Dorfnest at Kloten overpass

According to the call for tenders, the rail bridge was to be constructed with an "overhead falsework". This refers to an auxiliary construction which is installed during bridge construction works, which significantly limits the free space underneath the new bridge. The concrete formwork for this is suspended from suspension rods and then the bridge superstructure is installed. This solution requires an intermediate stage above the two columns and short edge stages. The intermediate supports can only be completed after tensioning of the whole bridge structure. As an efficient alternative, PORR recommended a "conventional, underlying rolled girder support with integrated protective tunnel" - a suggestion with which the client agreed. With this variation, no intermediate support is necessary between the platforms and no additional protective tunnel is required. The formwork, reinforcing and concreting works can be carried out "conventionally" with tower cranes. The entire construction process was to be overseen closely with a view to quality and schedule.

Numerous preparatory and accompanying measures were necessary for the implementation of the overpass construction scheme together with the two-track expansion. For one thing, closure of the platforms was essential. In the normal course of events, such closures would last for five hours at a stretch. For the assembly and disassembly of the protective tunnel and falsework, total closures of both platforms were announced. Another factor was the particular attention paid to the safety of all participants. To ensure this, slow traffic lanes were established on the construction site and extensive constructional safety measures such as protective fencing barricades along the platforms, protective scaffolding around the crane, a protective tunnel around the falsework and platform security measures were put in place.

A new 600m-long access road to the overpass construction was created. In addition, a line for the movement of the more than 50t drilling apparatus was established, primarily with an earth support shell made from hanging material prepared on-site, and the 175 drill piles with a diameter of 1,000mm were drilled out along this line, with piping. After the gradual excavation of the bored piles, the 350m-long pile row was coated with jetcrete. A more than 200m-long angled retaining wall was constructed between the bored pile wall and the abutment of the overpass structure, which served to delineate the required fill on the platform side. The extensive embankments on either side of the overpass structure were, for the most part, completed with the previously excavated earth, which was prepared on-site. A total of 27,000m³ of earth was excavated and just about half of it was used for the embankment construction and backfilling of the abutment area. The preparation of the excavated earth by adding missing components, and the re-use of this earth on-site, reduced the need for truck cartage by approx. 35%.

The entire project took place with ongoing rail operations, in order that rail connections on the heavily-used line should be maintained as usual. Elements of the works, therefore, were carried out during the short overnight break period.

Conventional construction process

The foundation of the overpass structure consists of two bridge columns with conical buttress heads, on which 16 piles with a diameter of 1,200mm have been constructed. The actual overpass structure was constructed in three sections - with lengths of 65m, 65m and 45m - in the usual sequence of "end-middle-end". The bridge superstructure consists of a total of 76 units of 4m-long formwork for the external trough walls, laid down on both sides of the floor formwork and attached to the falsework. The formwork units were already successfully established on another object and had to be individually adapted to the purpose.