© The Geological Society of London 2006 1 Engineering geological properties of the rock mass along the Kastela Bay sewage system

Abstract: Kastela Bay is situated on the Adriatic coast of Croatia, between the cities of Split and Trogir. The bay is 15 km long and 5 km wide and Ciovo Island lies at its mouth. The area is urbanized, densely populated, has heavy industry, and includes the cargo port of Split. At present, most of the waste water in the area is discharged untreated, and in an uncontrolled manner, directly into the sea. Therefore, design of a waste water treatment system has started. The system consists of a sewage system (30 km long), 18 pumping stations, a waste water treatment plant, and a waste water transfer tunnel through the Ciovo Island. The tunnel will be 2824 m long with a maximum overburden of 180 m. Engineering geological and geophysical investigations, exploration drilling and laboratory tests have been carried out in support of the project. These investigations have identified considerable heterogeneity in the clastic flysch sediments that occur in the coastal part of the area, and more uniform limestone formations on Ciovo Island where the waste water transfer tunnel is located. Generally, there is a clear zonation in the 15 m of the clastic flysch sediments. Three geotechnical units have been differentiated: Quaternary cover, and highly and moderately weathered bedrock. The tunnel on Ciovo Island will pass through well-bedded and karstified Turonian, Senonian and Early Eocene limestones. In terms of engineering geological properties and RMR classification there are 7 units. The investigation has also identified very low groundwater levels on Ciov