Open pit protection and foundation works for the CS Maslinica pumping station, Rabac
As part of the wastewater drainage system of the Labin–Raša–Rabac agglomeration, we made the open pit protection and foundation main design for the construction of the CS Maslinica pumping station in Rabac.
Site description and geotechnical conditions
The site is located directly along the coastline, in an area partly used as a parking lot and beach, within a typical karst environment with occasional rock outcrops. Geotechnical investigations showed that the ground consists of an approximately 8 m thick fill layer (sandy gravel with cobbles), underlain by moderately weathered limestone bedrock. The soil is highly permeable, while groundwater levels are directly influenced by sea level and tidal variations. In addition, the heterogeneity of the fill material results in variable deformation properties..
Technical solution for open pit protection and foundation works
The main challenge was the excavation down to approximately -3.0 m a.s.l. in highly permeable soil and in immediate proximity to the sea. Under such conditions, conventional dewatering methods were not effective, while watertight solutions would have required complex and costly construction. The presence of existing infrastructure further limited the selection of feasible solutions. For these reasons, a different construction approach was chosen.
The selected solution was prefabricated construction under water conditions. The pumping station structure was constructed outside the excavation and then lowered onto the previously prepared foundation soil under water. This approach avoided the need for complex dewatering systems, reduced construction risks, and enabled better control of construction quality. It also shortened the construction time and reduced the impact on the surrounding area.
Open pit protection was designed as a combination of sloped excavation (1V:1.5H) and vertical excavation supported by sheet pile walls. Sloped excavation was applied where space allowed, while in constrained areas reinforced concrete sheet pile walls formed by bored piles were constructed. The piles were connected with reinforced concrete capping beams to ensure overall stiffness and uniform system behaviour. In sections with greater excavation depth, additional stability was provided by self-drilling anchors. Slopes were protected from weather effects using protective sheeting to prevent erosion and degradation during construction.
After excavation to the design level, the foundation soil was improved by placing engineered fill layers of appropriate grading. The lower layer was constructed as a load-bearing granular layer, while the upper layer served for leveling and accurate positioning of prefabricated elements. Special attention was given to construction under water conditions, requiring suitable materials and installation methods. As vibratory compaction is not applicable under water, alternative compaction methods were applied, including compaction under self-weight, mechanical compaction by tamping and pressure applied by the excavator bucket, and repeated leveling in perpendicular directions to achieve the required surface.
Following the preparation of the fill layers, prefabricated reinforced concrete pumping station tanks were installed. After installation and final positioning, the tanks were filled up to the maximum operating level, providing surcharge loading of the fill in order to improve compaction and reduce expected settlements during operation.
Conclusion
The CS Maslinica project is an example of addressing complex geotechnical conditions in a coastal environment, where highly permeable soil and marine influence limit the use of conventional solutions. By applying prefabricated construction and an appropriate excavation support system, a safe and technically efficient solution was achieved.
