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On the Zagrad location, in the very vicinity of the Rijeka city center, the garageaccommodation- business complex is under construction, layout dimensions 90 x 60 m. The geotechnical conditions on site are very complex: on the lateral sides limestone rock-mass protrudes to the surface while in the central part of the location is a sinkhole with thick clay deposits. Regarding the location morphology, the construction pit being 2 m deep on the northern end of the location to the 5 m depth on the southern end - adjacent to the existing buildings and a traffic line-was designed. For the designing of the excavation, the support system and the building foundation design, a complex geotechnical investigation was performed. These investigation works included borehole drilling, geophysical measurements, laboratory testing of rock and soil specimens. In the northern end of the location, having the highest cut in the carbonate and flysch rock mass, investigation boreholes and seismic refraction were carried out, along the future excavation pit contours. The analysis of the refraction data was made by the inverse modeling (Delta T-V method). The results obtained by these methods were used to determine the geotechnical profile, the contacts between flysch deposits and carbonate rock mass. On the same basis the rock mass parameters for the walls of the future excavation pit were evaluated and used for the rock support design. On the lowest, southern, part of the site the main problem encountered was determining the location of the bedrock on which the foundation of the building was foreseen. On that part of the site investigation boreholes were drilled and ground probing radar scanning performed. This was allowed to determine cover deposits, weathered zones and basic rock masses in the soil. On the basis of the data collected, pit excavation methodology, rock support and foundation design options for that part of the site are taken. Due to the great variations in the cover thickness, the building foundation was designed as a combination of shallow foundations on easy-to-reach carbonate rock base, and deep foundations on toploaded drilled piles. During the construction of the pit and building foundations, the design assumptions were adapted according to the in situ situation.

The paper presents the construction of an open pit within the urban area, by application of the phase excavation concept and active design of the rock bolts reinforcement system for the flysch rock mass. By the end of the open pit excavation works, monitoring of the excavation stated great deformations in the area excavated in flysch deposits. The stability of the excavation was established by the well-timed intervention and additional reinforcement measures, without influence on the adjacent buildings.