During the interview with chief contractor Sander Tetteroo of De Groene Boog, one of the biggest challenges for the tunnel formwork was already discussed: connecting the walls to the tunnel floor. Due to the underwater concrete, the height of this connection is very inaccurate. Deviations of +/- 75 mm are not uncommon. Nevertheless, the formwork must seal this area properly. Hendriks engineer Martin den Hengst explains how this was achieved during an interview we had with him.
But first, let's return to the task at hand: a 2,235-metre-long tunnel with a central tunnel channel that had to be poured in 25-metre sections in a weekly cycle. That's quite a mouthful, which also illustrates the complexity of the challenge. With two sets of formwork, 90 pours were required, with surface assessment class B1 still having to be achieved after the final pour. This meant that the formwork to be developed also had to offer sufficient space to allow the formwork to be thoroughly cleaned and re-oiled after each pour in preparation for the next pour. "De Groene Boog's request initially led to an explosion of creative ideas, particularly with regard to ensuring a good connection to the uneven surface,"
Martin opens the conversation. "We quickly came up with a design for a kind of 'hydraulic caterpillar' that moves with the height differences of the tunnel floor, as it were. The formwork itself consists of a mobile tunnel formwork for the central channel, which is intended as an escape and service route, with two mobile half-tunnel formworks attached to it."
To ensure that the wall of the formwork is completely sealed against the floor, the lower part of the wall is constructed from 1.50-metre wall panels. Martin den Hengst: “The panels are fitted with a hydraulic cylinder at the top, which allows each panel to be adjusted by approximately 150 mm in height. At the bottom, each panel is fitted with a rubber strip that is enclosed in a tube. The panels themselves fit together hollow and tongue-and-groove so that they can move freely in relation to each other." Martin explains the process from initial sketch to detailed concept: "It starts internally, of course, but the customer is quickly involved in the further development. By working out the design in close consultation with each other from an early stage, the rest of the process runs much more smoothly. You notice that you reinforce each other. It's always good to be able to bounce ideas off each other in this way."
This sparring also proved useful in the final execution of the centre pin holes. "With the large amount of reinforcement used in the walls, it is always a question of how easily the centre pins can be inserted. It often happens that the reinforcement on the inside makes it difficult to insert the pin at the exact spot where it is planned. That is why we designed the centre pin holes to be flexible. The weaver only needs to keep a horizontal strip free at the height of the centre rings. With our flexible pin holes, the pin can then be moved 100 mm horizontally.”
The panels were developed for a concrete pressure of 60 kN/m2. A noteworthy feature is the solution that allows the concrete to be compacted after pouring. Normally, vibrating needles are used for this purpose, but these are not possible due to the high 'reinforcement density'. "The wall panels are therefore equipped with vibration motors," says Martin den Hengst. "These shake the formwork in a horizontal direction, allowing trapped air to escape from the concrete."
With a one-week cycle, it is important that no time is wasted unnecessarily. Once the concrete has hardened, the wall panel of the half-tunnel formwork can be pulled inwards by approximately 1 metre using hydraulic sliding frames. "This gives staff enough space to thoroughly clean and re-oil the formwork quickly for the next pour. The outer walls can be easily positioned using the crane. This means that no valuable time is lost."
Rails are used to move the centre tunnel channel and half tunnel formwork. These are adjustable to compensate for differences in height in the tunnel floor. The portals are equipped with steel double flange wheels driven by hydraulic motors. In the central tunnel channel, a mobile table formwork on heavy-duty wheels is also used, which is moved forward by a hydraulic winch. For sealing at the top, the table formwork is equipped with hydraulically extendable panels. Horizontal hydraulically operated extension strips are used to connect the deck of the half-tunnel formwork to the concrete wall of the central tunnel channel.
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