult to accurately estimate these stresses in highly fissured, overconsolidated clays, due to the influence of the fissured structure. In the current study, oedometer tests were performed on Søvind Marl, a plastic Eocene clay containing a large number of fissures and slickensides. Four incremental loading oedometer tests and four continuous loading oedometer tests were performed in order to determine the preconsolidation stresses. Multiple assessment methods were used to assess preconsolidation stresses based on the oedometer test data, results of which are presented and discussed on this paper. All of the test and interpretation methods yielded very similar results, with two possible values calculated as the preconsolidation stresses. However, only the upper stress bound is actually identified as being associated with preconsolidation; the lower bound is generated as a result of the collapse of the fissured structure in response to increasing stresses that were applied during testing. The upper bounds of the preconsolidation stresses of Søvind Marl were between 6300 and 8900 kPa, whereas the lower were between of 600 and 800 kPa.

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Results are presented of the analyses of the geotechnical properties of a fissured and highly plastic Danish tertiary clay: Søvind Marl. The analyses demonstrate the difficulties that can arise when dealing with this particular clay due to its unique characteristics. To characterize Søvind Marl and its pore water, samples were collected at multiple depths from five different borings located at the same site at Aarhus Harbour, Denmark, and all encountering the same Søvind Marl strata. Although Søvind Marl was deposited under circumstances very similar to those of London Clay, its properties show a poor resemblance to London Clay or other known clays. One of the key properties of Søvind Marl was found to be its extremely high plasticity, which can exceed 300%. Søvind Marl has high concentrations (up to 95%) of clay minerals, with a particularly large portion of smectite. Several key index parameters are presented and discussed including natural water content, Atterberg limits (plasticity in the typical range of 100%–250%), and calcite content (up to 65%). Results of vane shear tests and cone penetration tests are also presented. Oedometer tests yield two very different values for the preconsolidation stress level, although only one of these is correct. The dual calculation is associated with the fissured nature of Søvind Marl, which may confound traditional interpretation of test results, but must be taken into account for understanding of test results leading to proper classification of the material.

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