This paper presents the experimental study and numerical simulation of Paris plastic clay in a case study of Grand Paris Express project. Paris plastic clay is a highly plastic clay from the Sparnacian age (Lower Eocene) and forms the first tertiary deposits of Paris sedimentary basin, it has a very low permeability, is normally consolidated or overconsolidated, and is swollen in outcrops. To analyse and model Paris plastic clay, in-situ and laboratory tests, geotechnical analyses of test results, calibration of geomechanical parameters based on these analyses, 2D and 3D finite element modelling with the appropriate constitutive laws by semi-probabilistic and probabilistic methods were carried out. The monitoring allowed the comparison of measurements with the results of the numerical simulation to validate this approach.