Variation of CPTu Data in Finnish Soft Soils

Geotechnical design includes assessment of soil (strength) parameters, which are usually derived directly or indirectly from the field investigations and/or lab tests. In this study, the variation of CPTu data is studied. The focus was on the measurement uncertainty, and the transformation model uncertainty of correlation models for su, when the undrained shear strength is derived from the results. The CPTu data was taken from the database of Tampere University (TAU), and the data consisted of four soft clay sites in Finland. Each test site included three CPTu borings done close to each other. Three correlation models for su, calibrated in previous studies at TAU, were chosen for transformation models. The measurement uncertainty and the transformation model uncertainty was calculated for each CPTu boring and each transformation model at each site. The results showed that the repeatability of the CPTu is really good, as the obtained coefficient of variations (COV) for separate borings as well as for each site were low. The low COV- values indicate low measurement uncertainty in measured CPTu data, when the tests are done with care. Moreover, the results showed that the overall transformation model uncertainty was lowest for models based on the net cone resistance and pore pressure. For the third model the uncertainty was little higher as it included more uncertainty in the initial parameters.