Flysch deposits are common in Serbia. Approximately 15 % of the 250 km of highways currently under construction will be constructed in flysch. In the most southern part of the E80 project, close to the border crossing with Bulgaria, construction of 3-km long cuts, often over 30 m high, is foreseen by the design. The geology of this part of Serbia is mainly characterized by flysch deposits represented by a sequence of sandstone and siltstone, interchanging in different proportions. After excavation of a majority of the cuts, several slopes suffered from global and structural instabilities. This required re-design of one part of the cuts. For this purpose, a geological strength index (GSI) was estimated and the structural features of rock discontinuities were observed on the faces of the surface excavations. This information and data obtained from laboratory testing enabled the determination of five characteristic rock mass types. For every type, shear strength properties were determined based on three criteria: the Hoek-Brown failure criterion; the Mohr-Coulomb criterion; and an hyperbolic nonlinear envelope. During the process of converting the Hoek-Brown parameters to the parameters of hyperbolic envelope, the Levenberg-Marquardt algorithm (LMA) was utilized to solve for the nonlinear regression problem. Verification of the parameters was performed on several examples. The median angle pressure increases exponentially with the GSI value and ranges between the value characteristic for clays and well-graded gravel. The maximum angle difference is obtained for a GSI value of 30. The average normal effective stress acting on the failed slopes, expressed in the form of the stress level ratio (SLR), is below 0.5. This indicates that the curved part of the shear strength envelope is utilized during the shearing. As such, the linear segment overestimates the shear strength.
