Preconsolidation of Søvind Marl - a highly fissured Eocene clay

Determination of the preconsolidation stresses is a key tool in geotechnical engineering used to evaluate and estimate the behavior of soils. However, it has been proven difficult to accurately estimate the preconsolidation in highly fissured overconsolidated clays, due to the effect of the fissured structure of the clay. This article presents oedometer tests performed on Søvind Marl, a plastic Eocene clay, with a high presence of fissures and slickensides. Four incremental loading oedometer tests and four continuous loading oedometer tests were performed in order to determine the preconsolidation stresses. Used test apparatus and methods are presented. The differences between the testing methods and results are discussed. Several well-established methods were used to determine the preconsolidation stresses based on the oedometer tests. All tests and interpretation methods yields the same result; two apparent values can be determined as the preconsolidation stresses. However, only the upper boundary is deemed to be related to the preconsolidation, and the lower boundary arises as the fissured structure collapses during increasing stresses. The upper boundaries of the preconsolidation stresses of Søvind Marl are found in the range of 6,300 to 8,900 kPa, whereas the lower boundaries are found to be in the range of 600 to 800 kPa