The Port of Rotterdam is currently undergoing a massive extension called Maasvlakte 2. During the first phase 700 hectares of new port area will be created. A combination of hard and soft sea defences in the North Sea will protect the new port from the elements.
To realise this massive project, innovative design, techniques and equipment were necessary. A design was developed to ensure that wherever possible the sea defence would be soft, since a hard seawall is more expensive. One of the innovative design features of the soft sea defence is the use of Pleistocene sand. In other areas a hard seawall was needed and large quantities of rock fill were necessary. This demanded other innovations and the development of specialised equipment. For instance, although the multi-beam system has been an accepted method of surveying rock layers for quite some years now, Project Organisatie Uitbreiding Maasvlakte (PUMA) and Port of Rotterdam agreed that new research was necessary to quantify the differences between conventional survey techniques and the multibeam. This article describes some innovative survey techniques and equipment used at the Maasvlakte 2.
The Port of Rotterdam is currently undergoing a massive extension called Maasvlakte 2. Between 2008 and 2013, 240 million cubic metres of sand have been deposited from which 210 million m3 is dredged from a sand extraction area in the North Sea and around 30 million m3 comes from dredging the port basins and the Yangtzehaven. During the first phase 700 hectares of new port area will be created (Figure 1). A combination of hard and soft sea defences in the North Sea will protect the new port from the elements.
Beach and dunes with a length of 7.3 km form the soft part of the sea defence. The 3.5-km-long hard sea defence comprises 7 million tonnes of rock and around 20,000 concrete blocks weighing 40 tonnes a piece. PUMA (Project Organisatie Uitbreiding Maasvlakte) formed by Netherlands-based dredging companies Van Oord and Boskalis are responsible for the design, construction and maintenance of this immense project. The economic importance of the port extension is significant. The area added to the port in the 1970s, Maasvlakte 1, has virtually no room left for new companies and existing clients that wish to expand. The Maasvlakte 2 project will contribute to keeping the Port of Rotterdam in its current position as Europe’s most important port. With the construction of the 20-m-deep harbour basins and access channel it will be ready for the container ships of the future.
The design for the soft and hard sea defences was calculated to withstand a once-in-10,000-year storm with a wave height of 9 m coming from the north to north-west (348º) and duration of 12 hours (Figure 2). Wherever possible the sea defence would be soft, as the cost of a hard seawall is considerably higher. Still, during the execution of the project the design of the hard sea defence was further optimised, which resulted in lower construction costs. Model tests were carried out to verify the stability under various circumstances.
To prevent unsafe situations for shipping in the port entrance, criteria were set to the flow conditions for both the construction phase and the final layout. To analyse and quantify the effects, a current model was developed that can simulate the current in existing and future situations.
Soft sea defence
One of the innovative design features of the soft sea defence is the use of Pleistocene sand. By using this coarser grain of sand a steeper foreshore can be constructed, which requires less sand. The sand will be extracted till approximately –40 m NAP which is around 20 m below the current seabed level. This deep extraction method will limit the area affected by the construction activities. The row of dunes adjacent to the wide beach varies in height +10 to +13 m NAP. The new beach will provide room for recreation while the dunes will offer a lively habitat for nature.
Hard sea defence
The hard sea defence on the north side of Maasvlakte 2 was selected from a number of alternatives and comprises a so-called “stony dune with a reef of blocks” (block dam). The core consists of sand which is divided in two types. Finer sand (approximately 150 mu) located in the deeper part is covered by coarse sand (> 370 mu) in the upper part. Under the reef the sand is covered with filter material (0.3-35 mm). Next a layer of cobbles (20-135 mm) is placed. In the stony dune area the cobbles are placed directly on the sand creating a cobble beach with a 1:7 slope approximately 4 m thick. Under the concrete blocks two more quarry stone layers can be found: First 5-70 kg rock, secondly a top layer consisting of 150-800 kg armour rock.
A total of 7 million tonnes of quarry stone are required. Some 2 million tonnes of this rock are recycled from the existing block dam of Maasvlakte 1. In the surf zone 40-tonne concrete blocks are placed that measure 2.5 by 2.5 by 2.5 metres. In order to accurately place these blocks the development of a unique crane called the Blockbuster was required. To prevent the blocks from shifting, a toe construction of 1-10 tonne rock has been placed on both sides of the concrete block formation. The large scale of rock and concrete blocks that have been reused from the Maasvlakte 1 block dam contributed to a cost reduction and made the design also sustainable.
DETERMINING ROCK QUANTITIES
For the Maasvlakte 2 project a large amount of rock fill is needed to be placed above and underwater within relatively thin layers and small tolerances. The standard method for surveying rock levels is described in the manual on the use of rock in coastal and shoreline engineering (CUR 154) and requires the use of a semi-spherical foot as a reference. The greater part of the quality assurance on the sea defence works will be based on data acquired with a multibeam echosounder. The multi-beam system has been an accepted method of surveying rock layers for quite some years now. Research in the past pointed towards lower volumes being detected using multibeam surveys as opposed to the semispherical foot.
PUMA and Port of Rotterdam agreed that new research was necessary to quantify these differences – in particular for the rock grades that will be used for the Maasvlakte 2 project.
Source: Terra et Aqua, Number 131, June 2013