Making space: the LP4 project in Cuxhaven
The Liegeplatz 4 (LP4) project in Cuxhaven, Germany, consists of an extension of 85,000m² of new land adjacent to the existing Liegeplatz 3 (LP3). In May 2017, the DEME Group’s German subsidiary Nordsee Nassbagger- und Tiefbau GmbH completed the reclamation part of the LP4 project.
Nordsee is one of the companies within the joint venture that has carried out the LP4 project on behalf of Niedersachsen Ports (NPorts), the owner and operator of 15 ports on the German North Sea coast, including the port of Cuxhaven. The other partners are the German construction companies Bilfinger Marine & Offshore Systems GmbH (Bilfinger) and TAGU Tiefbau GmbH Unterweser (TAGU).
There were two main reasons for NPorts to initiate the project:
- to create extra space and allow for the future growth of the port’s roll-on roll-off (ro-ro) sea freight services – ro-ro traffic is an important activity in Cuxhaven, in particular for the export of cars. This activity has reached such a level that it is expected that there will be soon a shortage of space if no action is taken
- the efficient handling of offshore wind turbines – the offshore wind industry is becoming increasingly important for Cuxhaven. Siemens, for example, is building a new, large-scale wind turbine factory in the port.
A challenging reclamation
In August 2016, construction activities began with the ramming of the first piles of the sheet pile wall that constitute the new 290-metre-long quay wall. For the reclamation, some 1.1 million m³ of sand was used, which was dredged from several locations in the estuary of the Elbe river. The reclamation itself was challenging, not only from a geotechnical point of view, but also with respect to the coordination with the other activities on site.
Although the expansion has been designed to have a final level of between 6.25-6.75 metres above Normalhöhennull (NHN, the German reference level), a surcharge of several metres had to be applied during construction in order to speed up the settlement of the subsoil. Taking into account the surcharge, a level of 10.5 metres – and in some places up to 12.5 metres – above NHN had to be reached.
Realising a reclamation of such a height in an area with soft subsoil – and doing so within a restricted time period – contains high stability risks. The fact that this operation has been completed successfully and in a lesser time span than originally scheduled is largely to the credit of the DEME geotechnical engineers, the crew of the trailing suction hopper dredger (TSHD) MARIEKE, and the staff on the land reclamation site.
Smooth rainbowing
After the sheet pile wall had been completed, the MARIEKE started rainbowing sand behind this wall. Due to the soft subsoil, this had to be done carefully in small layers and with gentle slopes. In order to achieve this, the vessel was equipped with a double rainbow nozzle. Both nozzles were placed at different angles, allowing the ship to rainbow during low, as well as high, tide in the most efficient way.
During the rainbow operation, the MARIEKE didn’t stay in one place, but moved along the entire length of the quay wall. This allowed the vessel to optimally divide its load of about 3,600m³ of sand over the entire length of the reclamation site and spread it in equal and small layers. This operation required a high degree of craftsmanship by the vessel’s crew and close follow-up by the project’s survey team, using multibeam equipment.
Once a level of two metres above NHN had been reached, the MARIEKE shifted to pumping ashore through a floating pipeline. During the rainbowing and pumping ashore operations, the stability of the reclamation was constantly monitored by DEME and Nordsee’s geotechnical specialists. Besides the monitoring of the increment of the shear strength, the pore water pressure was also carefully observed.
Although it was taken into account that one or more pauses would be needed to allow the pore water pressure to fall, the monitoring results and subsequent calculations showed that even at the most critical level of seven metres above NHN, it was safe to continue the reclamation without a halt in operations. This resulted in the most efficient operation without any idle time for the MARIEKE.
A successful conclusion
While the reclamation work was being carried out, several other parts of the project took place simultaneously, including work on the quay wall by Nordsee’s joint venture partners Bilfinger and TAGU, and the installation of vertical drainage. This constituted another challenge for the project management team, involving meticulous planning and constant coordination to avoid the different activities interfering with each other.
After the completion of the reclamation, the civil works partners continued with the construction of the quay wall, including the installation of concrete piles and a concrete slab that will constitute the foundation of the quay surface. The project as a whole was successfully completed by early 2018.
