The existing Maasvlakte used to be protected to the south of the Maas channel by a block dam. This dam was constructed from quarrystone and concrete blocks. A total of some 2 million tonnes of riprap and over 20,000 concrete blocks were incorporated in the dam. Due to the construction of Maasvlakte 2, the existing Maasvlakte block dam will no longer be required. Once it has been dismantled, the 2 million tonnes of riprap and a total of 20,000 blocks will be recycled in the construction of the hard seawall for Maasvlakte 2. Only a few blocks are expected to fall short of the requirements set for recycling in the new seawall.
The hard seawall that will soon be protecting the north-western section of Maasvlakte 2 against the onslaught of the sea will be longer than the old Maasvlakte block dam. An ingenious technical design enables the contractor to erect a seawall that, although it is longer and uses fewer blocks, is at least as reliable as the old dam. The construction of this new seawall is in full swing.
The concrete cubes weigh over 40 tonnes a piece and measure 2.5 by 2.5 by 2.5 m. During the construction of the existing Maasvlakte, these colossal ‘dice’ were loosely dumped into the sea, so that they lie completely jumbled up on top of each other and on the beach. Picking up the blocks requires technical ingenuity. A large crane called a backhoe with a pontoon base does the job.
To pick up the blocks, the crane uses a special 'ripper grab'. This grab is up for the challenge of grabbing these massive blocks from every conceivable angle and raising them to the surface. The right combination of pressure and grip is vital in this context. Along the way, the desired oil pressure was determined for this operation and modifications were performed to ensure optimum grip. The majority of the blocks are located under water, with another share lying in the sand. Therefore, besides grip and pressure, sight is also very important. That is why the operator uses special survey equipment. An echoscope shows the operator the position of the block and the ripper grab below the waterline, in real time.
The positioning of the blocks in the new seawall is an extremely precise affair. The innovative and technically demanding design of the block dam requires the blocks to be meticulously placed. The PUMA consortium has developed a specially adapted E-crane for this purpose. This 'Blockbuster' places the blocks with a precision of 15 cm in their required position. Thanks to special survey and monitoring equipment, this can even be done without a direct view of the construction site, during bad weather and under water.