Construction Method

Preparatory works

Construction of the installation sites in Wannenboden, Reppisch Valley and Gänziloo were begun in June 1999 (treatment facilities for the tunnel, excavation and surface water, drainage systems with stormwater and wastewater pipes, surfaced site road systems etc.).

The tunnel will be driven downwards from the Reppisch Valley excavation site under the Uetliberg through to the Zurich-South interchange in the Brunau area (SG-DIE, MO-UET).
The upward driving of the Juchegg soft ground section (SG-JUC) will be driven from the Gänziloo excavation site (Brunau side).
The downward driving of the Gjuch soft ground section (SG-GJU), followed by the 500 m long Eichholz molasse section (MO-EIC), will be driven from the Wannenboden excavation site under the Ettenberg in the direction of the excavation site in the Reppisch Valley.

 

The Gjuch, Diebis and Juchegg soft ground sections

All the soft ground sections are being driven using the core construction method. The excavation support in the soft ground sections consist of steel arches (HEM 180 girders spaced 1 m apart) and 25 cm thick steel fiber reinforced shotcrete.
Some large-scale support measures will be necessary to cut through these ground conditions which, in geotechnical terms, are extremely difficult (injection, water table lowering, preliminary drainage bores etc.).

Dividing of soft ground cross-sections


The molasse sections

The Eichholz molasse section will be excavated in the extension of the Gjuch soft ground section. The crown of the tunnel will be driven by drill and blast, core, left and right side walls and base are excavated by ripping. The standard rock support is generally mesh-reinforced shotcrete and rock bolts.
In critical areas, particularly in the section driven under the SBB-Landikon tunnel, steel supports have been specified.

In the Uetliberg molasse section, the tunnel is being excavated using a tunnel boring machine (5.0 m diameter) followed by a tunnel bore extender (TBE) which applies undercutting techniques.



The tunnel bore extender will be used to widen the previously cut pilot tunnel to its full cross-section diameter of 14.20 to 14.40 m. The safety equipment, comprising of cable bolts, Swellex rock bolts, mesh and shotcrete (possibility exists to install steel) will be installed directly behind the boring head.

 
The sealing, the base, the service duct (prefabricated elements) and back fill on the sides will be installed from the back-up equipment.
 
The lining and the intermediate ceiling will be installed in stages in the rear.