Behaviour of Engineered Slopes in Flysch Rock Mass

Abstract

This paper presents some experiences during the reinforced cut construction in the flysch slopes for the Adriatic motorway near the city of Rijeka, Croatia. The Cretaceous and the Paleogene limestone are situated on the top of the slope, while the Paleogene flysch crops are on the lower slope and in the bottom of the Draga Valley where the motorway is located. Unlike limestone rocks at the top of the slope, flysch rock mass is almost covered by colluvial deposits, residual soils and talus breccia. The major part of the motorway was constructed by cutting of the slopes in flysch rock mass. Stability of the slopes was ensured by the reinforcement of the rock mass with rockbolts and appropriate supporting system. The self boring rockbolts, in combination of multi-layered sprayed concrete or reinforced grid construction, were used. The deep boring drains were installed for dissipation of ground water collected at contact of permeable cover and impermeable flysch rock mass. The interactive rock mass cutting design, based on the observational methods, was introduced during the construction. The initial design of slopes construction was based on the recommended Hoek–Brown criteria for heterogeneous rock mass such as flysch. Design properties of the flysch rock mass were adopted on the geological strength index (GSI) concept. During the phase of interactive design the appropriate measured equipment was installed. This active design approach has allowed the designer, based on the rock mass conditions and monitoring results, to change the support system in the slopes at some unfavourable locations. The measured values and the back analysis enabled the establishment of real rock mass strength parameters and deformability modulus.