Keywords: soil, constitutive model, calibration, uncertainty, laboratory tests.

The basic formulation of the Mohr-Coulomb model does not reproduce several phenomena that a real soil exhibits including: a) the evolution of volumetric plastic strain upon isotropic compression, b) either compaction or dilatation of the soil upon shear, depending on the initial density, c) stiffness increasing with mean stress. These phenomena are inherently built into the critical state constitutive models such as the modified Cam clay (CC) model or the hypoplastic model for clay (HC). Although these constitutive models are present in many geotechnical finite element programs, their use is still rather limited. The reason for this is that the parameters of these models are less known, and are seldom provided by the geotechnical laboratory. The ExCalibre web application available at https://soilmodels.com/excalibre-en/ was recently released to make the nontrivial process of calibration for these constitutive models easily available to everyone interested. To calibrate either CC or HC model, ExCalibre accepts an Excel worksheet with an arbitrary number of oedometric and undrained triaxial laboratory tests and fits the model response to the measured data. This contribution focuses on how the number of particular laboratory tests that the user chooses to upload to ExCalibre influence the obtained set of model parameters. For both material models and all soil samples the results show that the mean value of the calibrations with reduced laboratory protocols tests does not differ from the reference value obtained for single calibration run taking into account all available laboratory tests. This observation suggests that doing the calibration several times with a limited number of laboratory tests, i.e. one oedometer test and one undrained triaxial test, and then averaging the results yields very similar results as running the calibration just once with all available data. Second observation is that for both material models and most of the soil samples the coefficient of variation is less than 10% for all material parameters with exception of Poisson’s ratio whose coefficient of variation reaches 50% in case of the modified Cam clay model and 20% in case of hypoplastic model for clay.

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