Special Foundations for the new Hungerburg Funicular Railway, Innsbruck, Austria

Stabilization and Foundation using GEWI ^® Piles and DYWI ^® Drill Hollow Bar Anchors

Project
In 2005, after a detailed assessment of various project solutions, the Innsbruck city council decided to build a new funicular railway from the congress center near the city center to the district of Hungerburg. The existing “Nordkettenbahn” railway, which leads from Hungerburg to Hafelekar, the highest mountain in the Karwendel mountain range, was also fully reconstructed including all of its stations. The realization of this solution required a public-private partnership model (PPP) because the city was unable to raise the total project cost of approximately 50 million Euros.

The architectural challenge was to maintain the historical buildings to a large extent and to create a modern design for the stations that was in accordance with nature and the landscape.

Special Foundations using Micropiles
Due to the difficult ground and water conditions as well as the steepness of the terrain, special construction methods and micropile systems were needed for the foundation of some buildings and bridges.

Congress Station
The foundation is positioned at a depth of 5 to 7m, with the calculated ground water table located 1m below the top ground surface. Consequently, the foundation had to be stabilized to resist uplift forces. Due to limited space, a micropile solution for small drill rigs was chosen in order to achieve an adequate distribution of the uplift piles. The ground consisted of sediments of the river Inn: slightly silty to medium silty, sandy gravel consisting of stones in diameters of 300 to 400mm. Double corrosion protected GEWI ^® Piles with steel diameters of 40 and 50mm were installed at the site. The service load of the piles was between 345 and 540kN, with pile lengths ranging from 6 to 9m.

Alpenzoo Station
Due to the steepness of the ground, the basements of this station could not be excavated conventionally. To reach basement level, very large soil nail walls had to be constructed. Earth pressure acting on the retaining structure was not taken into consideration; therefore, both the soil nail walls as well as the micropiles were designed for permanent use. Due to the installation method of the DYWI ^® Drill Pile (uncased, only cement grout for corrosion protection, applied during drilling), a reduction of load using sacrificial corrosion was taken into consideration in the design calculation. This means that the ultimate load of a standard type R 51N DYWI ^® Drill Pile is 800kN, the reduced ultimate load is 721kN, allowing for a corrosion rate of 1.0 mm at the bar surface over a period of 50 years. At this location, the soil consisted of dense silty sand down to a great depth. The pile working loads were approximately 480kN with pile lengths of 6 to12m. The piles were subjected to alternating loads and had to be  rilled at various angles of up to 45°.

Track between Alpenzoo and Hungerburg Station
In this section, the track runs mainly on a steel bridge construction up to the Hungerburg Station. The soil conditions were as described in the previous paragraph: dense silty sands, partly with gravel. Therefore, the bridge columns are also founded on DYWI ^® Drill type R 51N micropiles with lengths between 5 and 8m.

Conclusion
This highly demanding project of constructing the new funicular in Innsbruck, Austria, exemplifies the multiple applications of micropiles using either GEWI ^® Piles or DYWI® Drill Hollow Bars.

These micropiles have proved to be very versatile in foundation, stabilization and anchoring projects in both urban and alpine areas. The foundations can be constructed in nearly all kinds of soils, including rock conditions and in difficult terrain, using lightweight drilling equipment. Technical advantages and commercial competitiveness are further benefits of this system.