Evaluating undrained shear strength of clay with DPSH-A - A down to earth assessment of the applicability in Scania, Sweden

Sammanfattning: The undrained shear strength of cohesive soils is an important parameter to assess in geotechnical investigations. It is normally evaluated with the cone penetration test (CPT) but problems with penetrating coarse, heterogeneous materials that is also hard, such as clay till, have pointed the interest towards other possible in-situ methods. This has led to studies assessing the possibility to evaluate undrained shear strength from dynamic probing. This thesis evaluates a suggested relation between the dynamic probing and the undrained shear strength of soft clays in Scania, Sweden. The undrained shear strength was evaluated by dynamic probing, CPT, field vane and piston sampling to obtained undisturbed samples for triaxial tests in the laboratory. The undrained shear strength evaluated with DPSH-A, according to the studied relation, is consistently lower than the same parameter evaluated from triaxial testing, indicating an inaccuracy of the relation. However, with only two triaxial tests conducted it is difficult to draw conclusions. The comparison between DPSH-A and CPT did not indicate any reliable correlation with the studied relation, albeit the scatter is large. The data set obtained by field vane was largely scattered and no indication of the accuracy of the proposed relation could be given. An intention was made to find better suited relations for Swedish soft clay, however R2-values of around 0,25 - 0,30 indicated that the data was largely scattered and no new relations could be suggested. Additionally, suspicions were raised that the data from DPSHA might be unreliable because of the fact that the DPSH-A was conducted in such a soft clay that the recommended minimum number of blows per penetrated unit was not achieved. This indicates a possible source of error, probably affecting also the final results and analysis. In conclusion, it is possible that the studied relation between the calculated dynamic tip pressure from DPSH-A and the evaluated undrained shear strength of soft clay could be accurate in soft Swedish clays, but further research is needed.