More than 1,000 t Post-Tensioning Systems installed in the Reppisch Valley Ventilation Center

DSI as partner of the JV Spannarbeiten, Uetliberg Tunnel, Zurich West By-pass, Switzerland

The 4.4 km long Uetliberg tunnel is a key structure for the new 10.6 km long Zurich West By-pass. In addition, the Zurich West By-pass closes a significant gap in the Swiss national road network between the A1 (Zurich-Bern) and the A3 (Zurich-Chur) expressways.

The entire Uetliberg tunnel consists of two parallel tunnel bores each underneath the Uetliberg and Ettenberg hills, one for each driving direction. In the valley between the two hills passes the Reppisch creek. This near-surface site of the Uetliberg tunnel in the Reppisch valley was used to build a ventilation center in an open cutting transverse to the valley. Following completion of the structure, the original natural scenery will be restored by completely covering the ventilation center. The recultivated bed of the Reppisch creek will then be positioned exactly above the center. The only thing that will indicate the existence of the ventilation center underneath is an open access road.

With a length of about 200 m, a width of about 55 m and a total volume of 180,000 m³ (according to SIA) the ventilation center will be significantly larger than "normal" ventilation centers. These dimensions are necessary for the planned extended functions of the structure over a total of four levels. The bottom level below the driving lane level will accommodate supply lines and cable trays for the power supply of the Uetliberg tunnel as well as sewer pipes for the tunnel drainage and fire hydrant supply pipes.

The subterranean space provides for a possible interchange of traffic between the two tunnel bores at the second level, the driving lane level. It is this level that is principally responsible for the size of the ventilation center. In addition to the two driving lanes and one emergency lane in the tunnels, the width of the ventilation center will be extended by an additional emergency lane.

The level above the driving lanes will accommodate the actual ventilation facilities. In case of fire, smoke and fumes can be removed from the driving space by suction and purged from the vent stack situated on the Ettenberg hill via a ventilation gallery. The only access to the ventilation center from outside will also be located on this level.

The fourth top level will accommodate the electromechanical facilities of the Uetliberg tunnel such as control systems and emergency power supply. The ventilation and air conditioning equipment installed there will also ventilate internal spaces.

Due to its extended function and considerable repair work required in case of fire, extremely high demands were made on fire protection. The fire protective panels are 10 cm thick, prefabricated and have a quality similar to concrete. Fire tests at temperatures of up to 1,350°C that lasted several hours showed excellent resistance and insulation characteristics. As a result, the temperatures at the level of the lower reinforcement and post-tensioning are effectively limited to less than 150°C.

The Reppisch valley ventilation center was designed as a monolithic structure. It was post-tensioned in the transverse and longitudinal directions and partly in the vertical direction. Joints are located at the transitions to the tunnels. The ventilation center is designed to resist uplift (as it is 14 to 18 m below ground water level) and after backfilling will bear superimposed loads of 2 to 15 m, including the Reppisch creek bed. DSI supplied GEWI ^® Reinforcement Systems to connect the walls and slabs. The ventilation center was continuously designed as a "bath tub".

In addition, the two upper storys accommodating electromechanical facilities will be protected against surface water by means of polymer bitumen waterproofing sheets (PBWS). All of the posttensioning operations for this challenging construction project were carried out by the Joint Venture Vorspannung LZ Reppischtal consisting of DSI, Stahlton AG and Geniteam SA. A total of approximately 70 km of posttensioning sytems were supplied and installed in this structure. In addition, special concrete with delayed setting was used for the 1.5 m thick foundation slab. The post-tensioning of the foundation slab with installed longitudinal and transverse tendons had to be carried out within a very short period of time, since the concrete had cured after three days.

Unfortunately, the construction site did not escape the flood that affected large areas of Central Europe in August 2005. Consequently, there were some delays in the construction schedule. For example, the production of the fire protective panels had to be temporarily stopped as a result of flooding of the precast concrete factory. Nevertheless, the original date of completion scheduled for summer 2006 shall be kept.