The main challenge in the construction of the 705 m long Vötting Tunnel: The conventionally driven section had to be excavated under a built-up area with an overburden of in part only 8 m. The project was further complicated by over-consolidated Tertiary soil consisting of gravels, sands, silts and clays in varying composition and distribution over the entire height of the tunnel.
In order to secure the loose rock in advance, a special rammed spile screen was used in the crown area. The screen was successively extended with 4 m long partially perforated spiles with a diameter of 50 mm, which were driven into the ground at an ascending angle of around 16 degrees. Next, mechanical packers injected the acrylate gels, each adjusted to the properties of the respective soil section, into the ground via the spiles. The excavation was carried out with round lengths of 2 m and subsequent shotcrete support.
Use of Two Acrylate Gels Over the Entire NATM Section
Two types of acrylate gel were used for grouting, which can be applied to a wide range of fine sandy and silty soils and whose properties can be individually optimised to the respective circumstances on-site:
Rubbertite – a low-viscosity injection gel grout with high flexibility, which is approved under groundwater protection regulations and exhibits a high degree of long-term stability
Variotite – a low-viscosity injection gel with high flexibility and very high expansion capacity
Thanks to the use of acrylate gels, grouting worked even in rock sections with high cohesive properties that could hardly have been injected with cement based grouts.
This method of advance was applicable across the entire spectrum of traversed soils. The pipe umbrella section originally planned for the last 140 m under a closed road with adjacent buildings was replaced by rammed spiles with the agreement of all parties involved. This section was also completed without any problems and on schedule.
The construction of the Vötting Tunnel demonstrated how highly specialised construction materials from the laboratory, together with innovative injection methods, enhance tunnel construction with safe, fast and economical alternatives.