Complex reinforcement puzzle for construction pit The Ensemble

In Amsterdam Southeast, construction is underway on The Ensemble, two "skyscrapers" with 592 rental units and approximately 20,000 m² space for offices and social and commercial facilities. DISA International was commissioned by combination HeSto to create the complex reinforcement puzzle in the construction pit by, among other things, managing the immersion operation. 

In March 2022, The Ensemble project was officially launched. HeSto, a combination of Gebr. van 't Hek and Roel van der Stoel, was commissioned to realize the construction pit. "This was partly due to our previous experience with complex works in underwater concrete with immersion reinforcement," says René Brouwer, business manager at Roel van der Stoel. "Because of the integral nature of the cofferdam, we brought together various disciplines at an early stage. For example, together with our client Cordeel Amsterdam and their structural engineer ABT, we further worked out the configuration of the immersion reinforcement, with DISA International contributing with their extensive experience in that field."

Sawing posts

The construction pit is composed of 140 meters of sheet piling AZ20-700 (final) and 170 meters of AZ26-700 (temporary) with a plank length of 14 meters. The grid further consists of 49 Hek combi piles around 530/650 gi with an octicon 430 core to 26 meters below NAP and 628 GEWI anchors around 50/200 to about 35 meters below NAP. The construction pit was initially dry excavated from ground level to about 6.50 below NAP. Then the pit was flooded and the final depth was reached from pontoon sets to minus 12 meters NAP. "DISA International's diving team cleaned up the sheet piles, Hek combi piles and GEWI anchors during the excavation process," explains René. "After the clean-up process, the diving team, using Roel van der Stoel's advanced pile/wire saw, sawed off and removed 290 pieces of existing precast piles vk40-45 to below the future underwater concrete." A gravel layer was then applied as soil improvement. 

Beam Grid

Before we could install the immersion reinforcement, we worked with HeSto to place a 2,500-square-meter beam grid in the tub underwater, says Marc Veldhuizen, Dive Supervisor at DISA International. "Four beams were also placed in the pedestal for each of the 50 piers." The beams were necessary because the immersion reinforcement is not allowed to lie directly on the bottom; concrete still needs to be placed between them (concrete cover), adds René. "So the accuracy of placement was very important, with a tolerance of 0 to 20 mm." The immersion reinforcement, in turn, was largely assembled on the construction site. There, too, dimensional accuracy was a major concern. "Recesses had to be made per basket for the existing piles and GEWI anchors, and these had to be measured out exactly." 

Limited vision

The immersion reinforcement consisted of 148 reinforcement baskets for the two towers, ranging in size from about 10 to 15 by 4 meters, weighing about 6 to 9 tons each. "These baskets had to be placed with great accuracy, with the horizontal tolerance being 0-50 mm," explains René. "The baskets were stacked in four layers on top of each other and placed using 200-300 tonne telescopic cranes." According to Marc, a complex puzzle. "Sinking rebar is a routine job for us, but this design was quite challenging with a lot of rebar overlap. Similar in that respect to our contribution to the A24 Blankenburg connection and the A16 Rotterdam, the Green Arch. It was a precision job, and that underwater with only 30 to 50 centimeters of visibility." A total of about 850 tons of reinforcement was accurately placed between the piles and GEWI anchors as part of the structure. Subsequently, approximately 6,800 m³ underwater concrete poured.

The construction pit of The Ensemble is now dry. HeSto and DISA International look back on a successful collaboration. "In the past we have successfully completed several projects, such as Valley in Amsterdam and the Haarlemmermeer Peak Storage. Now, together with Roel van der Stoel, we are creating a construction pit in the heart of Rotterdam."