Undrained shear strength determination and correlations on Søvind Marl

Undrained shear strength on Søvind Marl is found through undrained, volume constant, triaxial testing and is compared to field vane shear strength. Søvind Marl is extremely plastic and highly fissured Eocene clay found throughout Denmark. The fissured structure of Søvind Marl has an influence on both the preconsolidation and the undrained shear strength. Two apparent values of the preconsolidation stresses can be determined due to the fissured structure (Grønbech et al. 2015) which also considerably decreases the undrained shear strength.

Determination of shear strength of fissured clay is done through field testing or triaxial testing. Christensen and Hansen (1959) tested fissured Danish Oligocene clay and found the undrained shear strength, Su, be approximately 1/3 of the measured field vane shear strength, cfv. This correlation has since been used in Danish geotechnical practice with little to no further validation through modern triaxial test.

The measured undrained shear strength is normalized using SHANSEP. The strengths are compared to the measured field vane shear strength in order to determine μ (μ=Su⁄cfv ). The undrained shear strength is found to be increasing with increasing stresses up to a strength of approximately 450 kPa at a stress level corresponding to the lower limit of the preconsolidation stresses. The field vane shear strength is found to reach values of more than 1000 kPa. The ratio between the undrained shear strength and field vane shear strength was confirmed to be approximately 0.3 on Søvind Marl regardless of the stress level.