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The 1 million inhabitants of Croatia’s capital Zagreb can count themselves lucky to have high-quality drinking water that is drawn from wells around the city area. It is hence the city council’s utmost concern to maintain this quality by preserving the river Sava and the groundwater reservoirs thanks to the new Zagreb wastewater treatment plant.
(Published on September 2008 – JEC Magazine #43)
A 5.9 km HOBAS SewerLine® DN 1000 now transports all wastewater from New Zagreb (200,000 inhabitants) in the south across the river Sava to the treatment plant in the north-east of Zagreb. The facility and its supporting structures were built by Zagrebacke Otpadne Vode (Zagreb Wastewaters).
Three major sections
The pipeline consists of three major sections. MCP Culinecka is the first one, running from the treatment plant to the so-called Homeland Bridge. Open-cut installation with well-compacted gravel sized 0-16 mm was chosen for this part. The manholes used are T-pieces with nominal diameters of DN 1000/800 covered with stainless steel blank flanges.
Maintenance and repair work is made flexible by the mechanical couplings that join the manholes to the pipeline so they can be easily disconnected and taken out of their concrete housing at any time. The second section is the 900 m long Homeland Bridge that carries a road, a tramway and the 5 parallel pipelines (DN 1000) embedded in the marine. Four of the pipelines are for potable water and one is a CCGRP sewer main for New Zagreb’s wastewater. The 1 km long SewerLine® is installed on stainless steel supports with 5.85 m clearance. Since the only fixed point lies in the centre of the bridge, thermal expansion is provided for on both sides with the bridge moving up to 50 cm. KRESTA compensators are utilized here to absorb these movements.
This was a demanding task considering the relatively large pipe diameter, the pressure and line movement.
The magnetic field created by the tramway rails was an important factor when it came to choosing the right materials, as it can influence close parallel lines such as the sewer main. CC-GRP pipe systems are nonconductive, corrosion resistant, undergo hardly any thermal expansion, and have a long service life. Furthermore, the product’s low weight reduces the load on the bridge and facilitates pipe handling inside the marine. These advantages are what convinced the client to choose HOBAS CC-GRP instead of other available materials.
Another 4 km of CC-GRP BridgeLine® DN 300-400 and fittings were installed for the drainage of the Homeland Bridge. The third and final section of the line leads from New Zagreb to the Homeland Bridge. This 3 km long section runs through a water protection area with Zagreb’s springs which meant that the high-quality pipes needed to be protected in a watertight concrete channel.
Concrete supports hold the line every 5.8 m where it is fixed with stainless steel straps. An 8 mm thin EPDM layer was applied between supports, straps and the pipe to allow for small longitudinal movement due to temperature variations and to evenly distribute the pipe weight over the supporting areas.
The longitudinal stiffness of CC-GRP pipes both saves time and money as it minimizes the number of necessary supports. The easily jointed push-to-fit couplings contributed to the savings since no welding is required for HOBAS products. All sections of the pipeline were tested with up to 9 bar of pressure. This was quite demanding for the manholes because the DN 800 blank flanges fixed to the tees with no additional support had to withstand 477 kN (equivalent to 48 tonnes). Pressure tests on the pipeline inside the bridge were also hard to conduct because the bridge temperature kept changing, thus causing the length of bridge to vary constantly.
The project was highly demanding for all parties involved. However, HOBAS specialists were present at all stages, providing support to the design engineers on a number of details. Site advisors helped the contractor during pipe installation, deliveries were made on time, and on-site changes were implemented with flexibility to successfully complete the project.