2,100 m3 dumped in one weekend

Steel fiber underwater concrete floor for RIF010

Remediating and excavating a wet construction pit and then pouring an underwater floor with steel-fiber reinforced concrete is not an unusual job for Verboon Maasland. The place is: a canal in the heart of Rotterdam where RIF010 is being built. A nice job for Tom Maaskant, project leader for Specialized Projects at Verboon Maasland, who is responsible for a clean and solid floor for this wave pool. This was done in close cooperation with main contractor Mobilis, foundation company Kandt bv and diving company DISA International.

Verboon Maasland could get to work as soon as Kandt had completed the sheet pile walls of the wet construction pit. Maaskant: "The first part of our job was to remediate the dredging layer on the water bottom. Using pincers, we removed and disposed of a total of 1,200 cubic meters of contaminated dredge under remediation conditions from the cofferdam. The peat and clay layers in the construction pits were excavated completely separately on GPS, after which the boxes were cleaned and the bottom was made silt-free. The latter was a major challenge because there was no possibility for a wet depot in the area. Due to the skill of our people, this was eventually accomplished within the schedule. During the excavation work, a DISA International diving team performed soil and civil inspections on our behalf. Not an easy job because visibility was only 30 cm. In between, archaeological samples were also taken, a mandatory part in these parts. Fortunately for us, we did not run into an old city wall."

Sprinkled in one passage

To save time, the excavation of the construction pit was done in phases and in close cooperation. Maaskant: "On the side of the Market Hall, where the embarkation point for the cranes was, we started excavating. After there was enough safe space, Kandt started turning the piles behind us. With good coordination, you don't get in each other's way, even with large equipment in such a small workspace as this construction pit. With part of the piles in place and part of the pit shovel clean, we could use the diving team to clear the piles of grout residue and make them spotless. With that, the construction pit was ready for gravel spreading. The 30-centimeter thick gravel layer ensures that any silt residue is salvaged and cannot mix with the underwater concrete, as this is detrimental to the concrete quality. The gravel was spread using a wide bucket with the same machines used to excavate. During spreading, manual sounding was done with a sounding board to have immediate results. Thus, the entire construction pit was spread in one pass within the tolerances. That was not easy with a soil with slopes and corner twists."

Digital terrain model

Before Verboon Maasland could begin pouring, divers first measured all pile heights and pile positions and Verboon used a multibeam system to convert the gravel floor into a digital terrain model. Then a number of reinforcing strips of rebar were welded to piles and anchors were welded to sheet pile walls underwater to ensure that the concrete could not tear away from the sheet piling. In addition, sheet piles were spot-cleaned with a high-pressure sprayer at the point of contact with the underwater concrete floor for best adhesion. 

Steel fiber concrete

"And only then, once you've checked off the entire checklist, are you ready to pour the underwater concrete floor," Maaskant said. "Which began at four o'clock on a Friday afternoon and lasted until just after midnight on Saturday. 2,100 m3 of steel-fiber concrete in just under a day and a half. This concrete has strength class C30/37 and contains 30 kg/m3 steel fibers. You can pour a maximum of 80 m3 of this per hour, otherwise you run the risk of the steel fibers getting stuck in the pipes. Thus, the 75-centimeter-thick floor was poured within tolerance in one go."

"For us, the work was then over," the project manager concludes. "The post-measurement monitors showed that the concrete was maturing as expected. We are proud that we were able to put a strong foundation under this special project in this challenging environment."