The block dam is an important element in the construction of the hard seawall. With some 20,000 2.5-m³ concrete blocks, each weighing over 40,000 kg, the structure will remain unaffected by even the strongest waves. The blocks will come from the block dam that previously protected the existing Maasvlakte. They will be picked up by a floating crane (backhoe) and transferred overland to the construction site. After the quarrystone has been put in place, the Blockbuster will subsequently position the concrete blocks on top of the structure. Most of the block dam will be below the waterline. The waves will only break on and over the block dam during heavy water. To allow the contractors to build up a new block dam with the available number of blocks across a length of 3.5 km, a new method of stacking was devised. By positioning them more intelligently, the dam will require fewer blocks. This measure was necessary because the existing block dam needs to be replaced by a longer version.
A specialised land-based crane has been developed to carefully position the blocks in the block dam. This Blockbuster has a lifting capacity of 50 tonnes and can reach 63 m from the heart of the crane. And at this distance, the crane can still position the blocks with a precision of 15 cm.
The Blockbuster is a modified E-crane produced by manufacturer Indusign. The standard version of the E-crane – with ‘E’ standing for equilibrium – could not reach far enough. The seemingly simple solution proved to do the trick. By placing the moving section of the crane, the turret, on the main beam, which extends sideways, rather than in the centre of the track frame, the crane can extend to the required 50 m. A 360-tonne counterweight, formed by nine concrete blocks, provides the required balance. The track frame also needed to be adapted. Three double sets of caterpillar tracks ensure that the crane is sufficiently mobile.
In February 2010, work started on building up the crane on site. This was when the track frame’s first heavy connecting beam was hoisted on to the caterpillar tracks and the main beam was placed in position. This was followed by the counterweight, after which the main boom and the jib were fitted to the crane. In the summer of 2010, the first tests could be run and the contractor could gain experience working with the crane. PUMA has tested a number of grabs: a special grab was developed for the 40-tonne concrete blocks and the large chunks of riprap are positioned with the aid of a multi-blade grab. In this period, the contractor also extensively tested the precision of the crane and the survey equipment that measures the activities of the Blockbuster.
Furthermore, during the test period, the contractor also instructed a number of experienced crane operators, so that they can safely operate this exceptionally large crane and handle these extremely heavy loads. To complete the test phase, the operators worked on the ‘blind’ construction of a test section: using only the information on their screens, they constructed 50 m of block dam according to strict specifications. Two-thirds of the dam will be constructed below the water’s surface. This is why when placing the blocks, the operators rely on acoustic underwater cameras and an advanced version of the crane monitoring system.
In October 2010, the Blockbuster was driven onto the new land. It initially worked on the construction of a temporary structure protecting the hard seawall in the making. This was intended to limit, as far as possible, the washing away of sand and rocks during the 2010/2011 stormy season. Since 2011, the blocks are positioned in the definite profile from this strip.
The weight of the Blockbuster itself is some 1,200 tonnes; it has a 360-tonne counterweight and is approximately 30 m tall. Working at full capacity, the crane can position some 15 concrete blocks per hour – mostly under water. The Blockbuster will be working on the hard seawall throughout 2011 and 2012.