mass deformations and their influences on construction are observed during construction and even during service period. Observations of reinforced cuts along the Adriatic motorway near the City of Rijeka, Croatia, were conducted over a time period of three to seven years during construction phase and service period. Measured displacements reached significant magnitudes during the service period (up to 50% of displacements of construction phase) as a consequence of the time dependent behaviour of the rock mass. This thesis presents findings related to flysch rock mass weathering profile and its characteristics based on detailed geotechnical investigations and monitoring results coupled with numerical back analysis. It was found possible to detect the thickness of the flysch rock mass weathering profile by performing detailed geotechnical field investigations. According to the seismic refraction method that utilized the refraction of seismic waves on geologic layers and rock/soil units, the numerical model was divided into geotechnical units with similar geotechnical properties. To obtain deformability and creep properties of geotechnical units the direct approach to back analysis was used in numerical modelling. Numerical modelling was divided into two phases: modelling of construction phase using the linear elastoplastic Mohr-Coulomb model and modelling of service period using the visco-plastic Burger-Mohr-Coulomb model. Verification of the numerical model and parameters was based on comparison of simulation data and measured data of horizontal displacements at vertical inclinometer. Recommendations for the strength, deformability and creep properties of the geotechnical units of the weathering profile of a flysch rock mass are given together with guidelines for future investigations and research.