Zurich main train station, cross-city link lot 3.2 - shaft for south section

To ensure safe entrance of the tunnelling machine for the Weinberg tunnel target shaft of the southern section at the Zürich main train station, a 16 metre-long, 17 metre-wide and 17 metre-high seal block was created in the existing subsoil with low-pressure injections. The goal was to increase stability and reduce permeability of the soil. The top half of this seal block lies in the Limmat gravel (approx. 1,800 m3 of soil volume), the lower half in the lake deposits (approx. 1,500 m3 of soil volume).

To inject this soil matrix, 4 phases were used with various formulations and injection media. In addition to normal cement and bentonites, ultrafine cement, microsilica and silica gels were injected. The contractor determined the mixture by performing tests in advance in the laboratory.

In almost completely horizontal boreholes with casings from the previously created diaphragm wall shaft, plastic sleeve pipes were used at a sleeve distance of 33 cm. The high counter pressure of the water represented a particular challenge (boreholes up to 10 m below groundwater level). A special drill preventer was developed here. The injected material was systematically introduced to the subsoil using double packers via the sleeve pipes. An automated mixing and pumping system digitally recorded the relevant pressures and quantities. The success of the injection was analysed daily using this data and spatially represented.

Map

Divisions performance

Injections

Specific figures

Sleeve pipes

2'400 m

Bentonite

22 t

Normal cement

123 t

Ultrafine cement and microsilica (slurry)

81 t

Silica gel

120 m3