When port traffic in Rostock more than halved in the aftermath of the German reunification, the old Hanseatic town near the border of Poland decided to make a new start, to invest in modern port infrastructure, and to create additional land for attracting new companies. It soon proved a successful strategy, and demands from companies such as Liebherr geared up expansion plans.
The extension of Pier III is part of a major revival programme by the ‘Hafen- Entwicklungsgesellschaft Rostock mbH’, our client. The development company called for tenders according to specifications and design targets. The German DEME subsidiary, in a joint-venture with civil contractors, won the contract – thanks to a cost-saving innovative execution method that got the assent of the client. In developing this alternative method and as a consequence reducing final costs, support from DEME’s geotechnical department has been of crucial value.
Two specific challenges made this project a very complex one. In the first place, there was the presence of a soft subsoil, consisting of organic silt – 50.000 m³ of which had to be dredged. However, this could not be dumped into the Baltic Sea, a semi-inland sea with a delicate environment and a critical oxygen balance. Secondly, some 80.000 m² of geotextiles would have to be installed on the soft subsoil underwater, according to the initial plans. These challenges were overcome when DEME engineers developed and implemented an alternative working method.
The northern extension of Pier III was a complex project, for the challenging soil conditions, the various disciplines involved, and the many techniques that were applied: demolition of the existing northern wall; sheet-pile wall construction; dredging to obtain minimum depth and moving slurry-containing sediments; reclamation of the extension area with more than 1 million m³ of sand; backfilling; slope protection; installation of vertical drains; earth-moving works and placement of revetment. Within the joint-venture, DEME was responsible for all dredging and land reclamation, as well as the earth-moving and consolidation techniques.
The existing Pier III was extended by 1 km, by means of constructing an anchored sheet pile wall. A backhoe dredger then began dredging, with the material carried off by split barges. Behind the sheet piles, a zone of 11 ha was reclaimed with sand that had been dredged at sea.
The reclamation works were executed both by a diffuser pontoon, and by rainbowing and hydraulic reclamation by means of pipelines. Several trailing suction hopper dredgers (TSHD) in the 4.000 to 6.000 m³ class were assigned for winning the more than 1 million m³ of sand. It was dredged in a dedicated borrow zone in the Baltic Sea, at a transport distance of 17 nautical miles. Optimization and cost savings could be reached by innovation and an intelligent dredging management.
The bold approach, agreed upon after thorough research and in full agreement with the joint-venture partner and the client, made it possible to avoid the use of geotextiles after all. The soft subsoil was stored in a dedicated and controlled area. In order to facilitate consolidation, a total of 1,2 million m of vertical drains were put in the reclaimed ground. Finally, some 340.000 m³ of overload was placed to accelerate setting, a volume that was removed by dumpers in a second phase.