Annual report 2019

23

“THE CAPABILITY TO BUILD WITH CHALLENGING SOIL BALANCES IS IMPORTANT FOR THE FUTURE OF OUR INDUSTRY”

scale of the project involves a particular beneft for her and many other staff colleagues: “As technical designers, we are expected to be multidisciplinary on most Boskalis projects. But given the duration and multiple facets of this project, we have the opportunity here to focus on our own specialism for a while. That allows us to make a signifcant contribution to the project and, at the same time, to deepen our professional knowledge.” GIGANTIC CAISSONS The Design & Construct project is for a port area measuring 405 hectares that will be enclosed by 9.1 kilometers of quay walls consisting of gigantic building blocks known as caissons. Measuring 40 x 20 x 30 meters, these caissons – which are being produced in a massive casting facility on the project site – are some of the largest ever built. “After completion, they are hydraulically transferred to a floating dock and sailed to the unloading location. Eventually, tugs position them in the installation location and then sink them. Each individual cell is then flled with sandy material that is vibro-compacted.” SOLID FOUNDATION “A major challenge in the construction of the port area is the soft subsoil here,” Lescinski continued. “To establish a solid foundation below the caissons, we frst construct deep sand keys, up to 40 meters deep. We use all the dredged material as fll for the terminal site. We have installed a large containment bund to stop the material spreading. With a height of thirty plus meters, the containment bund is the backbone of the land reclamation work and a robust environmental mitigation measure. We are using about 13 million cubic meters of the best quality of sand to construct the bund, which was about 3/4 complete end-2019. Once the sand key is backflled with sand and vibro-compacted, we will then be depositing, and compacting, a layer of rock on top of

the sand. The next step is to position the caissons, which surround the terminal and serve as a quay wall, on top of this rock base. Once the caissons are in place, of which 27 are thus far installed, we put in geotextile and scour protection.” FILLING THE PORT AREA A total of about 130 million cubic meters of material will be needed on the project. “The amount of clay we dredge from the sand key is much less than what is needed for the construction of the entire terminal. The area will also be flled with clay and silt from various civil-engineering projects in Singapore,” Lescinski said. “The composition of that material can vary. Due to the different settlement properties, each type of material must be applied in the right location. We are responsible for the registration and management of all these flows and that entails sophisticated data management: we expect to deposit some 40 million cubic meters of this type of material over the next few years alone. That will result in a 25 to 30 meter thick clay fll, with up to 18 meters of sand overlying during surcharging.” UNSUITABLES “At one time, we would have classifed the soft soils used for the construction of the terminal site as unsuitables,” Lescinski continued. “The decision to use them is a direct consequence of the need to reduce reliance on sand and global benefcial re-use trends. Singapore is certainly playing a pioneering role in this respect,” Lescinski said, who expects a similar approach to be used more often in the future. “Sand is becoming an ever-more valuable resource globally, after all, and there are more and more restrictions for disposing incompetent material offshore. The capability to build with challenging soil balances is important for the future of our industry. The more new land you build, the more often you will have to resort to groundbreaking solutions of this kind.”

ANNUAL REPORT 2019 – BOSKALIS