Constructing The Green Connection with and without the use of an intermediate support

Abstract

In the Netherlands, a lot of traffic congestion occurs on motorways. This problem is most severe nearby larger cities. Utrecht is one of these cities. To improve traffic flow in this area, a huge masterplan is designed by Rijkswaterstaat called “A27/A12 Ring Utrecht”. One part of this masterplan consists of the motorway A27 at Amelisweerd. Here, the A27 is situated in a U-shaped concrete structure and must be expanded at both sides. Across the motorway a deck structure is going to be constructed with on top a public garden. This structure spans the total width of the A27 for 249 meters and is called The Green Connection.
According to the original design of Rijkswaterstaat, this deck structure should be realized with an intermediate support. It will be advantageous to omit this structure, since it has a complex execution method. Therefore, it’s investigated if The Green Connection can be realized without the use of an intermediate support.
Then, the execution aspects of the original design will be discussed more thoroughly. The first challenge is constructing the extended parts which is explicated according to 11 main tasks. These tasks seem to be relatively straightforward to execute. After realizing the extended parts, an intermediate support must be constructed. It is found that the existing foundation lacks bearing capacity by far. A new strengthened strip foundation with extra foundation piles must be realised in the middle of the motorway. Due to the boundary conditions (such as the water pressure beneath the structure and permanent drainage is prohibited), the only possibility left is to construct small building pits, compartments, within the existing structure. Such a compartment has a rough length of about 20 meters, will be about 6.5 meters wide and must be constructed 13 times.
Thereafter, the deck structure should be assembled. Three alternatives in methods of assembly are outlined and discussed with the help of the same key-words. All three methods could be realized. But, it’s important to indicate that with some extra investments, the remaining space for traffic could be maximized during assembly.
After discussing the execution aspects of the original design, the technical feasibility of the single span deck structure was investigated more thoroughly. In the Preliminary Study was deduced that two structural designs seem to be a possible solution in constructing The Green Connection. It turned out that the box beam design seems to be advantageous. Although this judgement is substantiated with preliminary calculations and an overall execution plan, it still required more research. Therefore, a reliable structural design is performed for a 75-meter span beam which can be used as a single span deck structure. It does exceed the boundary condition of 280 tons which was posed initially with 8%. However, no optimizations have been applied to this design. If the enumerated optimizations are performed, a beam can be designed according the boundary condition and possibly even less.
When the original design of Rijkswaterstaat is compared to the single span design, the differences are quite straightforward. In essence, the question arises whether the extra money of constructing a single span deck structure outweigh the money which can be saved by leaving out the intermediate support and constructing the extended parts 25% narrower.
Rijkswaterstaat has performed a design with an intermediate support. In this thesis, the feasibility of this design is investigated, and in particular this support. The only possible method of execution in realizing the support is upon condition that a temporary applied drainage system will be feasible and approved by the authority. When the authority states that draining ground water is prohibited, the original design isn’t feasible anymore. In that case, the single span design isn’t just an alternative to the original design, but is the only feasible solution.
Concludingly, providing the applied principles of this thesis, it is strongly recommended against constructing an intermediate support within the existing U-shaped concrete structure. Since the structural reliability of a 75-meter span beam is proven, an intermediate support wall is redundant. Therefore, the single span design is less risky, less time consuming and less expensive compared to the original design.